Boeing Test Pilot

Boeing has a new blog to follow the development of the new B777-200LR. I especially liked this article, by Boeing 777 test pilot Suzanna Darcy-Hennemann. She is the chief project pilot for the -200LR, and I hope she writes again later with more details on what is tested and what kinds of adjustments her trials lead to. It's very rare to see an aviatrix of her age in the industry. I wonder when and where she started and what she's done along the way.

I agree with her about the beauty of the early morning. It almost seems a crime to slice into the quiet stillness of the dawn by firing up an engine, and the air is so smooth. The earlier I can depart, the better. I made a flight services briefer laugh once by stating my flight plan departure time as "as soon as I can tell the mountains from the sky."

Intake Deflectors

Attentive readers know that the air intake on a PT6 is near the back of the engine, but remember, that's a description only of the engine. The engine itself is a little thing. If it were a shell, I'm not sure I could squeeze myself inside it. It only weighs twice as much as me, and it's made of steel and titanium, while I'm made of squishy girl bits. The engine is housed inside a nacelle with a great big scooplike hole in the front, underneath the propeller, and that is where the engine air enters the cowling.

It is undesirable to have snow, ice or sand entering the engine, so each nacelle is equipped with an intake deflector, a hinged screen that drops down from the top inside the scoop at an angle, to block about 70% of the intake area. The effect is to deflect any solid particles downwards. Air still enters, and comes around the corner of the deflector and enters the engine, but solids can't make the corner and rattle out through a small door in the rear of the cowling. I was going to compare it to a mass spectrometer, but analogies work better when they reference things people are familiar with, like Q-tips and bacon. Intake deflectors have nothing whatsoever to do with Q-tips or bacon.

The air can negotiate the corner around the intake deflector, but there is a loss of pressure because the ram air pressure is reduced and the airflow is disturbed by the presence of the deflector screen. Extending the deflectors reduces engine torque by a pound or two. That's only a few percent, so it's tolerable. The deflectors are required during flight in visible moisture (i.e. rain, snow, mist or clouds) while the air temperature is below five degrees. Some operations even wire the deflectors in the permanently deployed position, to avoid maintenance problems with the complicated deployment and retraction system which I am about to describe.

The cockpit control for the intake deflectors consists of a 3-position toggle switch spring loaded to the centre OFF position. When the pilot pulls the switch down to the EXTEND position, that opens a solenoid in each nacelle, allowing bleed air pressure into the actuators for the intake deflectors. The bleed air pressure pushes the deflector plates down towards the extended position. The motion of the deflector plates pulls cables to open the previously mentioned small door at the back. After three to five seconds the deflector plates reach the fully extended position and engage with spring loaded levers that lock the plates in place, with pins. The pilot can release the switch and allow the bleed air valve solenoid to deenergize.

A pair of microswitches connected in series on each nacelle activate the deflector indicators, causing them to display EXT when the deflectors are locked in the extended position and the small door is open. The indicators are blank when the door is closed. Power for the indicators, and power to operate the bleed-air solenoid comes off the right DC bus bar.

When the pilot lifts the switch to the RETRACT position the solenoid valve is opened again, extending the deflector plate far enough to disengage it from the locking mechanism. sort of like how you have to pull down on a projector screen in order to roll it up again. The release lever electrical solenoid energizes, rotating a cam and disengaging the locking pins. Full retraction of the locking pins operates yet another microswitch, which deenergizes the bleed air valve solonoid and allows retraction springs and ram air pressure to push the deflectors back to the up position, closing the rear doors at the same time.

The pressure required to extend the deflectors against the force of the springs and ram air is quite high, so N_G must be above 80% for successful extension.

Some early models didn't have the door in the back: I guess they just collected a pile of ice or sand inside the nacelles. Normal deflectors use a coarse wire screen, but for desert operations a finer screen may be used.

Accident Investigation

David asked me how long after the accident it took me to figure out what had happened, and I have to admit it wasn't until that evening when the eureka moment arrived. It was fun piecing together what I knew and suddenly realizing the accident sequence. I mean, I knew I was being hit by something: it made a loud noise, and I could immediately tell I was injured, but the what happened how took some deduction.

Everyone from the first fire department paramedic to the doctor was asking about my neck, but it was fine. People expressed surprise that I didn't have whiplash-type neck injuries. I've thought about it and realize that I sit very erect while driving, with my head already almost against the headrest, so there was no significant travel for my neck to whip through. My injuries seemed to have been sustained as a result of the second impact, on the car ahead. And what disturbed me was that I had no memory of seeing my car close in on it. I was asked "did you take your foot off the brake when you were hit?" Surely I would remember the sight of my car ramming another after I had come to a complete stop behind it, and would remember trying to brake, or honk a warning, or something. I just remember seeing the car ahead of me not pull away with the others when the light changed, and it was still a decent distance away from where I remembered my front bumper being located. I knew I was lucid and clear-headed. There was no way I had lost consciousness or had any memory loss. So why hadn't I seen myself hitting the car ahead? Did I close my eyes?

That evening I had someone help me inspect my more minor injuries. I had had my seatbelt on, of course, but I could find no bruising on my hips or shoulder: something you'd expect to find if I had been thrown forward hard enough to break my back. Had the inertial reel seatbelt failed? I've never liked those things. Give me a five point harness that I can cinch down tightly, and that holds me equally on both sides. But if the seatbelt had failed I would have struck the dashboard. I have no injuries on my chest or face. I do have cuts and bruises on my shins and knees, but the front end damage was not enough to compress the engine towards me.

I had assumed my spine had been compressed by being bent forward further and faster than allowed for by its design limitations. That's not what happened. The initial impact from behind slammed me against the back of my seat and broke it, leaving me flat on my back ramming the other car feet first. I couldn't see it because I was lying on my back, thinking "damn, I just got rear-ended." The concussion of the second impact was in line with my spine, travelling up my feet and legs. My car stopped and the car in front continued further, like two marbles trading momentum, and the force of the impact swung my broken seat back up a little so I could just see the car ahead.

It felt as satisfying as a good murder mystery to have worked that out. That's why there were broken pieces of plastic dashboard on the floor, and scrapes on my shins. My doctor says that's why my leg muscles are probably sore, not from the awkward way I've been walking. The human body can take a lot of G-force, so long as it isn't directed along the wrong axis.

Engine Instruments

When you look in the cockpit of an airplane and are taken aback by the bewildering array of instruments, part of the problem is that a good proportion of the instrumentation is doubled or more. The engine instruments are multiplied by the number of engines. The flight instruments are multiplied by two for two pilots, sometimes with tertiary redundancy in standby instruments. There may also be more than one of the same navigational instruments. Faced with this array, the casual observer can be left unable to identify something basic, like a clock.

The engine instruments are usually right in the middle of the panel, in a column n wide, where n is the number of engines. I'm going to go over what I need to know about the engine instruments on my beastie.

Torquemeter

The torquemeter indicates the pressure being delivered to the propeller reduction gear. It's marked in psi with a red line at the maximum of 42.5 on the earlier models and 50.0 on the later ones. It is driven by a cylinder assembly mounted on the rear reduction gear. Gearcase oil pressure is delivered to the cylinder. Increased torque drives a piston which increases the oil pressure in the cylinder. The oil pressure is compared to gearcase pressure, in order to compensate for a decrease in pressure with altitude, and the difference transmitted (1A 26V AC) to the cockpit gauge. If AC power fails, the indication drops to zero.

These are the top centre gauges on the dashboard, right under the fire handles. Engine torque is the main measure of engine power, but to actually determine horsepower output, propeller speed must be considered.

Propeller RPM

Propeller RPM or Np is reported by a tachometer located at the front of the reduction gear. It generates its own power, so no aircraft electrical buses are involved. It is indicated on gauges right below the torquemeters, measured in percent, even though 100% doesn't necessarily mean anything, as max rpm is 100% on the early models by only 96% on the newer ones. The gauge shows marks every two percent, with the numbers displayed and larger marks every ten percent. There is also an inset gauge that shows 0 to 9 around in one circle, indicating 1% intervals more clearly than the little hash marks. It is important to set rpm precisely.

Shaft horsepower is obtained using the formula:

SHP = (Np x torque) / 172.77  (for the PT6A-27)

and

SHP = (Np x torque) / 170   (for the PT6A-20)

The numbers 172.77 and 170 are the thermodynamic constants for the respective engine models, along with whatever is needed to make the dimensions of SHP work out. And you know that when they give the divisor to five significant figures that you'd better not set the torque to only three sig figs. "38.671 pounds torque set, captain, sir!" Um, yeah. There's also a custom made circular slide rule that will help do the calculations, but I don't have one. Yet.

If you're beginning to get the idea that you don't conduct a takeoff in this airplane by shoving the power levers up to the firewall, or even up to the red lines, you're right. The PT6A-27 is derated for this airplane, meaning that you act as though it's a 620 hp engine when it's actually capable of 680 hp. (If you used full power, and one of the engines failed, the airplane would be uncontrollable.) You calculate the appropriate setting and set that at takeoff, whether or not the power levers can still go further forward. If you cannot achieve the torque required without exceeding T5 or N_G limits, then you have a maintenance problem and you go take a coffee break while someone in coveralls deals with it.

T5

This is the temperature at station 5 of the engine, between the power turbine and the compresser turbine. As I mentioned before, it's not the hottest zone of the engine, but it's much easier to measure temperature here. Sort of like the old joke about the drunk looking for his keys near the lamppost. Under test conditions it is possible to measure the temperature in the hottest part of the engine, station 4, where the maximum is 1050 degrees. A relationship was found between there and T5, so that by observing T5 you can accurately predict the corresponding T4. So the pilot memorizes this:

Phase of Flight	PT6A-20 T5 max	PT6A-27 T5 max
Takeoff & Single Engine Emergency	750	725
Max Climb	725	695
Max Cruise	705	695
Idle	685	660
Starting	1090	1090
Acceleration	850	825
Max Reverse	750	725

T5 is measured via eight thermocouples wired in parallel, and is powered by the voltage produced by the temperature difference across the probes, so requires no aircraft electrical power.

GG RPM

This is the rotational speed of the gas generator, the rear shaft of the engine. It is sensed by a tachometer generator driven by the accessory gearbox, at the rear of the engine. Typing "accessory" just gave me a flasback to grade eight English, when the teacher insisted that accessories meant items that complement a woman's outfit, not "extra stuff you get, like stuff to make your car better." I don't suppose he would have been any more receptive to "extra equipment that runs off an engine drive shaft, like pumps and tachometers." The scale works the same way as the RPM gauge. You just have to remember that this one is at the bottom, under the ITT gauge.

Fuel Flow Gauges

These run off 26V AC and show fuel flow in pounds per hour. The fuel flow transmitters are on the fuel lines above the fuel strainers and before the fuel emergency shutoff valves. If AC power fails, the indication freezes.

Oil Temperature

Left and right DC buses power oil temperature gauges, right above the oil pressure gauges. The probes are on the accessory gearcase.

Oil Pressure

Each oil pressure gauge is two gauges in one. The first reports pressure measured at the accessory gearbox and transmitted by 26V AC to a needle that travels over a circular gauge. AC failure results in a zero pressure indication. Coloured bands show pressures acceptable at idle power only (yellow, 40-80 psi) and at engine speeds 75% N_G and above (green, 80-99 psi). A pressure sensor located near the oil cooler will illuminate a 28V DC low oil pressure caution light if the pressure drops to 40-42 psi. The light goes out again if the pressure rises to 44-46 psi.

Write Off

The car is damaged beyond economic repair, which means will now be auctioned for recycling. If it were an airplane they'd tow it into a hangar, and it would be flying again in a few weeks.

This is the first vehicle of any kind that has been destroyed while I was in it, but some of the airplanes I have flown have been later destroyed under the command of other pilots. It's an odd feeling. I know it doesn't go away. I once helped an 84-year-old pilot look up what had happened to the airplanes he had commanded in the 1940s and he had the same disappointment to discover that once faithful ship was now probably pop cans or siding. You have proof of your connection to the airplane, inked in your logbook, but the airplane no longer exists.

I can think of three aircraft from my logbooks that have been destroyed, one due to loss of situational awareness during an approach on a rainy night, one that had been loaded beyond its centre of gravity limits with marijuana, and one that was involved in a midair collision. They, like my car, were mostly just metal and plastic. The human beings inside were flesh and bone, but I didn't know them, hadn't become familiar with their workings, had never written their names in my logbook. This post isn't about them.

Heating System Malfunctions

As discussed previously, the heater combines the pneumatic system (bleed air) with the electrical system (powered valves) and manual control valves. The function of the system depends on the speed of the airplane, the outside air temperature, the engine power, and even what the passengers choose to do with their gasper outlets. There's a lot that can go wrong.

If it's a hot day, everyone opens their gasper vents and then disembarks without closing them. Lets say the next trip takes us somewhere considerably colder, with a smaller passenger load, so there may be a number of unattended vents spewing cold air into the cabin. Remember, we have no control over that air supply. This problem can be controlled if I close all the gasper vents during the preflight inspection, during cold weather or before a high altitude flight.

If you get cold easily and want to annoy your fellow passengers with an excessively hot cabin, sit in row one on the right side, turn on your gasper vent and direct the flow of air towards the cabin air temperature sensor. It's a box on the forward bulkhead just behind the copilot (right seat pilot). It's sometimes just sticking off the wall and sometimes recessed behind a little grille. It will report a too cold cabin temperature and, assuming the heating system is on AUTO, the system will try to compensate by increasing the temperature. Similarly if cargo obstructs the temperature sensor, the cabin temperature may be reported as too warm, and you will be too cold.

The same air supply that provides cabin heat--nice to have, but not as essential to aircraft operation as, say, functional wings--provides air for pneumatic airframe deicing. A dual pressure switch ensures that sufficient air pressure is available for deicing. If the bleed air pressure from the engine drops below 25 psi, the cabin heat valve will not open. If the valve is already open, it stays open, but if the pressure drops below 20 psi, the valve will motor closed, and stay closed until the pressure is back above 25 psi.

On the ground, the deicing services are not needed, so in order to get heat in the cabin, the pilot needs to set enough torque to get that 25 psi of bleed air. Increase engine N_G until the PNEUMATIC LOW PRESSURE light goes out (at 16 to 18 psi) then add another two pounds of torque. Make sure the vent fan is on.

On the ground, the ram air valve should be at least slightly open in order to move the heated air out of the silencer plenum into the cabin. Otherwise there isn't much point heating the air, unless the pilots just want a toasty warm cockpit floor. Once the air under the floor reaches 300 degrees fahrenheit a DUCT OVERHEAT light (5A CB L DC bus) illuminates to remind us to open the ram air valve.

In flight the ram air valve is usually kept slightly open, but if it's really cold, like latitudes greater than retirement age cold, the ram air vent can be closed, because the hot air entering the system acts like an ejector to drive air recirculation. The air recirculation intake is just above the floor by the base of the captain's seat. And all this time I thought that was just a convenient place to lose your pen. It also melts things you leave up against it, because the air doesn't always circulate exactly the right way.

Stale air normally vents out through ceiling grilles and leaves the aircraft throught he aft baggage compartment. If you have the Venezuelan burrito eating team on board, airflow can be increased by turning the heater to OFF, fully opening the ram air valve, fully closing the cabin air valve and increasing airspeed. In an emergency, the aircraft will be ventilated faster by opening the cockpit windows, but the fumes will come forward through the cockpit instead of out through the rear.

Seagull Test

I'm just testing Blogger's photo hosting to see what it looks like. Does anyone have any trouble seeing this image. Does it make the page too slow to load. Do you like seagulls? Do you think the one in the middle has only one leg?

Update: re-edited to put the photo in the centre and see if two narrow columns of text are less annoying than one. Perhaps the seagull had one leg tucked up to keep it warm, like I would put one hand in my pocket or inside my parka, doing a one-handed task on a cold day.

Ought to Be a NOTAM

A movie set for the latest remake of War of the Worlds depicts the wreckage of a B747 strewn over a destroyed town. The entertainment section of my local newspaper claims that the set, on the Universal Studios backlot, is so realistic that pilots overflying it have called ATC to report a crash.

I can't find any evidence that the story is true, so it might be something that Universal made up as a publicity stunt. The pictures look pretty convincing to me.

Is It Hot In Here?

A day spent with ice packs on my spine seems a good one to talk about heaters.

A pilot I know flying this airplane in a moderate climate confessed to not understanding how the heating and ventilation system works, and I think that's common, as the chief pilot had simply decreed "don't touch this control at all, and only adjust this one if you really have to." Lets see what's so complicated.

The heating system and the ventilation system use most of the same ducts, and I'll start with ventilation. Early models--and remember most of them are still flying--had separate ram air scoops for the passengers and for the pilots, but later models had one bigger scoop for both. Passengers can adjust gasper outlets (those little twist-to-control air blowing thingies) above their seats, and according to the manual the pilots don't control the flow of air reaching the gasper outlets. Control freaks that we are, we could increase the airflow by putting the airplane into a high speed descent, but we'd rather not.

I can see from the schematic that there is one more way we might affect cabin air flow, at least in the later models with the shared ram air scoop. The duct just inside the scoop divides into two channels, one of which goes straight to the cabin gasper outlets while the other leads to the manually controlled ram air valve, controlled with a knob by the captain's right ankle. In the up position, the valve is open and ram air is being freely admitted to the heating and ventialtion system. Lowering the knob closes the valve until in the fully down position no ram air is going to the heating/ventilation. You'd think in that case more air would be available to the gasper outlets. Perhaps the airflow is already so great it makes no difference. I'll have to experiment on hapless passengers some time and find out.

Whatever it does to the gasper outlet supply, the ram air valve admits air to a space (called a silencer plenum) under the cockpit floor from where it can go to the pilots' footwarmers, the windshield heater outlets, and the cabin air control valve. If the cabin air control valve is open, then the air passes through it to the cabin baseboard outlets.

The knob for the cabin air control valve is inconveniently located on the floor behind the co-pilot's seat. This was the "never touch" knob mentioned above, and from the location it seems like the manufacturer didn't want us messing with it either. Pull it up to close it, and down to open it. (You didn't think it was going to function in the same direction as the ram air valve knob, did you?) So the cockpit and the cabin are sharing ventilation air. The manual recommends than in hot weather we fully close the cabin air control vent, keepig all the ventilation air for ourselves. We are, after all flying the airplane, and all you're doing is looking out the window or reading the paper. In case we feel guilty for depriving you of ventilation air, the manufacturer assures us that even if we shared it, the low velocity air exiting the baseboard outlets wouldn't cool you much. We also have little electric fans blowing air on us, and we can open our windows.

The same model number that combined the passenger and crew ram air scoops added an electric fan (20A L DC bus) inside the air duct to provide cabin ventilation on the ground. It should be turned off in flight.

The heat for the heating system comes from the hot bleed air takes out of the engines at station 3. It is routed from each forward engine nacelle, and up through check valves to a common duct in the cabin roof. From there bleed air intended for heating is routed forward along the left side of the fusilage, through the bulkhead that separates the cabin from the cockpit and up to an electrically controlled (5A L DC bus) valve under the cockpit floor. From there it may be injected into the heating and ventilation system, just after the ram air valve.

That is all relatively simple. Now we get to the heating controls. The heater has three modes: OFF, MANUAL and AUTO. The OFF mode closes the cabin heat valve and supplies no heat to the system, but if contamination of the bleed air is suspected, the bleed air switches should be selected off too.

The MANUAL mode allows the pilot to operate another three position switch marked MANUAL WARM, HOLD, and MANUAL COOL. The switch is spring-loaded to the HOLD position and gives the pilot direct control over the setting of the cabin heat valve. It takes about 30 seconds for the valve to motor between fully closed and fully open, so presumably with practice you can turn the the heater from OFF to MANUAL, then hold the switch down to MANUAL WARM counting "one Mississauga, two Mississauga ..." until it reaches the desired openness. It would take quite a bit of practice, because the temperature thus produced depends on your engine speed, outside air temperature, aircraft speed and ram air lever position.

The AUTO setting is supposed to maintain a constant cabin temperature in flight, despite all those variables. Why does the MANUAL setting exist, then? I told you: pilots are control freaks. (Did I mention that it bugs the heck out of me that since the last Blogger upgrade a space is enforced between the end of a blog entry and the byline? Damnit, if I wanted a space I would have put one there.) We'd rather be too hot or too cold than not be able to control a valve ourselves. The automatic heating control works a lot like your house thermostat, except that the dial is just labeelled COOL and WARM with a little arced arrow between, rather than having exact temperatures marked on it. A temperature sensor, in the back of the cabin in early models and at the front in later models monitors the temperature. There's a little fan mounted over the forward temperature sensor in order to prevent air stagnation around it. The outside air temperature, cabin temperature, and silencer plenum temperature are compared via a Wheatstone bridge--hey I remember that from highschool physics, never thought I'd care about it ever again--to the temperature selected on the dial.

I'll save malfunctions related to this systems for another day.

If you're ever a passenger on an airplane small enough to have no flight attendants (as this one is) and you are too cold or too hot, wait until one of the pilots looks back into the cabin--we're supposed to from time to time to make sure you're not rioting or having an orgy--and make eye contact then signal your discomfort with either mimed shivering or exaggerated fanning yourself. If we're not completely clueless we'll try to do something about it. Another tactic is to write a little note and pass it up to the front row. In this day and age you have to be extremely careful that "tell pilots it is too cold in the cabin" isn't misread as "tell pilots I have a bomb in my carry-on" so please use neat penmanship.

"Hijackers"

One of the challenges for airlines is to hire people who are mature, responsible, intelligent, make good decisions, and who chose aviation as a career. Why would anyone with any brains decide to be a professional pilot? The answer seems to be a combination of hooked on flying, and arrogant enough to believe that everything will go his way, so that the career will work out. And then the airline has the problem of a bunch of arrogant pilots.

Several years ago Air Canada took over the faltering Canadian Airlines International and merged the two companies. The seniority lists were merged too, discounting the years of service of ex-CAI pilots. The original Air Canada pilots say, "if we hadn't rescued you, you'd hav no job at all, and would have had to start from the bottom of the seniority list." The ex-CAI pilots say, "We've put in as many years as you have and are part of the new merged company, why shouldn't we have the same credit for time served?" The acrimony is so great that the rift between the two factions will not be closed until, and may outlive the day that the last pre-merger pilot takes his retirement.

The Boeing cancellation was the result of a playpen fight between "Original Air Canada" and ex-CAI. They couldn't agree on how to get paid for the new shiny toys, so they tossed them out the window so neither of them gets them. No wonder I couldn't find any sane reason to cancel the order. One spokesman was calling the other faction "hijackers." The company is trying to call the union back for another vote.

Oddly, someone claimed that because OAC pilots outnumber ex-CAI pilots eight-to-one on the bottom one-third of the seniority list that proves OAC pilots were unfairly treated with respect to seniority merging. That made me blink. It proves nothing. OAC pilots outnumber CAI pilots at the company as a whole. At the time of the merger, Canadian was laying off, not hiring, so had very few low seniority pilots. Low seniority ex-CAI pilots have also likely had a greater rate of attrition, going to Canadian North, and other companies that have become defacto ex-CAI havens.

Even when I know the reasons, I can't identify with either faction. I remember sitting on the balcony of an ex-CAI pilot's home, and listening to him explain the hardships the seniority list merging had visited upon him. I was taking a midday break from a flying job that paid $300 in a good week, and nothing in a week of rain. I just nodded and made sympathetic noises while I sipped my lemonade and watched his horses try to get into his swimming pool.

Compression Fracture

The morning started beautifully. I got to sleep in because of a later than usual start time. I had a delicious breakfast. The weather was glorious. On the way to work I stopped for a fill up, and won a can of Pepsi on the scratch and win ticket that came with the gas. Continuing to work, I was ahead of schedule so a red light was of no concern to me. I braked gently and waited behind another car.

My window was open and my radio wasn't on, but I didn't hear any braking. I never saw the car that struck mine from behind at what police say was 60 km/h. The ambulance attendants said that it was just as well I didn't see it, because there was nothing I could do to escape, and tensing up in anticipation of the impact would have made my injuries worse. The driver's seat had collapsed somehow but I could still reach to turn off the ignition and put on the handbrake. The horn didn't work to summon help, but the driver ahead, whose car mine had been pushed into, came back and helped me call for an ambulance. The fire department came first, determined that it was not necessary to cut the car open to extricate me, and turned my care over to the ambulance paramedics. They asked me lots of questions, including hard ones like my name and the day of the week, loaded me onto a clamshell stretcher, fitted me with a collar to prevent my neck from moving, and took me to hospital. After half an hour or so in a corridor I was taken to an exam room and poked and prodded and asked all the same questions I was asked in the ambulance. They sent me for x-rays, and finally told me I had a compression fracture of one of my vertebrae. It's the one roughly behind my belly button, maybe a little higher.

The doctor showed me the x-rays. I could see a column of ghostly tubes aligned one above the other, and I could clearly see that one of the tubes had a ridge around it that the others lacked.

"Imagine if someone took a pop can and lightly tapped it with a sledgehammer," explained the doctor. "It would compress down a little bit, becoming shorter, with a wrinkle around it. That's what has happened to your vertebra."

"Like the fuselage damage when an airliner lands hard on the nosewheel," I said. But he wasn't familiar with the recent Skyservice incident.

"There's not much we can do to help you, just manage the pain, and let you heal."

"So the wrinkle just fills in? Goes away?"

"No, it stays. You know how people get shorter when they get older? Well you just got acutely shorter."

The ambulance attendants had asked for my height to adjust the stretcher. Guess I got it wrong. It took me a moment to remember that acute means "sharp, sudden" and not "severe".

The police had the remains of the car towed to an impound yard. I sent someone to collect my bag from the wreckage and mentioned, "plus there's a can of Pepsi you can have for your trouble."

When he brought the bag, he brought cellphone pictures showing front and back end damage, no broken glass and not a lot of crumpling, but a crease through the body that gave me company in the being-a-bit-shorter-than-yesterday department. It's probably a write-off, and I've had it since the first week of my aviation career. Finally, he produced the can of Pepsi. I turned it around and held it up to the light to discover a wrinkle running half way around the circumference of the aluminum skin of the can. Three for three: me, the car and the can.

This entry outs me to anyone at work who has been reading this blog, but really what's in here that anyone couldn't guess? I have flip-flopped several times on my decision to post this personal story, but it's definitely part of my career. I won't know for a few weeks whether the injury will cost me the job at Ichneumon, or make it difficult for me to continue my current job. It will definitely cost me a few weeks of revenue at the peak season of the year. Can a person develop an Advil addiction?

More Spinning Things

The PT6A engines drive three-bladed, constant-speed, full feathering propellers. Both rotate clockwise, viewed from the rear. A typical cruise setting of 76% Np is equivalent to 1672 rpm. A tachometer on the side of the reduction gearbox gives a cockpit indication of propeller speed.

Airplanes that work properly are really fairly easy to fly, and anyone can learn to fly straight, turn, and go up and down in an hour or so. The taking off and landing bit can be a bit trickier, but really a few hours and you can do that, too. The hard part is continuing to fly the airplane when things stop working, so most of learning about a new airplane is learning how things work, and what they are likely to do when they stop working.

If you have two engines, and one stops working, the non-working one contributes only drag to the operation, holding back its side of the airplane, and the working one continues to pull its side forward, causing the airplane to yaw towards the dead engine.

To make things worse, Murphy's Law ensures that any non-workingness occurs at the most inconvenient moment possible. For an engine failure, such a moment is when you are using the engines to climb at a low speed, so the axis of rotation of the propellers is angled up relative to the oncoming airflow. This means that the plane of rotation of the propellers is not perpendicular to the oncoming air, and thus the downgoing propeller blades meet the air at a steeper angle than the upgoing ones. The steeper angle of attack means more pulling force for the propeller on the downgoing side. That's the right side for both propellers, but the closer-to-the-middle side for the left engine and the further-from-the-middle side for the right engine. That means that if the left engine fails, the remaining engine has more turning moment than would be the case if the right engine had failed. For this reason, the left engine is called the critical engine.

To reduce the drag of the dead engine, its propeller can be feathered. That means that the blades can be turned parallel to the oncoming air. Normally in flight, oil circulating through the propeller hub holds the blades at an appropriate lower angle, opposing both a set of rotating counterweights and the feathering springs in the hub. When the pilot selects the propeller control to FEATHER, by pulling the propeller lever full aft, a pilot valve cuts off the oil supply to the hub and the pressure drops. If the propeller is rotating, counterweights drive it to the feathered postion, of 87 degrees to its plane of rotation. If the propeller is not rotating, the feathering springs alone will do the job.

Later models incorporate an autofeather system. If the autofeather system is selected on, both power levers are far enough forward that the engine would normally produce at least 86 to 88 percent N_G, and the torque of one engine decreases to 11 psi for at least two seconds, the autofeather solenoid valve opens and oil is dumped from the hub, allowing the feathering springs and counterweights to drive the propellers into feather. The pilot has to be careful not to pull back the power lever of the failed engine--the normal reaction-- because that would inhibit the autofeather mechanism.

To test the autofeather mechanism, the pilot (on the ground) sets up all the pre-conditions for the autofeather to work: both engines running, autofeather selected on, power levers moved up to 88% N_G, torque on both engines indicating 20 to 30 psi, and waits for the the amber ARMED light to illuminate. Then the pilot lifts one of the AUTOFEATHER TEST switches, which cause the system to not notice if a power lever has been pulled back. Then the pilot pulls back the power lever on the same side as the test switch and waits two seconds. The propeller should feather. The pilot then repeats the procedure for the other propeller.

That is about one twentieth of what I have to know about the propeller system. This system makes me wish it were a jet.

Weapons in the Cockpit

Someone e-mailed me a link to what I assume is a joke cabin PA. The person who sent it merely wrote, "I want to fly with this pilot." The first time I watched it, the audio was turned off on my computer, so I was watching video only, scrutinizing the clouds waiting to see a lightning strike, a narrowly avoided oncoming aircraft or a roll of toilet paper thrown out of the cockpit. When nothing happened, I activated the sound and replayed it, discovering it to be a pro-gun statement.

Pilots who fly armed should be proficient and confident in the use of their weapons. A pilot who is not a good shot would probably do more harm than good. A pilot who would hesitate to use the weapon when the need arose, would simply be supplying a weapon to the attackers. Skill could be assured by adding firearm proficiency to the already long list of initial and recurrent training items that airline pilots must demonstrate proficiency at, but what about the attitude?

To a certain extent, the pilot-in-command attitude already suggests the attitude of someone who would not hesitate to shoot someone who threatened the safety of an aircraft. Thats the attitude that causes a pilot to take whatever action necessary to assure the safety of a flight, to take personal responsibility for everything that happens on that flight and to be able to take control from another pilot, even a senior one, if a dangerous situation is not being corrected. But some people are repelled by the idea of carrying guns.

Should pilots be allowed to carry guns if they are competent and willing to use them? After all, someone who is not ready and willing to use a firearm in defence of the aircraft probably wouldn't want to carry one anyway. Some people would say that someone who is eager to carry a gun onto the flight deck shouldn't be allowed one. I believe that the risk of a pilot coming to harm because he or she didn't have a gun on the flight deck is lower than the risk of accidental harm resulting from the presence of a gun on the flight deck. It's not that the risk of gun accidents is high: it's probably very low. But the chance of needing the gun is vanishingly low, because hijackings are very rare. Police--a more highly trained gun carrying group--have gun accidents, but that accident risk is worth it to them, because police have a far greater likelihood of needing to use the gun, daily and over their careers.

With reference to the linked video clip, a flight attendant should not be armed, especially not known to be armed, because passengers are close around him for legitimate reasons for the whole flight, so he is far too vulnerable to having the weapon taken away.

I wouldn't object to taking weapons training to carry a gun on a flight deck. (There are very few areas of skill or knowledge that I would decline an opportunity to learn). I once worked for a company which responded to post-September-11th concerns by arming me with--I swear I'm not making this up--a big stick. The stick was already present for another purpose, but the person training me indicated it with all seriousness as my weapon of defense against unlawful interference.

Spinning Things

Ezekiel saw a wheel a-spinning, way in the middle of the air.

In response to a question on engine outputs, I'll talk about gyroscopic instruments and what makes the gyroscopes spin.

The airplanes I fly use compasses that are not hugely different from the compasses that got Cabot and Cartier across the Atlantic to Canada. A magnetised component seeks to align itself with the Earth's magnetic field, indicating the orientation of the observer on a compass card. The compass is prone to large errors while accelerating or turning and and is hard to read in rough air. For this reason, the magnetic heading is transferred to an instrument which is not north seeking, but instead keeps track of where north is by remaining stable in its orientation while the airplane turns around it. The instrument is called a heading indicator. The pilot must update the heading indicator on the real heading periodically throughout the flight, but in return doesn't need to squint at a bobbing compass for every heading.

Another instrument, the attitude indicator, gives a cockpit indication of the aircraft's situation with respect to the horizon: nose up, nose down, and/or banked left or right. This is useful when the real horizon is obscured by darkness or clouds.

Both these instruments maintain stability through the operating principle of the gyroscope: a wheel inside that spins so fast that it maintains its orientation instead of turning with the airplane. It is gimballed to allow it to do that, and the motion of its turning relative to the case around it are transferred through ingenious levers to the face of the instrument. A third instrument also contains a gyroscope, but it takes advantage of the principle of gyroscopic precession--the tendency of a gyroscope to change its motion around a different axis than the one where a force is applied. This post is not about these instruments, nor about the gyroscopic principles, but merely about what keeps the gyroscopes spinning inside.

Early gyroscopic instruments were powered by a venturi tube mounted on the outside of the airplane. Some of them are still out there. The tube looks like an old fashioned car horn, flared out at the front and then narrowing in the middle. As the airplane flies (it doesn't work on the ground) air is forced by the pressure build up at the forward flared part to accelerate through the middle part, which causes a pressure drop in the middle part. A line leads from the middle of the tube to the instrument panel, so the pressure drop creates suction through the line. The suction force is applied tangentially to the gyro wheel in such a way as to keep it spinning. The rim of the gyro has little dimples in it to take advantage of the suction. It's like using a vacuum cleaner to pick up dead fish. That's not a very useful analogy but it's too hilarious not to link, and does at least document that suction can do work.

That method of spinning the gyros has certain weaknesses, like the fact that you can't test to see if the instrument is working until after you've taken off, or that the venturi tube is sitting on the outside of the airplane where the rain and snow and ice are. The airplane I plan to be certified on in a couple of months has never used that method to keep its gyros turning, but has used every other method I know of, so its history shall be my explanation.

The early serial numbers used pneumatically driven flight instruments, I believe simply directing the 18 psi regulated bleed air against the gyro wheel at an appropriate angle. That is to say, they would blow instead of suck to keep the gyros erect. They, um, stopped doing that because of moisture problems. I think I can relate.

Later they routed the bleed air through a venturi, just like the kind that used to be mounted on the outside of airplanes, and used the suction thus created to drive the gyros. They seem to have reverted to bleed air pressure for serial numbers fifty-eight to one forty-nine but those persistent moisture problems drove them to try something else. For a while the accessory gearbox drove a dry suction pump and then finally they settled on electric gyros, using 115V AC. The turn coordinators, as a backup, usually run off 28V DC, from the L and R bus bars for the L and R side of the cockpit (the pilots have one each).

My prayed-for future employers probably own aircraft from every serial number range, most of which have been refitted with avionics that the manufacturer didn't dream of, but if I'm familiar with what might be there I have a better chance of knowing what might or might not work in what kind of failure.

Quadruple Pump Failure

If you haven't read the posts before this post yet, skip this post and read down until you get to the post that ends by asking you what you think will happen if all four boost pumps fail. Unless you don't want to play the game.

Clearly something bad might be expected to happen if all four pumps in a system as complicated as that described below should fail n'est ce pas? You'd think. The thing is, if the #1 and #2 boost pumps fail in both tanks, leaving zero operating boost pumps, then four caution lights illuminate. No, the engine doesn't stop. The engine-driven boost pumps that deliver fuel to the fuel control unit and hence the engines at up to 850 psi is quite capable of lifting the fuel out of the tanks, through the fuel filters, through the fuel flow indicators, through the emergency shut off valves, to the engines.

You're not supposed to run it that way for long though. Without the low pressure boost pumps, the high pressure boost pumps will cavitate and wear excessively. Ten hours of operation with less than 5 psi pressure supply to the high pressure boost pumps, and they have to be overhauled. So the DC electric boost pumps boost fuel up to the engine driven pumps at about 22 psi, and then the engine driven fuel pumps pressurizes the fuel and shoves it into the engine.

Day, Redefined

This is the longest day of the year, and the post time corresponds to the solstice. I enjoyed the old Canadian legal definition of day:

"day" - means the period beginning one half-hour before sunrise and ending one half-hour after sunset and, in respect of any place where the sun does not rise or set daily, the period during which the centre of the sun's disc is less than six degrees below the horizon.

I love living in a country where the daily rising and setting of the sun is not necessarily a given. The sun coming up in the morning is often catalogued among the world's indisputables, like unrest in the middle east, bears shitting in the woods, and the Pope being Catholic. It's not unique to my country of course. Other countries also have regions that endure six months of night.

A couple of years ago they changed the legal definition of day, so it's pretty boring. It no longer discusses the possibility that the sun may not rise in the morning.

"day" or "daylight" - means the time between the beginning of morning civil twilight and the end of evening civil twilight.

But they can't change the fundamental nature of the country.

In far northern latitudes, when you consult the GPS page that is designed to display sunrise and sunset times, it doesn't give you a time at all. All you see is the word NEVER. That is so cool I whole this whole post in order to report it.

First Saudi Arabian Aviatrix

Saudi woman Hanadi Zakariya Hindi has completed commercial pilot training at the Mideast Aviation Academy in Jordan and already has a job. Her training was sponsored by the Saudi Prince Alwaleed ibn Talal, and she will be flying private jets for his Kingdom Holding Company.

It doesn't say what type she is to fly, but an airliners.net search suggests that the prince doesn't mess around with little planes like Gulfstreams. Hindi's ride might be this Boeing 767, this Airbus 340 or this Boeing 727.

Saudi women are not permitted to drive cars, but apparently no one ever thought to ban them from flying airplanes, so Hindi (and her sponsor) have swooped through a loophole in the society. The article doesn't say what her uniform will consist of. The link above shows here with her hair loose. Here's another picture of her with a simple headscarf and another with a more severe head covering. And here's a head-to-toe covering (not Hanadi Hindi, but who could tell?) that I'd never want to have to wear while flying an airplane.

I suspect this is more about the political manoevering of Prince Alwaleed than the career ambitions of Hindi, but maybe I'm just cynical. Curiously, all the articles refer to her as "Captain Hanadi Hindi" so that makes me wonder, who is the male FO who reports to this green-out-of-training cause célèbre? I'll take the job, if the prince has a shortage of male applicants.

Fuel Pumps

We've filled the tanks and we've measured the fuel. We know how the engines work. Now we've got to get the fuel out of the tanks and into the engines, which are located on the wings, above the cabin. I think this installment is going to be boring and/or confusing, but this is my blog so I get to write what I want.

As I mentioned when I described the fuel tanks, there are two boost pumps in each collector cell. These are submerged 15 ampere electric pumps, with the #1 pumps powered from the left DC bus and the #2 pumps, or standby pumps, powered from the right DC bus. As you can doubtless deduce from the name, under normal conditions the #1 pump in each tank operates and the standby remain idle.

There is a pressure switch associated with each boost pump, triggered to illuminate a caution light if the pressure upstream of that boost pump falls below two psi. While the #1 boost pump is operating, both the #2 boost pump and its associated caution light are inhibited. If the #1 boost pump fails, the pressure at the #1 boost pump pressure switch drops, and BOOST PUMP 1 caution light for that tank will light up. So will the BOOST PUMP 2 caution light, but only momentarily because the #2 boost pump loses its inhibition and starts pumping. (Wow, did I just manage to make the boost pump autochangeover system sound lewd?) The pressure rises across the #2 boost pump pressure switch and so the BOOST PUMP 2 caution light goes out.

During the preflight inspection the pilot checks the function of the pressure switches by checking that all four boost pump (BOOST PUMP 1 AFT, BOOST PUMP 2 AFT, BOOST PUMP 1 FWD, & BOOST PUMP 2 FWD) caution lights are illuminated before the boost pumps are selected on. There's even a test for the autochangeover system: holding both boost pump switches down to the TEST postion simulates a failure of the #1 boost pump. The pilot should hear the #2 boost pump come on, and the BOOST PUMP 2 caution light should not be illuminated.

If the #1 boost pump should fail, but the autochangeover system doesn't work, the pilot can turn the standby pump on manually, with a switch labelled STDBY BOOST PUMP EMER AFT (or FWD).

There is another indirect way the boost pumps can be turned on and off. If the pilot moves the fuel selector to feed both engines from the same tank, then a valve (5 A CB on the R DC bus bar) opens connecting the two tanks. Both boost pumps in the selected tank are turned on, and both boost pumps in the non-selected tank turn off. Unless the corresponding STDBY BOOST PUMP EMER switch is selected on. In that case fuel would be supplied from both tanks. There has even been a case when a pilot left the STDBY BOOST PUMP EMER AFT switch on while the tank selector was set to BOTH ON FWD, and the #2 boost pump the aft tank was stronger than the forward tank #1 and #2 boost pumps put together, so the engines were fed from the aft tank until the aft low level light went on, alerting the pilot to the situation. (Quick: how many pounds of fuel were remaining in the aft tank when the low level light went on? And if there was an ejector failure?)

So, to recap, if the #1 boost pump fails, the #2 boost pump comes on automatically. If the autochangeover system fails, the #2 pump can be turned on manually. If both boost pumps in the same tank fail, both engines can be fed from the other tank. So what do you think happens if all four boost pumps fail?

Why Not?

Air Canada pilots rejected the big Boeing purchase the company planned, because the new type was tied to a change in the way pilots would be paid for flying what type of airplanes. I'm really disappointed.

Come on people, all those beautiful new airplanes? I can't wait to get my hands on one. The better the company does, the more money there is to pay pilots, and forcing them to buy second-hand, less inefficient airplanes isn't going to increase anyone's paycheque.

Maybe I'm just an ignorant youngster because I haven't transitioned from ogling the big shiny airplanes and saying, "I just want to fly!" to saying, "I just want to be paid!" but hmm. Well, if any Air Canada pilots read my blog, leave a comment, or send me an e-mail and tell me how the new Boeings would have hurt you. They sure wouldn't have hurt me. I was looking forward to flying one, one day.

Update: The CBC story says that compensation for training is the issue. It would be especially juvenile for me to say, at this point, "no one would have to pay me to learn to fly a Dreamliner." That's how this industry got into such a disgusting mess in the first place: pilots like to fly so much that we fly first and worry about being paid later, so we get paid only enough to eat, because training costs go up if your pilots are starving to death all the time.

Got Gas?

I left off a discussion of the fuel containment system noting that the fuel indication system was a topic for another blog entry. You may have been sceptical about the idea of an entire blog entry on the gas gauges. It's somewhat more complex than a little needle over an arc with an F at one end and an E at the other, like in my car.

It starts even more simply than that, with an aluminum dipstick. I'm not sure why "dipstick" is such an insult, this is a state-of-the-art, custom made piece of high technology. When the airplane is on level ground and the boost pumps have been turned off for at least 15 minutes the levels of the different fuel cells can be assumed to be equal. The dipstick is calibrated so the wet mark shows the total weight of JP-4 or Jet B fuel in the tank. (Other grades are denser, so the same level equates to a greater weight of fuel. You'd have to multiply by the relative density to convert.) It's tricky to actually read the level of jet fuel that shows on an aluminum dipstick. Another aviatrix taught me to breathe heavily on the dipstick, creating a sheen of condensation, before putting it in the tank. The fuel level shows up clearly after you remove it. I may have to come up with another system in hot climates.

There is a fuel capacity probe in each of the eight fuel cells that determines the fuel quantity presence based on capacitance. Not capacity: capacitance. A capacitor is a device that stores energy as an electrostatic field between two conducting plates separated by a non-conducting material. Its ability to store energy is called its capacitance. The property of an insulating material that increases capacitance over that of a vacuum is called its dielectric constant. So change the material separating the plates and you change the capacitance. In a fuel capacity probe, the size and separation of the plates remains constant, and the capacitance varies with the fuel or lack thereof in the space between them. So a change in the amount of fuel in a tank changes the capacitance of the probe-fuel-probe capacitor and induces a current, which drives the fuel gauge.

I wouldn't have thought that this would serve as any better a method of measuring quantity than a level detector, but I would have been wrong. Remember the airplane may be fuelled with any of a number of different turbine fuels, of different densities, and the density of the same fuel will change with temperature. Apparently, the dielectric constant is related to density, such that the fuel gauges are calibrated in pounds and read accurately regardless of the grade ortemperature of fuel used. I understand that this may not be completely linear and that some temperature corrections may be required. I lack some information here.

The fuel gauges run off the 115V AC bus. If power to the gauge is lost, the needles do not fall to zero, but remain at the same indication. Pilots say "DC dies and AC lies" to remember the behaviour of failed instruments. You can test the calibration of the gauges by pressing the IND TEST button near the fuel selector. The needle should fall to the zero indication and then return to the same fuel indication.

The AC system is backed up by DC-powered low level warning lights that illuminate when the fuel level falls below 75 pounds in the forward tank or 110 pounds in the aft. If the ejector system has failed and the collector cell has the same level as the other cells, the low level sensors will give false indications and trigger at 330 pounds in the forward tank or 440 pounds in the aft tank. Good test question, that.

There's also a fuel flow meter for each engine, calibrated in pounds per hour. Each is powered from the 26V AC bus, and is protected by a half amp fuse. Sometime I'll tell you a story about why I will never forget that it's a half amp fuse.

Weasel

Someone sent me a page out of a newspaper, and as I took it out of the envelope I thought I was busted. There was a full-page article on the short-tailed weasel, a burrowing mammal local to my correspondant's area. Its fur is brown in the summer and white in the winter, so it would make a poor pet. No matter what colour furniture and carpets you had, you wouldn't be able to hide the shedding in spring AND fall. Then I turned the page over to discover the article I had been intended to read. It was about an airline pilot who is probably also white in the winter and brown in the summer, like many caucasian Canadians.

Wings and Things

The airplane I want to fly has one wing on each side, and at the back there's a tail, which pilots sometimes call the empennage, because otherwise people might understand. This blog entry is about all the hinged surfaces hanging off the wings and tail.

The primary flight controls--ailerons, elevator and rudder--are mechanically linked to the cockpit through cables. The ailerons are kind of big and heavy to move just by pulling on cables, especially if you have a cup of coffee in the other hand, so two geared servo tabs, one located on the inboard trailing edge of each aileron, deflect in the opposite direction to assist in the movement. The rudder also has a geared tab, at the lower edge, for the same purpose. The pilot is expected to have enough leverage to move the elevator herself, so the elevator has no geared tabs.

All three axes of control have cockpit adjustable trim, with one trim tab each, located at the outboard end of the left aileron, the top of the rudder, and at the left side of the elevator. The trim controls are conveniently located so that both pilots can reach them easily, especially if the captain can comfortably dislocate her own shoulder and the first officer has an extra long left arm. The purpose of trim is to use aerodymanic force to hold the controls in the desired position during flight. Elevator and rudder trim is mechanical, while aileron trim is electric, powered from the left DC bus. Easy to remember, because the trim tabs are all on the left, except for the rudder trim tab, but that would be the left if the airplane banked over towards the other trim tabs, to put them all together.

The difference between trim and geared tabs is that the geared tabs are used to help MOVE the controls, and trim is used to HOLD them in place. Almost all airplanes have trim, at least on the elevator. Larger ones have geared tabs or some other way to assist the pilots in moving the control surfaces.

On most airplanes the ailerons hang off the back of the wings, but these ailerons are a little different. One reference calls them "unique." The flaps run the entire span of the wing, except for the bit in the middle where the cabin is obviously in the way. The fore flaps are hinged directly off the wing rear spar. Don't be fooled by the term fore because they are still trailing the wing, it's just that there's another set of flaps aft of the fore flaps. The ailerons are actually attached to the outboard fore flaps, and their differential movement increases when flaps are extended.

If you can follow that, wait until you hear how the flaps work. Remember that the fore flaps hang off the back of the wing, so the trailing flaps hang off the back of the inboard fore flaps. The outboard fore flaps operate through a range of zero to twenty-six degrees, the inboard fore flaps extend up to forty degrees, and the inboard trailing flaps go from zero to sixty degrees. According to some peculiar rule of averaging, this equates to a nominal flap range of 0 to 37.5 degrees. You'd think the two inboard flap deflections would add up to a total deflection of 100 degress and curl back under itself, but when you look at the aft inboard flap, it seems to move from being about ten degrees below the horizontal to being about 85 degrees below horizontal. Now that's 75 degrees of travel, not 60 and not 100. So I can't quite figure out how the flap deflection is measured. There is only one flap lever for all these flaps, and it is graded from 0 to 40 degrees. I guess it's rounded off. The point is: this airplane has a heck of a lot of flaps.

When you extend flaps on an airplane, the nose has a tendency to pitch up. This airplane has a flap-elevator interconnect tab: a servo tab on the right side of the elevator that automatically deflects up as the flaps extend, to counteract the pitch up tendency. The tab has a range of 12 degrees up and down.

Oh and the flaps are hydraulically actuated, which means in the event of a hydraulic failure they will not extend, and may slowly retract due to leakage and aerodynamic pressure. They retract more slowly than they extend, and they don't extend all that fast, so you don't get massive pitch changes. If there is leakage in the poppet valve (whatever that is) of the flap control mechanism, the flaps may droop to the extended position while the airplane is parked overnight. It would be a good thing for an FO to notice a discrepancy between the flap selector postion and the flap position during the morning walkaround.

The other things trailing off the wings and tail are static wicks. I once encountered a Transport Canada inspector who literally pulled one of the static wicks off an airplane with his hand, and threatened to write up the airplane for missing it. Like the crooked cops in one of those old movies who pull you over and kick in your headlight, then cite you for driving with one headlight.

Not Up In Smoke

I was chatting with an Air Canada pilot who said that Air Canada has trouble hiring their quota of francophone pilots. I blinked.

"Why? There are thousands of pilots in Quebec. There are French-Canadian pilots all over the world!"

"Yes, but they all smoke." Not all, of course, but far more quebecois smoke than anglos do.

Me, with a puzzled look, "So?"

"Air Canada doesn't hire pilots who smoke."

Wow. It's not like you could hide your smoke-wizened lungs from the scrutiny of an airline medical. I'm not a francophone, at least not much beyond basic communication, and I'm not a smoker either. This could be useful information to keep your 13-year-old from taking up smoking.

Bleeding Hearts

In order to describe conditions inside, different parts of the engine are referred to by numbered stations. Station 1 is the air inlet. Station 2 is the entry to the compressor. Station 3 is the exit from the compressor. Station 4 is near the exit from the combustion chamber. Station 5 is between the compressor turbine and the power turbine. I believe station 6 and 7 are respectively the exit from the power turbine and the exhaust port, but I'm not sure. An additional station 2.5 is defined between the axial and centrifugal stages of the compressor.

At both station 2.5 and station 3 compressed air is deliberately bled from the system. You'd think after going to all the work of compressing it, you wouldn't want to let any out, but you do, for two completely different reasons.

At lower power settings, the axial compressor is more efficient than the centrifugal one, so at low power the axial compressor delivers more compressed air than the centrifugal compressor can accept. That would lead to a buildup of pressure at station 2.5, creating a back pressure that would cause the axial compressor airfoil blades to stall, completely disrupting the calculated flow of air through the engine.

So the manufacturer has included at station 2.5 a compressor bleed valve which automatically opens at low power settings and closes at higher ones, becoming closed at about 80% N_G. The pilot has no way of controlling the valve, and the only action she has to take is to allow the engine speed to stabilize at 85% N_G for five seconds before applying takeoff power, in order to allow the bleed valve to close smoothly. When the compressor bleed valve is open, compressed air is just dumped sideways out of the engine into the atmosphere.

If the bleed valve were to become stuck open, the pilot wouldn't notice it during ground operations preparing for takeoff, because it's supposed to be open then. When takeoff power was set, the pilot should notice that engine torque is lower than it should be, but that N_G and the temperature at station 5 (T5) is increasing, as the fuel control unit increases the fuel flow in a fruitless attempt to increase the torque. It's never going to get there because the air it needs to burn is being dumped overboard. If both pilots are too busy looking out the window and thinking "I love this job" (new first officer) or "I hate this job" (disgruntled captain) and miss the engine indications, the result could be a compressor overspeed, which would wreck the engine. The opposite problem, of the valve closing at high power but failing to re-open at low power is apparently very rare on this engine, but would result in a compressor stall.

Meanwhile, at station 3, compressed air is bled out of the engine to be put to work. Air is routed from the engine through DC electric shut-off valves into a duct in the roof. At cruise power, bleed air is at about 450 degrees C and 80 psi. It is used as-is if the heater is in use and/or routed through a heat exchanger, air filter, and pressure regulator to provide cooler 18 psi air for pneumatic systems.

The switches that control the bleed air supply actually only control bleed air available to the heating system, airframe deicing, and the pneumatic autopilot. Air for deflector operation and, in older models, the pneumatic flight instruments is obtained before the bleed air valves, so that the deflectors and flight instruments work even with the bleeds switched off.

The Joy of the Chase

The English hunt foxes on horseback. Hounds pick up and track the scent of the fox across the countryside, and the riders urge their horses to follow, galloping across fields and leaping over hedges and walls, thoroughly enjoying the pursuit of the fox. When they catch it, I think they cut off its tail and give it to someone, but they don't actually do anything enjoyable with the fox. The whole aim of the exercise is the pursuit. Lately this has started to worry me. Not the fox hunting itself: I think they banned it in England. But you see, the fox is a burrowing mammal.

Sometimes wanting is superior to having. I've really had a grand time these last few years, suffering for my art, as it were. I have the art of pursuing chief pilots down to a science. I know the script. Now I'm a little frightened of my own impending success. Once I catch the fox, I hope it doesn't bite me.

Burning Hearts

For an airplane, the airframe is the body, the engine is the heart and the avionics are the brain.

The airplane of my affections has two hearts, a pair of Pratt & Whitney PT6-A20 (or PT6-A27 for later models) engines. These are is reverse-flow free turbine engines with a three-stage axial compressor, a one-stage centrifugal compressor and one power turbine. Let me elaborate.

Engines work by taking advantage of the fact that when you burn fuel in air, the products of the combustion take up a lot more space than the reagants, both because there are more molecules, and because the heat of the reaction makes the gases expand. The more fuel you burn, the more power the engine produces, but in order to burn more fuel you need more air, so part of engine design is devising ways to cram more air inside. Going fast through the air helps, but it's useful if the engine to work while the airplane is standing still, too, so we compress the air before burning it.

The picture shows a larger version of the PT6, with two power turbines. The propeller is mounted on the front of the engine, the left side of the picture.

An axial compressor stage consists of a spinning disc called a rotor, and an adjacent non-spinning disc called a stator, each fitted with very carefully engineered airfoil blades. Intake air is accelerated and compressed by the spinning rotor and then further compressed as it slams the stator, sort of like your car would be if you drove off a freeway ramp into a concrete wall. The stator also helps to prevent the airflow from spiralling around, keeping it parallel to the axis of rotation of the rotor, hence the name axial. As one rotor plus one stator equates to one compressor stage, a three-stage axial compressor is like a row of half a dozen doughnuts, with every second doughnut spinning around. In this case the first spinning doughnut is titanium and the subsequent two are stainless steel with cadmium frosting. The blades on multistage compressors get smaller and closer together as the pressure increases down the line.

Air exiting the third stage of the axial compressor enters a centrifugal compressor, an additional doughnut in the row. As it spins, it flings the air outward, into a chamber called a diffuser, which slows it down, thereby compressing it further. At take-off power, this air has a temperature of 280 degrees celsius and a pressure of 103 psi.

This compressed air enters the combustion chamber where fourteen nozzles add fuel to the continuously burning mixture. The temperature in the combustion chamber is so high that the walls of the combustor can't actually withstand it. The airflow is made to provide a cooling layer that keeps the fire away from the walls. So turbine fuel is burning at 1050 degrees celsius, less than six feet from my head, and what's keeping it in there is air. Whee.

Instead of burning through the combustor, nacelle, or my head, the hot expanding gases are directed to the compressor turbine, which they spin at a rate that can exceed 37,500 rpm. That's called N_G, N for number of revolutions per minute and G for gas generator, another term for compressor. The compressor turbine is on the same shaft as the compressors discussed above, and is what provides the power for their rotation. About two thirds of the power generated by the engine goes right back into turning those compressors. The accessories gearbox, also on that shaft, runs useful things like the oil pressure pump.

After the compressor turbine, the gases pass through the power turbine which spins its own shaft at up to about 33,000 rpm, called N_F, supposedly F for fuel. (F for fuel and G for gas: about as useful as GUMPS, eh?) The shaft drives the propeller through a 15:1 reduction gearbox, for a maximum propeller speed (Np) of 2200 rpm.

The limiting component for engine temperature is the vane that guides the expanding gases to the compressor turbine blades, but it's so hot there that the temperature (T4) cannot be reliably measured. The temperature is measured instead between the compressor turbine and the power turbine (T5), and the pilot memorizes a little table of values for T5 that correspond to the maximum allowable temperature for T4 at various phases of flight.

Free turbine refers not to the price of these things, but to the fact that the power shaft has no physical connection to the compressor shaft. The ends of the two are aligned, but there is about five millimetres of space between them, and they rotate in opposite directions.

Reverse flow means that air goes in near the the back of the engine and exhaust comes out near the front, the opposite of your regular jet engine. The exhaust is still directed backwards, though and its discharge adds an extra twenty-nine horsepower of thrust to the engine output.

Any questions?

It's A Date

Bill called back. I have a course date. The course covers the aircraft systems portion only. That means I won't learn the SOPs (Standard Operating Procedures: the script for things to say and do so that any crew can operate the aircraft safely together, even if they've only just met) or indoctrination into company policy, or receive any flight training until I have a job offer.

Until my course is over, I resolve to stay out of internet chatrooms, and quit reading my referrer logs. The former is an obvious time waster, and the latter leads to me browse fascinating new sites that link to mine, or copy google searches and find even more cool time-wasting things to read. I thought I'd stop blogging, but blogging is an effective way of studying, because I have to write, think, and proofread. So I think I might blog more. Plus coming up with ways to make things like the propeller synchronizer amusing to the casual reader will require me to know it intimately. "The left propeller is the master and the right propeller the slave." Hmm.

I'll report back on the course, when that happens. If there's a job offer I'll wait until I'm qualified and have completed my probation before asking my new employer about a corporate blogging policy. And if they don't like me, and send me home, I'll tell you more about holding patterns.

Open Mike Night

Michael Oxner just blogged about funny radio conversations, including some pieces of controller-to-controller dialogue. Controllers tend to be pretty sharp folk, and pilots usually appreciate their witticisms, even when they are mocking us. Sometimes though, you'd think someone whose every working moment is broadcast and recorded would have more restraint.

Very late one night I was diverting to an airport where I hadn't originally intended to land. I called the FSS in order to amend my flight plan and ask for NOTAMs. This was back before Flight Services was centralized in each region, so I was calling a couple of guys in a local flight services station.

"Middle of Nowhere Radio, GXYZ"

After a long pause (I'm sure I was the first traffic to call them in in an hour) they responded. I made my request, and then there was some scratchy clunking and a profanity-laced question and response.

"Can you tell what the f#!@ that bitch is saying?"

"F#!@ no. Radio's f#!@ed or something."

I had a pilot-rated passenger sitting next to me, also an aviatrix. We gasped, then laughed. I switched over to the other communications radio and responded in my most formal pilot tones.

"Middle of Nowhere Radio, GXYZ, check your difficulty reading my previous transmission. Is this comm clearer?"

Another pause, and then very very attentive service. He realized that the mike was stuck during the side comments on the intelligibility of our radio, and was probably crossing his fingers that I wasn't going to take offense. Relax, unknown flight services guy. I've never actually uttered the phrase "mark the tape." The only time I've ever wanted to preserve a record of a conversation is the funny ones, not to file a complaint.

Advanced Chicken Counting

Don't count your chickens before they hatch.

It's a silly saying, really, because if you've got a bunch of eggs that might hatch into chickens, you really should count them, to make sure you have enough heat lamps, and chicken feed, and chicken water bottles, a big enough properly-fenced chicken enclosure, and your chicken veterinarian on speed-dial. It's probably also a good idea to count them so you can tell if anyone is stealing any eggs when you aren't looking.

This isn't the first time I've been this close to a job. I've even been this close to a job with this particular company before. I know how these things work. But that knowledge isn't going to stop me from getting excited about it. I need to be ready to leave my current job with no hard feelings, and be physically and mentally ready to take on the new job.

What the maxim should say is don't count on your chickens before they hatch. Don't make purchases based on the money you expect to get from selling them. Don't depend on the chickens hatching. And don't spend your every waking moment counting them or blogging about them, either. You have better things to do.

Fuelish Wisdom

Talked to Bill at Ichneumon. He knew who I was, acknowledged that he was supposed to schedule me into a course, but seemed in no hurry to do so. I talked to a couple of Ichneumon pilots and they indicated that courses get scheduled at the last moment. So my next few days or weeks will consist of studying the airplane and bugging Bill.

And you get to study along with me.

Today's topic is the glorious fuel system of the airplane I hope to be flying soon. (No prizes, Canadian aviators, for guessing which company Ichneumon is, so please don't name it in the comments. You can just smile smugly at how obvious it is.) The engines can burn pretty much any kind of turbine fuel, it can even burn avgas formulated for piston engines, althought the lead isn't good for the turbine blades, so it's limited to 150 hours per engine overhaul cycle. You could probably run it on barbeque lighter fluid in a pinch but I wouldn't want to be the one to have to open all those little bottles to pour the contents into the tanks. And explaining it to maintenance afterwards might be tricky, too.

The manufacturer even designed it so I wouldn't have to climb up on the wing to pour the lighter fluid in, as the airplane has its two fuel tanks in the belly, and the filler necks along the left side. The forward tank feeds the right engine and has a capacity of 151 imperial gallons, while the 164 gallon aft tank feeds the left engine. Fuel cannot be pumped from one tank to the other, but one can choose to crossfeed, supplying both engines from one tank.

To prevent all the fuel from sloshing to the front or back of the airplane, each fuel tank is divided into four interconnected cells, numbered one through eight from front to back. Each cell contains a fuel capacity probe and has two vents, but cells #4 and #5 are the collector cells. and also contain electric boost pumps to send fuel up to the engine. The way they collect fuel from the other cells is quite clever. The boost pumps can deliver 450 pounds of fuel per hour at 22 psi, more than the engine can possibly burn. The excess flow is pumped around a loop going back to the collector cell again, but on the way it goes through an ejector, sort of a one-way T-joint connected to the manifold between the four fuel cells. The force of the fuel being pumped past actually draws fuel out of the other three cells, so that if everything is working properly, the collector cell should remain full until the other three cells are empty. A level control valve in the collector tank inlet standpipe prevents overfilling of the collector tank.

If the ejector becomes blocked, the boost pump (and its back up) fail, or the level control valve jams shut, the fuel will not transfer and the fuel level in the four cells will become equal. With an inoperative ejector system, extreme nose up or down attitudes produce greater changes in centre of gravity and risk uncovering the boost pumps, but normal aircraft operations can continue. The FUEL LOW LEVEL light will illuminate early, but that's part of the fuel indicating system, and a topic for another blog entry.

Airline Names

If you've just stumbled across this blog or are unfamiliar with the airlines in Canada, you may be wondering why Canadian airlines have such odd names. They don't really. Only in this blog. To protect my anonymity, their secrets, and my job search strategy, or possibly just to make this blog more amusing, whenever I mention an airline in a context not involving a news story I replace the company name.

I could call them Airline A, Airline B and so on, but that doesn't draw you into my world. So I've named them after burrowing mammals. That's right, furry animals that dig holes. To be strictly accurate, some of the animals prefer to appropriate existing holes rather than do their own digging, and many of the names are actually alternate names for the same animals, but I'm in a country that had a Skyward and a Skywords, a Pacific Coastal and a Coastal Pacific, an Air Labrador and a Labrador Air, and an Air Canada and a Canadian Airlines. I think my way is easier. I cycle through the alphabet, so the first twenty-six airlines, each start with a different letter. That way if you can't distinguish between an Ichneumon and a Quoll as easily as between an A320 and an A319, you can just go by the letters.

There is no correspondance between the characteristics or names of the animals and those of the airlines. I simply grab the next animal off my list. Aardvark to Zibellina and then back to Armadillo. If somewhere in Canada there is an airline named after a burrowing mammal then (a) why? and (b) I have never worked for nor applied to work there. Amusingly, this site is on the first page of search results for "burrowing mammals" on google.ca.

In addition, the chief pilot of any airline I talk about is arbitrarily named Steve. Most of them seem to be named Steve anyway. Any other other staff I need to speak to receive pseudonyms. Someday I'm going to walk into an airline office and be told "You'll need to speak to Steve Fox about that," and I'm going to have to explain what is so funny.

Weak-Kneed with Glee

I got a phone call this morning from Steve Ichneumon, asking if Bill Ichneumon had called me yet. Well no he hadn't. "I told him to call you," said Steve. "He said you were hard to get a hold of. He's obviously lying. I'll remind him to call you." I am rather hard to get a hold of, by dint of the fact that I work ridiculous hours, rarely have a day off, and my cellphone is turned off while I'm flying.

Ichneumon wants me to come to a groundschool course. It's not a job offer, but I'm sure I can make the grade. This is my big break. It's like having George Lucas call you and ask if Spielburg has called you yet. Or however it works in Hollywood. But forget the explanatory similes. I really, really, really want to work for this company, and it looks like I will. After the conversation I literally fell on the floor crying with happiness. Then I ran around screaming and pumping my fists in the air. Then I cried some more. You really don't want to know this about someone who might be flying your airplane, do you? Sorry.

Now, in yet another way that makes our industry silly, I have to get a hold of as many people as I can who work for Ichneumon, to get all the information that will be taught at the groundschool course, so I can learn it in advance. Only the most naive pilot would wait until the actual groundschool course to learn things. Blogging may diminish over the next month or so.

Bill hasn't called back yet. I'll call him first thing tomorrow morning.

Clip-On Ties

A pilot I know recently got his first airline job and came by to tell us about the process. He was going on line the next day and admitted that he was heading home to try on his new uniform.

The uniforms are pretty much all the same. We all wear black or navy polyester pants, a black or navy polyester tie, and gold or silver barred epaulettes to slide over the buttoned shoulder loops on the white pilot shirts we already own. No one but a newly hired pilot ever grinned in so much delight at the opportunity to wear so much cheap polyester. Third tier pilots don't wear hats. They're traditionally too young to be balding, so they don't need one.

This pilot reported that, given the choice between a clip-on tie and a real necktie, he had chosen the real tie. I laughed, having made exactly the same decision, for what turned out to be the same reason. There are probably valid safety considerations that should have directed us towards the clip-on, but you look at the ties, and then you look at the stores person issuing the uniform, and you think, "If I'm going to wear a three dollar tie, it's at least going to be tied in a real knot."

Fate Laughs On

I was thinking today about the course I took that coincidentally included the Badger Airlines training staff. On the first day there was a getting-to-know-you session where pairs of participants 'interviewed' each other to tell the class about the other. I was paired with a Badger training captain, and one of the things he asked me about was career goals. I didn't say straight out, "I want your job," but it's what I really wanted.

I'd envisioned my career starting, as it has, at the lower echelons of the industry, and progressing to a company like Badger. I knew all along that aviation was a pyramid scheme, and that not everyone gets to stand on the top wearing the uniform of a jet captain, so I accepted that my ascent might stop at Badger or its ilk. I was, and am, prepared to complete my career becoming an expert on a niche airplane, the go-to girl who knows every in and out of the aircraft and the company's destinations. I'd run the groundschool and become a company training pilot, and check pilot. And I'd eventually get a new car. You know, the idle fantasies of the student pilot.

What hit me this morning is that I have certainly at some point said words almost exactly matching, "I want to work with the Badger training department." And of course, for four days in May, I did. I got exactly what I asked for.

I guess I need to be more specific. I want to fly a Boeing jet. But I don't want that to come through a real life scenario from a really bad movie, where I'm on a trip and the pilots become incapacitated and one of the passengers has to land the plane. Exciting as that might be, I don't want my dreams fulfilled via loopholes. Although that could happen too. It always works out pretty well in the movies.

Oh, and I've reconsidered. For a just-once opportunity, I'd rather fly a DC3 than a B787. But for a permanent job, I want the new Boeing.

(And before you remind me, I know I've been neglecting my mammals. Too many twelve hour days lately. I can't compose cover letters or make coherent phone calls when I consider that I've already republished this twice after finding I'd accidentally typed the same word twice, and then accidentally typed the opposite of what I meant. Plus at work today it took me two tries to spell VFR. And yes, it's pronounced vee-eff-arr.)

Corrosion

Not long ago, a man who was training to do structural repairs on aircraft was watching, along with his instructor, as I performed a preflight inspection of an airplane. I pointed out a few of the things I was looking for, but I was a little self conscious, feeling that these guys probably knew how to take the airplane apart and put it back together, while all I knew how to do was ensure that all the bits were attached in the right order. Then the instructor asked the student a simple aerodynamics question which he couldn't answer, plus the student misidentified an aileron, so I realized I wasn't enduring as advanced a scrutiny as I had believed.

A few minutes later I discovered a few spots of corrosion, raising bubbles in the paint on one side of the fuselage. I pointed them out, surely a point of interest for my observers. You don't have to have any training or know that my airplane was made of aluminum to know that corrosion is an undesirable event. It weakens the structure. I heard the instructor ask the student what kind of corrosion it was. Kind? Um ... I didn't know corrosion had names, except that ferrous corrosion is "rust." I knew that the aluminum skin of my airplane was attacked by a combination of moisture, pollution, and the effects of different metals being in contact. I see corrosion forming under the paint, especially at joints and near rivets. The paint gets rough, then bubbles up like badly applied wallpaper. Eventually, or if you press on the bubbles, the paint flakes off, revealing that the aluminum underneath has turned white and flaky. When corrosion forms at an edge, such as at the trailing edge of an aileron, you can see the aluminum lifting into separate layers, like a mille feuilles pastry. If that happens you can crumble the edge of the metal with your fingers. The student wasn't able to answer the question about the type of corrosion, so when I got home I looked it up.

There are a number of different sorts, some variations on the others and often working at the same time in the same spot. Some lists separated out more types of corrosion, or omitted or combined some in my list.

Pure aluminum is corrosion resistant, but pure aluminum is not strong enough for airframes, so aluminum alloys are used in airplane construction. The outer surfaces of the alloy sheeting is often coated in pure aluminum (Alclad) or anodized. The surfaces are also typically primed and painted, but the corrosion still gets in.

General or uniform corrosion, occurs when an exposed surface is consumed equally through reaction with air and moisture. It is the least dangerous sort.

Penetrating pit corrosion is the same kind of degradation to the metal, but instead of being even and on the surface, it occurs when moisture enters localized disruptions to coating, and creates cavities that may go deep down, or cut sideways under the surface.

Galvanic corrosion occurs when two different metals are connected in the presence of moisture, and they become the terminals of a battery, which erodes its own surface through the same principle as electroplating. Because alloys are made of combinations of grains of different kinds of metals, there are always metal-metal boundaries, for intergranular corrosion, even if there is not a steel bolt connected electrically to the aluminum frame. Corrosion products build up along the grain boundaries, making the metal exfoliate into the leaves I described above.

Another form of corrosion is fretting, when parts rub on one another and grind particles from one another. You see this at hinges, and where cowlings rub as the engine vibrates. It creates a fine gray dust that streaks backwards on the airframe, and comes off on your fingertips as a gray smear, almost like grease.

I found useful information on types of corrosion at Avstop, Boeing and Charles Stuart University.

Apparently the structural course was almost over, so the student was almost qualified. I hope he was really good at riveting. Remember: someone graduates at the bottom of every class of doctors, and of every class of maintenance engineers.

After I wrote this, but before I posted it I learned about Connie, a female Canadian aircraft structures mechanic. I've linked to *Corrosion of the Week*, an annex of Connie's main blog, Connielingus. The latter contains some sexual content, but I think you can handle it. There's airplane fixing stuff there, too. I never realized before that repairing airplanes could be romantic. Connie, any corrections or additions to this post are more than welcome.

Taken Alive

A few days ago Virgin Airlines flight 45 was diverted to Halifax from its original destination of New York, because it was inadvertantly transmitting a hijack distress code. Apparently Halifax has more highly trained counterterrorism personnel, or no one at NORAD cares if Halifax gets blown up. It's not a high-profile place like New York. Apparently Canada is now the standard place to divert possibly hijacked aircraft.

How it happened is easy. Every pilot has probably experienced the phenomenon of the transponder transmitting a different code than the one we've set on the face of the little box. We're briefly amused by the fact that our radar signal shows up on ATC radar screens as a B747 or a Sikorsky 76, and then we try to fix the problem. Usually turning the little digits away and then back to the assigned number does the trick, or recycling the transponder (turning it off and then on again). If it doesn't, it becomes maintenance's problem. It's usually stuck on 0000 or the last code we were assigned, and is only a nuisance. In this case, the crew was unlucky enough that out of 4096 possible combinations, their box fixated on the one that is means the pilot has a gun pointed at his head.

So despite repeated assurances that it was only a transponder problem, that the aircraft was under no threat, they were escorted to Halifax by a couple of CF-18s. I'd hate to be the one making the cabin PA to explain that one to the passengers.

I think ATC took the correct action. I might not even have remarked on the story except that the media has done something unexpected. Ordinarily such stories pussyfoot around the issue of the distress code used, and even the equipment used, to indicate that a hijacking has taken place. You notice I haven't even stated it, so trained am I to make at least a pretence that this is inside information. It's hardly a big secret. It's published in textbooks. It's required knowledge to get any kind of pilot licence. But it's not traditionally shouted from the rooftops. Yet the media stories this time around all give out the code. This one initially had a sidebar listing ALL the reserved codes, but I notice now that they have reduced it to a description of the transponder with a mention that certain codes are never assigned.

Now we pilots will have to come up with an even BETTER secret decoder ring.