Post Mortem

Well what a clusterfailure that was. Imagine if that vacuum and indication failure had all happened over the course of once turbulent flight in night IMC across the mountains. Suction failures are extremely dangerous. The suction-driven instruments are central to the instrument scan and experiments and studies of actual accidents show that they have a very high chance of killing you. The "ninety percent" figure sometimes quoted is misleading: accidents attributed to vacuum failures (about three per year in the US) result in fatalities 90% of the time. That's because there aren't many ways to crash an airplane due to spatial disorientation without it being deadly. More than ten percent of pilots who experience vacuum failure in IMC do get out alive, but I'm not going to guarantee I would be one of the survivors.

Here's what a vacuum failure can look like.

The gyrating blue and black instrument is the attitude indicator. It won't necessarily look that energetic in a failure. Its slide to one side or the other may look exactly like a functional instrument indicating a real wing drop, prompting the pilot to bank and pitch in an attempt to return the aircraft to straight and level flight. The pilot must fly by the turn coordinator, the tippy little white symbolic airplane at bottom left. It indicates rate of turn, not bank angle, so doesn't respond as instantaneously to changes in attitude.

I had difficulty identifying the failed vacuum system because its own indication system did not work properly. When I felt that the heading indicator was not working properly, I suspected the vacuum system, but the indicator lied to me, telling me the system was okay. When I shut down an engine, the associated vacuum failure indication appears immediately, leading me to believe that the same thing would happen in the case of a real suction failure. The two vacuum pumps are in two different engines, had widely separated serial numbers and had been installed by different people at different times. I had never heard of one vacuum pump failing and taking out another one.

If an electric instrument has a power loss, it immediately throws up a flag, so I know not to trust it. There is no failure indicator on the heading indicator or attitude indicator and without other reference, it is really easy to follow the dying instrument. The autopilot tried to follow it, too. With TWO failing vacuum pumps, there was no pressure differential across the shuttle valve to pull the autopilot out of the system. Flying across the mountains to the FBO where company has shipped the replacement pumps, I have post-it notes over the failed instruments, still the only indication, apart from their unresponsiveness, that they are not working. When practicing flying on instruments, if the pilot can see the real horizon, even a little, the task is much easier. Even if you try to ignore it, you subconsciously draw on a lifetime of doing what your species does: visually determining which way up you are. While it's easy to design an indicator that is invisible until electrical power is removed from it, I guess it's harder to calibrate one to not enough suction. Maybe it could depend on gyro rpm. This FAA document claims that "attitude indicators with failure warning flags are available in newer model aircraft and are slowly making their way into the general aviation fleet as retrofits," but I think they may be talking about replacing vacuum-driven AIs with electric ones.

The cause of the double failure is eventually determined to be poorly-installed air-oil separators. Oil was able to enter the dry vacuum pumps and damage the vanes. I'm not sure if the indication issue was because it was a gradual failure, or because oil got in the indicator, too. It is all functional now, but now the vacuum pumps are in phase, which makes me nervous, because that airplane has flown about 900 hours since that happened, and I'm back in it. This article seems to think that vacuum pumps fail every 500 hours or so, but our mandated time between overhaul is definitely 1000 h.

Late Spring

An airplane in Canada is legally required to be equipped with a working oil pressure gauge for each engine. When you start a piston engine, your eyes go to the oil pressure gauge. You're waiting for the pressure to come up. Given that it's the first thing you do on starting an engine, it's one of the first things I learned as a student. (Right after, "in order to start an engine, you have to turn the key to the start position and hold it there until the engine has actually started," which was explained to me immediately after I did it the wrong way. Yeah, I was pretty dense).  If the pressure doesn't come up, you're supposed to shut down the engine and not do anything else.

I'm not sure I've ever followed that instruction. I am starting an engine that I have just verified has oil in the crankcase and does not have a huge puddle of oil on the ground underneath. And I know how hot or cold that oil is, from 'so hot that I had to use a rag to handle the dipstick, and couldn't really see a level anyway', to 'I just spent twenty minutes deicing and I can't feel my toes or fingertips'. If it's the latter, yeah, I know that oil isn't going to come dancing through the tiny line to the pressure gauge in any big hurry. I will give the oil a while to warm up, and thus the oil pressure more time to increase.

In retrospect, I wonder if disciplining student eyes to go straight to the oil pressure gauge was a way of training us to be vigilant for a hot start when we move on to turbine engines.

High oil pressure can be a sign of a blockage, or you're running summer weight oil too far north, so instead of distributed cooling slipperiness, the oil is a viscous glob having trouble getting forced through some part of the system. Yeah, oil that is too thick can cause high or low oil pressure indication, because unless you're short on oil, both high and low oil pressure indications are caused by poor oil circulation. The pressure is high one place and low another place. Or it's the gauge. Low or high oil pressure, it's usually the gauge.

Even when its probably not the gauge, so long as there isn't oil splattered all over the airplane, and the engine didn't quit, the pilot rolls up to maintenance and says, "I have a low oil pressure indication, but it's probably just the gauge." You never want to give maintenance the idea that you are terrified of the airplane they gave you. I'm not sure why we do this. Perhaps I should try the opposite strategy sometimes and insist that ... oh wait I did that last week with a stuck propeller cable that turned out to be ... maybe I'll admit that one another time. While instantly diagnosable by someone leaning into the cockpit from the right-side door, it really wasn't visible to someone strapped into the left seat. Anyway, under-reporting of the severity of our concerns seems to be a pilot thing. Maybe because maintenance likes to show us to be wrong, so if we show little concern, they will work hard to find some way it could have killed us. Which is what we want them to do.

So my story is, I report unequal oil pressure on several consecutive flights in this airplane. It's in the green on both engines, I concede, but used to be equal, I think. I didn't remember there being a split, and I would. A pilot scans the engine instruments far more times than she knows, because we only notice we're doing it when something is wrong, and the two needles for the same thing on the two engines not being in the same position on their adjacent dials comes up as wrong, even when they are both in the green. The oil pressure indication on the right engine is inching up slowly. If it were a single engine airplane I probably would not notice. That's something I say a lot, but I keep reporting these things, because sometimes it matters. It's remarkable what a tiny thing can be a symptom of.


Eventually the needle reaches the yellow and they take it apart and find out what was wrong with it. An oil pressure gauge works the only way one logically could but I was still kind of flabbergasted when I realized that a little pressure line comes off the oil system, ducting a bit of the engine-pressure oil right to the instrument in my dashboard. Its internals consist partly of a ball bearing and a spring, and the pressure of the oil on the ball bearing compresses the spring, driving the needle.  The spring was worn out.

Two Odours, A Taste and A Mess

I'm trying to take notes on my days and then turn them into blog entries later. Sometimes the notes are a little too cryptic.

Pulp mill smell. Tour buses. Fragrant weeds. Milkshake. Starter gasket leak.

Clearly I landed in one of the two hundred seven Canadian towns that is within smelling distance of a pulp mill, was obstructed, amused or otherwise influenced by the presence of tour buses, smelled some weeds, drank a noteworthy milkshake and ended the day with oil dribbled all down the front of my engine nacelle.

I remember the oil leak. Seeing oil coming out of the engine that far forward made me suspect a propeller governor, and that's a biggie. But careful examination showed that the oil was not coming from the propeller hub. There was oil pooled inside the cowling, but not very much. I photographed and then cleaned up the evidence, and sent the photos to maintenance. (Oh my god modern technology is so wonderful. The first time I had a maintenance problem at a remote site, I drew a picture of the affected components. Yes, I took an actual piece of paper and a pen and looked at what was wrong and reproduced it with technology barely above Neanderthal (I had a ballpoint pen, not a fire-blackened stick) and then faxed it to the PRM, and I thought that was a pretty savvy use of modern technology. You can't do things like that with a telex machine, after all). Maintenance in the pre-digression instance agreed that a minor oil leak from something other than the propeller at the front of the engine was not a grounding item and I flew it until we reached a town where expertise, parts and availability of maintenance personnel all came together to fix it. We already had an appointment there for troubleshooting on a fuel control unit that my own company maintenance couldn't get to behave quite right.

The maintenance engineer in question was experienced, but at his first day on the job at a new company. That meant he didn't have a dozen things he was supposed to be doing, nor people who knew what he was best at to interrupt him for his help. It also meant that he was keen to show his skills, and supervised by people who wanted to make sure he was doing it right. I think it worked out quite well for all of us. I endured several hours of airport appreciation time--perhaps this is where the milkshake or the weeds came into play--and they gave me a T-shirt along with a functional, properly fuel-controlled and non-leaking airplane, at the end of the day.

The rest, I don't remember. I clearly wanted to tell you about it, but there have been too many towns, odours, weeds, vehicles and beverages to distinguish any in particular.

One of Many Things That May Matter

Remember when I used to obsessively post every detail of almost every flying day? There must have been readers who thought this was a carefully crafted fictional narrative and not a true account, because of the minor details that became recurring themes and that finally ballooned into crucial matters of life and death. There were however thousands of minor details that never turned into anything, because that's the way life is, but we forget those ones. Here's a detail of the sort that occurs every day. Maybe it will be a cool precursor to an inflight incident or a great maintenance catch. Probably it's nothing at all. It's my job to take such things into consideration, day in and day out. I start with background on how the system works.

Very small airplanes, and some older large ones, have a fixed pitch propeller, a single artfully-shaped piece of metal or wood that bolts to the drive shaft and delivers thrust as its airfoil shape spins through the air. Experimentation and science shows that a propeller like this is most efficient at a given angle of attack (or angle of incidence, as the British call it), one particular speed. One can get propellers that are optimized for climbing (useful for a drop zone plane), or for cruise (useful if you rarely need the best climb but want more speed or fuel economy on longer flights), but if you're willing to spend a lot more on a propeller, you can get the best of both worlds: a constant speed propeller.

On a constant speed propeller, each blade is a separate entity, set in a hub in which it can pivot. The pilot sets the desired rotational speed of the propellers and then counterweights within the hub swing out if the propeller is going faster than the set speed and swing in if the propeller is underspeed. When the weights swing in they open a valve, allowing pressurized oil to flow to the hub, twisting the blades to a finer (flatter) pitch. (Americans call it low pitch, I think.) For a given speed and power setting, a finer pitch propeller will turn faster, getting it back on speed. If it goes too fast the counterweights will close the valve, and the propeller blades move towards a coarser pitch, eventually going all the way to the feathered (maximum turning resistance, minimum forward drag profile) position if oil pressure is lost. I can command this drop in hub oil pressure by pulling the propeller lever all the way aft to the "feather" position. The ability to feather is another huge advantage of a constant speed propeller: if one engine fails I can greatly reduce the drag it causes by feathering its propeller.

The oil that does this job is the same oil kept in the crankcase, the oil that performs the regular functions of cleaning, cooling, lubricating and sealing my engine. At start up, this oil is the same temperature as ambient air, and thus it's a little sluggish. The propeller is not feathered during a normal engine shutdown (pins lock the propeller pitch as rotation slows below 1000 rpm) so the oil that was acting to keep the propeller blades at fine pitch before shutdown is still there at start up. Cold oil is not good at circulating nor at moving through little valves, but should I have an engine failure at the worst possible time--right after take-off, low altitude, low speed, nose up attitude, max weight, poor terrain ahead of me--that is what I will urgently need the oil to do. As soon as I recognize an engine failure after take off, and before the propeller slows to below 1000 rpm, I'm going to be pulling the appropriate propeller lever back. I don't just trust that the lever will do its job though. Before the first flight of the day I let the engine warm up enough that the oil can circulate, and then I pull each lever back to the feather position and then forward again, quickly, but just enough that I can see that the rpm is falling in response to the command. I'll do this three times with each propeller, alternating so the rpm can recover in between.

On this particular day I observe that the left propeller doesn't feel quite right on feather check. It doesn't take longer to react. The RPM comes down the same amount. I just don't feel like it's going to feather if I ask it. That's so ridiculously subjective I don't even know what it is. The resistance must be subtly different in the cables. It's the sort of thing I could ignore. There's no quantity check or measure I can point to that it doesn't meet. The left propeller is due for its 500 hour desludging soon. That's probably all it is. But I mention it to maintenance anyway. It's odd what a pilot can sense without knowing how or what. I once told a maintenance unit that the propeller control on a newly overhauled engine didn't feel quite right. I don't know if I had any more specific comment than that, and I don't know whether they took a closer look on account of my statement, or by coincidence, but they told me a few days later that the wrong propeller cable had been installed.

And that's all of this story that I know so far. There are a thousand such stories. Hiccups, unexpectedly difficult starts, flickering gauges, and funny noises and only a few do we ever know the cause of or have any concern about ever again.

SouvenAIRs

Company has rush-ordered a new heater, we're almost due for scheduled maintenance, and the weather is turning bad all over the province, and it's already bad in the neighbouring ones. It's a rare convergence of circumstances that suggest so strongly that we fly the airplane back to base, so we check out of yet another hotel and fly home VFR.

The co-owner welcomes us in, then I tidy up the airplane. I don't track down a vacuum cleaner before closing time, but I do neaten up the seatbelts, pick all the muffin crumbs out of the carpet, find my headset bag and stow my headset in it, and refold all the charts properly and sort them all back neatly in the racks where they can be reached for the next flight. When I'm travelling with an airplane, a lot of garbage ends up in locker where I store cleaning supplies. That's because I'll be preflighting, open that locker to get out a cloth and 210 spray (a type of plastic polish for airplane windshields), clean the windows, and toss the spray bottle and used wipe back in. Then I'll check the oil, get out a funnel and a couple of litres of oil. They're actually 946 mL, a quarter of a US gallon: that's the standard size all over North America, and it looks like even in Europe, maybe because the crankcases on American-made airplanes hold an even number of US quarts. Anyway, we call them litres, even though they aren't. I'll add the required oil, then when I get tired of waiting for the last drips to come out of the bottle into the crankcase, I put the lid back on the bottle tightly, and toss the bottle in the locker with the funnel and the other empties. Empty bottles don't weight much and the home airport has some kind of environmental disposal for them, so rather than run around a strange apron looking for a FOD bin, I haul them all home.

As I do so, I'm always amused by the way the bulging and squashed sides of the various bottles tell the story of the trip. Putting the cap on tightly seals in those few millilitres that didn't drip out while the bottle was inverted over the crankcase, but mostly what it seals in is air. Air at the pressure of the aerodrome where I added the oil. If it's a sea level airport, the sides of the bottle bulge out at higher elevations. If it's a mountain airport, pressure at a lower elevation crushes the bottle. I'm easily amused.

I clean things up best I can, report the minor snags (right engine has almost double the oil consumption to the left, noticeable split in the throttles to maintain equal manifold pressure above 10,000', and some hydraulic seepage). Then I am "released" from call for a few days. This will give me time to get my camera repaired, take my friend's kids to the amusement park as I've been promising for so long, and do my laundry.

Back soon with clean underwear!

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On the subject of this stun gun, it's interesting to see that the seatback pockets are the same security hole for JetBlue as they were for Victory Airways. Whenever we'd boarded with something that wouldn't get through security, but we were going south, we'd pop it in the seatback pocket, get off and go through security and have it waiting for us back on board. CATSA never swept the airplane itself.

Thirty Minutes of Oil

Oil: I'm supposed to put Exxon 2380 oil in this airplane and it goes all around and makes things slippery, cooled, cleaned and sealed. Done. Yeah, right.

C'mon, Aviatrix, it's not that bad. Half an hour on the oil system and then you can go play with your toy flight simulator. Minimum oil temperature for start is -40. You don't need units there, the Fahrenheit and Celsius scales cross at -40. I'd be cross, too. In addition to the standard four functions of oil referenced above, oil pressure on this airplane takes part in propeller operations, specifically normal blade adjustment, negative torque sensing, and the unfeathering pump. It's used in three parts of the anti-icing system, and it has gauges and annunciator lights associated with it.

Oil lubricates the reduction gear, so that the propeller rpm can be 2000 instead of the 42,000-odd rpm of the main engine, and oil pressure in the propeller hub drives the propeller towards fine, against the pressure of the feathering spring, using the usual system of flyweights governing a pilot valve to keep the propeller on speed. Feathering dumps the oil from the hub, allowing the feathering spring to drive the blades all the way to 89%, while negative torque adjusts the blade angle to windmill in the 18-28% range, waiting for the pilot to either feather or attempt a restart. It's a kind of half-hearted autofeather.

Anti-icing has already been described, oil circulates in the oil cooler inlet lip, and then if its temperature is still above 55 degrees it goes to the oil cooler, else it goes through the filter and back to work. Fifty-five is the oil's lucky number because that's also the pressure differential across the oil filter that will cause it to bypass the filter. Oil heat also anti-ices the lower part of the engine air inlet, provides the heat for the fuel anti-icing heat exchanger, and makes the airplane nice and warm if you put your hands against the nacelle after shutdown.

Would you believe that that took me thirty minutes? It did include going off to get another book to look at a diagram of the propeller system. I need to know more than this, but I need a break.