Monday, February 27, 2006

Euphemism of the Night

From a pilot job advertisement ...

We are looking for a responsible, self motivated person that understands and accepts the demands of the industry. [italics mine]

Ouch. Um. I won't be applying for that one. Convenient that they coded it so clearly, as it would be awkward to say flat out "Applicants must be willing to fly over gross, with known aircraft defects, in unsuitable weather."

If you want more to read, AC Pilot is sometimes a little dry, but I laughed until it hurt when I read about the Sandwich Snack.

Saturday, February 25, 2006

The Icing on the Cake Wing

When I was a kid I used to protest when the grownups would knock all the beautiful icicles off the building overhangs. They were so shiny and magical in their elaborately overlapping patterns. But the grownups said they were dangerous, because they could fall on someone and hurt them. Grudgingly I had to agree that I would not want a giant icicle to fall on me, but I still liked to look at them. To me, a frozen waterfall is one of the most beautiful things in nature. All that power crystallized for the winter.

I have to admit to secretly admiring airframe icing. It's like a tiger: deadly dangerous, yet exotically beautiful. John, Sam and Shawn have recently blogged on the dangers of ice. Shawn has a great picture of leading edge ice. (More icing porn, please Shawn.) So in the wake of learned, cautionary posts by some of my blogging colleagues, I have the audacity to say "but ooh, isn't it pretty?"

I think it's fascinating the way, despite the rush of air past the wing, that rime icing develops forward into elaborate shapes. If the departure airport was above freezing, during the climb the fuel is still warm and warms the wings around the fuel tanks, so that ice forms first on the areas between the tanks. During descent, the fuel has been chilled to the temperature of the cruising altitude, and ice forms where befoer there were gaps. I like to watch the ice patches shrink to nothing as we descend into warmer air.

I feel terribly guilty admitting this, as ice causes so many accidents. I almost deleted it instead of posting it. I know the effect of ice on performance, and respond promptly to icing conditions, but I can admire the sleek beauty of the tiger even as I chase it away from the village, can't I?

Thursday, February 23, 2006

Phone in to Win

Here is the ADF story I promised. I heard it from a pilot at a training course, years ago.

The crew is flying a heavy Boeing into Vancouver, the captain's home town. It's a VFR morning, and Vancouver has an ILS so they don't need the ADF. They have had it tuned to a local AM radio station since before the top of descent, and are listening to the tunes, the traffic, the weather, and a phone in contest. Tenth caller wins! The captain pulls up his cellphone out of his flight bag, turns it on, and dials in.

"Congratulations! You're the tenth caller."

It turns out that this is the same radio station that many of his colleagues listen to. In fact people he knows are listening to it at that very moment. They recognize his voice. And they recognize the steady whoosh-grind sound of the jet engines as heard from the cockpit. And of course everyone knows you're not supposed to be using your cellphone in an airplane. Especially while you're the one landing it.

The DJ asks the captain his first name, and he gives it. The DJ can hear the background noise, too, but he can't place it. "What are you doing right now?" he asks.

There's a bit of a pause, and then the captain replies "I'm ... driving ... a .. truck."

No word on what the the prize was, or even if this really happened. But I like the story.

Also, apparently Cockpit Conversation is now the third Google result for "how to impress your teenage girlfriend." In the interest of helping those who come here for that, I'd suggest respecting her, planning ways you can spend time together, spending time with her when you pay attention exclusively to her, and not to your ipod or Nintendo, and introducing her to your friends when you meet them while you're out. Oh and get your sister's help on what to wear.

Wednesday, February 22, 2006

Voice of Command

I had an optometrist appointment. Unless you're lucky enough to have never needed corrective lenses, you know the drill. While I'm seated in the chair, the optometrist projects one line of an eye chart on the opposite wall. There's a contraption in front of me like something out of Kubrick's A Clockwork Orange and as the optometrist flips levers to change the arrangement of lenses in front of my face, she asks, "Which is better ... one ... or two?"

I'm concentrating on the rows of letters with Aviatrix-like intensity, tossing back "one," "two," "one," as quickly and efficiently as I can. It's a task I've been given. It matters to how well I'll be able to do my job. I take it seriously.

She twizzles a big dial and creates a whole new lens configuration, then projects a different line of letters. "Can you make out any of those at all, even by guessing?"

I focus for a moment and then I realize I know the whole line, but it's cheating. The letters are a repeat. "I have this one memorized from a few minutes ago. Show me another one the same size."

"Please!" she says, like a mother prompting a child for correct manners, but she's quite offended. "I'm the one who gives the orders around here."

I apologized right away, then thought about it some more while I was having my pupils dilated, and my retinas scrutinized, so I could elaborate on my apology. "It's no excuse for rudeness, and I'm really sorry, I think it's like when I'm flying, and I'm focused on the task, the language is very terse. 'Flap ten' or 'say wind' or 'pull up and go around'. It's done for clarity and efficiency, but you are right, it is rude in any other context."

I think she grasped the idea, because she then described an analogous situation. She had been observing an operation on one of her patients, and the surgeon, whom she knew well and had always thought a well mannered person, was being so rude to the OR nurses that she was shocked, but the nurses didn't seem to mind.

I would have nodded, but I still had my chin wedged in the Orwellian optometry device. "They understand. It's the same kind of thing. It's just the way people speak in that environment." What, she thinks operating theatres are like M*A*S*H? I thought that the surgeon intent on the operation and spitting one word commands like "scalpel!" was such a cliché that everyone knew it. In this case it would have saved a lot of aggravation and hurt feelings if I had said, "Could you please show me another one the same size? I can't tell if I can see it, or just recognize it from before."

I have to admit that sometimes I get fed up by how verbose people can be when they ask you to do something. "Aviatrix, bring me a case of oil from maintenance," is not a rude request. I think I already blogged on hearing a flight attendant explain to another that, "The pilots seem rude, but it's just the way they talk. They talk to each other like that too."

The good news is, my eyes are just fine.

Tuesday, February 21, 2006

Over New York, Again

Way too many job hunting stories lately. I'm hoping it will pay off later with some great training and new job stories. But for today, a joke and some equipment stuff.

The joke is a supposedly true story from the Overheard in New York website.

Stewardess: The plane is about to land. Please everyone turn off your iPods.
Man: Why does she just assume we all have iPods?
Lady: It's New York. Even the people asking for money have iPods.

--United Airlines plane, LaGuardia

The airline pilots probably have iPods, too. Not like us lower echelon pilots who just listen to AM radio on the ADF.

I thought I'd already told you about the ADF, but a blogger search of this blog only turns up an example of replacing it with a round GPS unit. ADF stands for Automatic Direction Finder. It's an old-fashioned, but still in use navigational instrument. The pilot tunes the onboard ADF receiver to the frequency of a navigational beacon (NDB) on the ground, and a needle on the face of the instrument points on a compass card to the direction of the station, relative to the airplane. (The needle does its pointing automatically, hence the A in the name. The older instrument that the ADF replaces didn't indicate the direction to the station until the navigator had carefully adjusted the loop antenna to the strongest signal.) To ensure that the ADF is receiving the correct station, the pilot monitors a morse code transmission from the station and matches the dashes and dots to the ones the chart indicates her chosen station should be emitting.

It just so happens that the navigational beacons in question transmit their morse code identifiers on frequencies in the AM radio band. There are people out there who enjoy monitoring NDB signals, just for a hobby, but when it comes to listening enjoyment, most people prefer a less monotonous type of audio signal. (Is it just my perception, or are the British more prone than other nationalities to observational 'collections' like trainspotting or counting car number plates?) Any ADF receiver can be used to listen to local AM radio stations, while flying the airplane. I have a story about this, for later.

Monday, February 20, 2006


With two weeks left in my plan, I haven't yet acquired the perfect interview suit. Shame isn't it? Especially as I have no sceduled time off during the hours when the stores are open, between now and my interview. The general rule with interviews is that you dress one level up than you would dress on the job. So if the job would require you to wear a filthy t-shirt and jeans, you wear a clean t-shirt and jeans. If the job requires a clean t-shirt and jeans, you go polo and khakhis. All the way up to if the job requires a suit, you wear a damn fine suit (not a tuxedo or evening gown: that's where the rule breaks down). Well my job requires a shirt and tie, so guys wear a suit. But when I wear a suit, that's a skirt and heels. It seems to derail the object. I want the employer to picture me flying his airplane, not stalking the catwalk. Slacks I think.

Getting dressed in the mornng is more complicated for girls.

So as not to dedicate an entire post to my wardrobe, I'll also tell you of today's ATC weirdness. Approaching a busy control zone VFR, I received an instruction from the controller to remain clear of the airspace. He suggested I orbit over a named landmark. Problem was that the named landmark, depicted on the chart and clearly visible on the ground was unambiguously inside the control zone I had been ordered to remain clear of. As the "remain clear" was an instruction and the "orbit over" was a suggestion, I resolved the ambiguity by orbiting over something else, outside the control zone. A moment later I was cleared to enter the zone for landing. After landing, I asked the (less busy) ground controller about the instruction, and then the first controller's voice came on the ground frequency explaining that he thought it would be safer to orbit over the landmark he named, and that while yes, it was technically inside the control zone, that he had expected me to disregard the part about remaining outside, and orbit over his landmark. "But," he conceded, "I can see how that could be confusing." No kidding.

Sunday, February 19, 2006


While making my New Year's resolutions, I wrote:

I'm going to pretend I have my first airline interview on March 1st.

Today's e-mail included more than twenty-five invitations to chemically improve my sex life, and one short e-mail with the subject "Interview." I opened it, expecting it to extol the virtues of "a fine luxury wrist accessory," but instead it said,

You have been selected for an interview ...
It's not an airline, but it is a very valuable step towards where I want to be this fall, and is a sweet payoff for the effort I have put in. At the very worst, it is interview experience.

I can't tell you too much about the company, because if all goes as planned, it will become the new company that I don't tell you anything about, because I work there. I will let you know how the interview went, and then you'll have to read between the lines to see if I got the job. You know how.

Saturday, February 18, 2006

Best Range in Wind

The range of an aircraft is how much distance it can cover with the available fuel. The last post I did on range covers the air range - how far the airplane can go through the air. But the air is usually moving, so the air range isn't the same as the ground range. If there is a fifty knot [nautical mile per hour] tailwind at the flight altitude, then over the course of an hour, the airplane will travel fifty miles further over the ground than through the air. And if it's a fifty knot headwind, the ground range will be fifty miles less than the air range, for every hour of flight. And of course it's the ground range that determines whether you get to Anchorage before the fuel gauges get to E.

As soon as there is a headwind, ground range will always be less than air range, but one can somewhat offset the negative effect of a headwind by speeding up. The handwaving argument says that even though increasing the fuel flow will increase fuel consumption per air mile, increasing the ground speed will minimize the fuel flow per ground mile.

The easiest way to prove that an increase in speed could increase range, is to consider the extreme case. Imagine that the airplane is travelling at its best range speed, at a true air speed of 200 knots, but it's in a 200 knot headwind. The effective ground range is zero. If the fuel will last four hours at this fuel flow, then after six hours, the airplane will have gained no ground. Increase the fuel flow such that the fuel on board will only last five hours, and the airspeed will increase. It doesn't matter how small the increase in airspeed is from that increase in fuel flow, it translates to a positive groundspeed over those five hours: maybe it's going forward at 20 knots, for a total of 100 miles range. Increasing the fuel flow further, so the fuel would only last four hours would increase the speed further, but if the groundspeed at the new fuel flow was 24 knots, then the range over the four hours would only be 96 miles.

The trick is working out how much to increase the fuel flow over best air range to offset the wind. Here's the theoretical method using the fuel flow versus true airspeed graph discussed earlier. Look at the x-axis, the airspeed axis. Change the scale so that instead of true airspeed it reads in ground speed. So if there's a fifty knot headwind, the zero knots point of the original graph becomes minus fifty, the fifty knot point of the original graph becomes zero, and the hundred knot point of the original graph becomes fifty. Basically you're subtracting the headwind from every notch of the scale. Now, and this is slap-myself-in-the-forehead obvious but I didn't think of it: find the best ground range by drawing a tangent from the groundspeed zero (the true airspeed fifty knots) to the thrust required (fuel flow) curve.

In the airplane, I know how to use that theory to work out a practical best range speed by making a little table of fuel flow versus airspeed, and I know the approximate zero-wind best range speed (it varies with loading, altitude and temperature) of the aircraft I fly, and then using a rule of thumb to compensate for the wind. I can now see that I should be using the GPS groundspeed to obtain the best range speed. In a large jet, the weight change with fuel consumption is significant enough that for optimum range, the best range speed needs to change as the weight decreases. I imagine Dave has an onboard computer that compares his INS groundspeed with the metered fuel flow and can tell him the best range, as well as the most economic speed to fly at every moment. I don't have one of those, but I'm looking forward to learning all about how to use one.

Friday, February 17, 2006

Jazz, City Centre and Politics

I'm posting two political stories in a row. Perhaps I'm thinking that once I'm a big-time airline pilot I'll have to keep my mouth shut about such things, so I'm getting it all out of the way now.

The main airport serving the Toronto Metropolitan Area is CYYZ, the Lester B. Pearson International Airport. Like many large international airports, Pearson is not technically located in the city it serves, but rather a $40 cab ride away. Toronto has another airport, located on an island near the city centre. Air Canada Jazz operates a few flights in and out of the island airport, Toronto City Centre, including a direct flight from Ottawa, the national capital.

Plans are always rumoured about the possibility of building a bridge, or a "fixed link" as politicians call it, to connect the island to the city, thereby allowing arriving passengers to drive away from the airport, rather than taking a boat. That hasn't happened yet, but construction of an improved ferry terminal has been announced. Jazz planned to expand service to the island.

The next move in the game was for the Toronto City Centre to give Jazz notice of termination of its lease, cutting off access to the airport. Jazz says it will suspend service to the island airport effective March 1st. It's all a lovely political mess with the port authority, the city, the province, the airline, the airport's new owner and the outraged condo-dwellers all fighting over the amount of traffic and who will make money from it. Not that I'm any different. I look at every news story with a critical eye: does this make it more or less likely that I will get a new job?

Thursday, February 16, 2006

Spy vs. Spy

I had to laugh when I read this piece on the ongoing legal battle between Westjet and Air Canada. It's like something out of Mad magazine.

Many ex-Air Canada employees still enjoy standby flight privileges on the Air Canada. They have a computer login ID and password in order to view the space available on flights, so they can identify the ones most likely to accommodate standby passengers. Westjet hired a few ex-AC people and then initiated a clever ploy whereby they used the still valid IDs and logins to systematically track passenger loads on Air Canada, competitive information that allowed Westjet to manipulate their own schedules to best attract customers from Air Canada. Air Canada found out and sued, and Westjet countersued, and so on. The long sordid tail includes everything from e-mails coded "007" to people going through each other's garbage. As far as I understand it, Westjet says that it's Air Canada's fault because they knew Westject was spying on them and didn't do anything about it.

And in other news, Air Canada is accused of further intrigue related to price fixing for internatinal cargo.

Wednesday, February 15, 2006

Maximizing Air Miles

Right after I set up the background aerodynamics to talk about jet performance, I saw Dave's latest post on a real life situation involving setting maximum range speed to offset unforecast headwinds. This topic corresponds to two of my best aha! moments while reading this textbook, so I'm going to seize Dave's story and festoon it with trigonometry in order to cement my knowledge and share it with you.

Dole and Lewis treat jet performance before propeller aircraft performance, which confused me at first, but now I realize it's the logical progression. A jet is actually simpler than a prop plane when it comes to performance. Jet fuel is converted directly into thrust. Thrust is a force, and the thrust produced by a jet depends on three things: the speed of the intake air (V1), the speed of the exiting exhaust (V2), and the mass flow of air through the engine (Q). If you want an equation, you can use this one: Thrust = Q(V2 - V1). The faster the jet is going, the more air will go through the engine, and the greater the intake air velocity, but the exit speed remains pretty much constant at different airspeeds. That decrease in acceleration through the engine almost exactly cancels out the increase in airflow, so that at a given RPM, a jet can be considered to produce the same amount of thrust regardless of airspeed.

Remember the graph of total drag versus airspeed described a few days ago. Here's a graphic of the curve from wikipedia. It doesn't show the high end compressibility, but you'll get the idea. Because thrust counteracts drag, the y-axis can be relabelled in thrust units, making the same total drag curve into thrust required for level flight (Tr) versus airspeed. We can add another line to the graph representing thrust available (Ta) at max power. It's a straight, horizontal line, because jet thrust available does not vary with airspeed. The line runs across the graph and intersects the thrust required curve somewhere on the high airspeed end of the upward curve. When thrust required equals thrust available, you have just enough thrust to sustain level flight. The line and the curve intersect at the highest airspeed available for level flight. At lower airspeeds, there is excess power, so the aircraft can accelerate or climb. At higher airspeeds there is not enough thrust to overcome the drag, so the airplane would have to be descending to sustain that airspeed. At lower rpm a smaller amount of thrust is available, but it's still a straight line to determine the maximum cruising speed. Set the thrust low enough and there are two points of intersection with the Tr curve: one at the high end for maximum cruise at that thrust setting and one at the low end, representing the lowest speed you can fly with that thrust. Fly any slower and you need more thrust to overcome induced drag. There is one thrust setting that is so low that when you plot its horizontal line on the graph, the (L/D)max point at the base of the U-shape curve rests on the line. Set Ta any lower and it wouldn't intersect the Tr curve at all. Where Ta intersects Tr at the Tr minimum, that's the minimum fuel flow required to sustain flight, and the corresponding speed is called best endurance speed. Fly at that speed to stay aloft for the maximum possible time before running out of fuel.

That's not the fuel flow Dave set. He wasn't trying to stay in the air long enough to finish showing the onboard movie. He was trying to get to Anchorage before his fuel reserves dropped low enough to require paperwork. Note also that he wasn't trying to get there before the bars closed: he had to reduce fuel flow and slow down to set best range speed. As Dave put it, groundspeed/fuel flow x fuel remaining = range. If that is unfamiliar, realize that when you divide nautical miles per hour by pounds of fuel per hour, the per hours cancel and you get nautical miles per pound of fuel. Nautical miles per pound times the number of pounds you have left tells you how many nautical miles you have left.

Another way of putting it is that to get the furthest distance possible using the available fuel, you want to burn the minimum number of pounds per nautical mile. You want the smallest possible value of fuel flow divided by groundspeed. That's flipping it over from the way Dave did it, but that's how it works best with the graph. If you can increase the airspeed without increasing the fuel flow by a larger factor, you win. And if you can decrease the fuel flow while keeping the airspeed from reducing as much, you win, too.

The same y-axis that yesterday time I called drag, and two paragraphs ago I called thrust, could just as soon be labelled fuel flow. Thrust overcomes drag, and fuel flow determines thrust. So we have a graph of fuel flow versus airspeed. Note that this is airspeed not ground speed, the difference being the considerable headwind Dave was fighting. As it stands, the graph is concerned only with the speed through the air, the result of the fuel flow. I'll get to the headwind after solving the zero wind case.

For any point on the curve, its distance right of the y-axis represents the airspeed and its distance up from the x-axis represents the fuel flow required to sustain that airspeed. That is a somewhat duh statement, as it simply restates what is plotted on the graph, but I'm setting up my first aha. Consider any point on the curve and draw a vertical line from it to the x-axis. The length of that line is fuel flow. Draw another line from the origin (zero-zero point on the graph) to the point under consideration. That line is the hypotenuse of a right triangle. The base of the triangle runs along the x-axis and its length represents airspeed. Fuel flow divided by airspeed equals fuel per distance, but now you can see that it's also the perpendicular divided by the base of a right triangle. If you know trigonometry you see what is going on, and if you don't, this should be enough to make you rush out and learn, because the perpendicular of a right triangle divided by the base is equal to the tangent of the angle between the base and the hypotenuse. The tangent of an angle decreases with the angle. Now remember that we're looking for the lowest possible value for fuel per distance. So now all we have to do is find the line that goes from the origin to the curve, making the smallest possible angle with the x-axis. Inspection quickly reveals this to be a line tangent to (i.e. barely touching) the curve.

There's a way to realize this without the trigonometry. Lets say we start at the (L/D)max point at the base of the curve, with the lowest possible fuel flow and a low airspeed. If we add a little more thrust, we get an increase in airspeed. Add a little more, a little more airspeed. The goal is to stop adding more thrust at the point when the airspeed increases less than the fuel flow does. That's the point where the curve bends away, curving more up than forward, the point described in the final two sentences of the previous paragraph.

I had known the handwaving argument that the best range speed could be determined from a tangent drawn from the origin to the thrust required curve, but I hadn't noticed how to mathematically prove of it. I like it.

The speed and fuel flow represented this way corresponds to the maximum distance the aircraft can travel through the air with a set amount of fuel: its air range. My other aha! moment in this chapter was an even simpler demonstration of how to adjust the best range speed to compensate for a head or tailwind: best ground range. I'll write about that soon.

Tuesday, February 14, 2006

No Delay

When pilots are ready to take off, we taxi up to the hold short line that separates the taxiway from the runway and announce ourselves. If it's an uncontrolled airport, we just have a good look to see that there is no one coming and then broadcast our intentions on frequency. At a controlled airport, we are announcing ourselves to the appropriate tower controller. Something like "Flight whatever holding short on bravo two," meaning that we're on taxiway B2. Some people just say "Flight whatever ready to go" and make the controller work out where they are.

The controller has many possible responses. He could say "wait," meaning that the airplane should stay where it is. (When they appear to completely ignore our call, the message is the same). Better is "taxi to position runway zero one," meaning that we can cross the hold short line and position ourselves on the runway in readiness for take-off, but still need to wait for a take-off clearance. Variations on that include "line up and wait," "position and hold," or "taxi to position behind the Navajo, number two for departure." You can see from the last that sometimes the controller lines up more than one airplane on the runway, so that they can be launched sequentially without the need to wait for each pilot to taxi to position. It can take a few moments as we don't want to go too fast, especially in high winds or with ice and snow on the runway.

The controller may issue the take-off clearance right away, even as we are taxiing up to the hold short line and haven't called yet. He's juggling arriving and departing traffic so that everyone gets the use of a runway, and no one cuts anyone off, even if an aircraft has to make an unexpected go-around. Sometimes the take-off clearance is modified with instructions like "short delay approved" meaning that it's okay to sit on the runway for a few moments, completing checklists, before applying take-off power and getting the heck out of Dodge. Sometimes the instruction is "cleared take-off no delay." If you require a delay on the runway then you must refuse that clearance, with something like "unable, holding short." The "no delay" clearance usually means that there is another aircraft on final approach to land.

This is all a long set up for something that made me laugh the other day. I'm on final and a little Cessna is "cleared take-off, no delay" from the runway I'm aiming at. He doesn't move for a moment, and I think he hasn't heard the clearance, but then he slooowwwwly taxies to position. The tower controller prods him off the runway in time for me to land, with "Papa Quebec Romeo, that's the slowest 'no delay' I've ever seen."

Saturday, February 11, 2006

Airbus Testing

An Airbus A380 has been undergoing cold weather perfomance testing in Iqaluit this week. It's one thing to engineer a product and another to see if the passengers are going to freeze to death because of a taxiway traffic jam during a cold snap in Ottawa. Good to see that Airbus is checking it out.

According to CBC:

Since the A380's tail stands eight stories high, it instantly became one of the tallest structures in Iqaluit. And with a capacity of more than 500 passengers, it means the Airbus can comfortably hold about 10 per cent of the capital's population.

It must be drawing quite a crowd. I'd like to see it at my airport. Canada has seen unseasonably warm temperatures right across the country this winter, but the -25 in Iqaluit should be cold enough to allow them to complete the tests.

Thursday, February 09, 2006

A New Winner?

Here's the game. The following message was delivered to an e-mail address that I have been using for at least six years. As internet-savvy computer users, you decide whether clicking on the links in the e-mail will lead to an employment opportunity, or to a porn site with pop-up ads for pharmaceuticals.

From: Human Resources

Dear Aviatrix,

A career opportunity matching your profile for a Pilot position is presently vacant.

If you would like to apply online and haven`t met our recruiting team in the past six months for the mentioned position, click here or click Jobs to consult the list of other positions currently available.

If you do not wish to receive further job notifications, please click here to access your profile and desactivate the check box labeled "Please advise me of similar career opportunities".

We thank you for your interest in Air Canada.


Air Canada Recruitment

Replies to this message are undeliverable and will not reach the Recruitment Department. Please do not reply.

The e-mail used my real full name, including middle initial, and the "" domain appears exactly as it does in my inbox. In the original, the links go to a long address not at the domain. There were also two attachments to the mail: "this_mail_in_html2.htm" and "this_mail_in_html3.htm". So what do you think?

Investigation reveals that this is genuine Air Canada recruitment mail. Why do I feel like they want me to stuff envelopes or be in their internet porn movie? Has Air Canada outdone WestJet in the "make your recruiting e-mail look like spam" sweepstakes? The only thing missing is a request to send money to a box office in Nigeria.

Wednesday, February 08, 2006

Lift Does Not Act Up

Comments on my last post revealed some confusion regarding the role and direction of the lift force on an airplane in flight. I now attempt to dispel and elucidate. The following is basic theory of flight, not anything dramatic or unique to jets.

Lift is the component of aerodynamic force that acts perpendicular to the direction the airplane is actually travelling. (The movement of the airplane through the air, relative to the air, is the same thing as a wind blowing towards a stationary airplane. Leonardo da Vinci figures that out). The direction and strength of the airflow caused by the motion of the aircraft, but before the airplane actually disturbs the air, is the relative wind. To repeat: lift acts perpendicular to the relative wind.

When the airplane is in level flight, as depicted in that ubiquitous four forces diagram, the flight path is parallel to the ground, so the lift does act straight up and thus is in direct opposition to the weight. Thrust balances drag, (and all the moments I haven't mentioned balance out), and thus there are no unresolved forces. Disturb that equilibrium and the airplane will accelerate (speed up, slow down or turn) in the direction of the unbalanced force. The change in motion will result in a change in the forces, until either the forces are balanced, or the airplane disintegrates or hits something.

It doesn't matter which way it is going or if it's an eight engine bomber or a glider, the lift vector is perpendicular to the relative wind. Really. In the glider, relative to still air, the unaccelerated flight path is always slightly down. (The glider can go up if the air it's in is moving up (a thermal or updraft), or if the pilot raises the nose to zoom (that's actually a technical term) the aircraft, trading airspeed for altitude, but I'm talking about the stable, sustainable case.) It applies equally well to a glider or an aircraft with power at idle.

So your flight path is inclined below the horizontal at an angle I'll call gamma (γ). There is no thrust. Drag acts directly backwards along the flight path. Lift acts perpendicular to the flight path, so up and a little forward. Weight acts straight down. We wave our wand of trigonometry and resolve weight into two vectors. One, of strength weight x cos(γ), acts perpendicular to the relative wind, directly opposite lift. The other, of strength weight x sin(γ) acts along the flight path. (I'm using x for multiplication because * is nerdy, and I'm pretending that non-nerds are going to be following my trigonometry). Inventory the forces according to the direction they pull, and you discover that the only force opposing lift is weight x cos(γ). A cosine is always less than one, so lift is less than weight.

You will also see that weight is the only forward force. That is why the flight path must be angled down: if gamma is not greater than zero then there is no force to offset the drag.

Shove the nose of your glider down, accomplished by lowering the elevator such that the airflow hauls the tail up, and everything changes. It's complex, more complex than the next few sentences indicate. The angle at which the wings meet the air decreases, decreasing lift. The unbalanced weight accelerates the airplane downwards. The change in direction changes the relative wind, changing the directions of the components of lift, drag and weight. The higher airspeed changes drag and lift. The airplane continues to accelerate until the change in drag can offset the change in the forward component of weight, and the new lift offsets the new perpendicular component of weight. Or the glider is still accelerating as it lawn-darts into the ground.

If instead, you haul the nose of the glider up, by shoving the tail down, what happens? There is the same complex dance of changing forces. Lift momentarily increases, because of the increased angle of attack, and the glider accelerates upward. Weight doesn't change, but with an upward flight path, now there is a component of weight that acts backwards along the flight path, and what is there to act forward? Nothing. Lift acts perpendicular to the flight path, opposite weight x cos(gamma;) but as weight and drag both slow the glider down, lift decreases. You can keep hauling the nose up to increase the angle of attack, and that will work right up to the critical angle, when the glider stalls, and if you don't get it flying again the only forces acting will be weight straight down and drag straight up. The weight will accelerate the glider downwards until the drag increases to equal the weight. Or you hit the ground. That's a good way to land a small airplane, by the way. Just arrange for the stall to occur just above the ground, so that you hit the ground before the plumetting starts. If you're going to be plummetting, you want to do it in something really draggy, like a parachute. With that, the drag equals the weight at a speed that is low enough to survive the impact.

If we want to go up for long, we need another force along the flight path. That's thrust. So here's our aircraft with a flight path inclined upwards by that same angle gamma (I hope html does γ on everyone's computers.) As always, drag is opposite the flight path and lift acts perpendicular to it. Weight (W) is still W(cos(γ)) perpendicular to the flight path and W(sin(γ)) backwards along the flight path, just as in the previous paragraph. Thrust we'll consider aligned with the flight path. For the aircraft to be in equilibrium, forces along the flight path must balance and forces perpendicular to the flight path must balance. Along: Thrust = Drag + W(sin(γ)). Perpendicular: Lift = W(cos(γ)). Once again the cosine is always less than one, so the lift is less than the weight, and the steeper the angle of climb, the less lift is required.

Another way of explaining what is happening is that in a climb, the thrust is helping to hold the airplane up, and in a descent the drag is helping to hold the airplane up, so not as much lift is required. That explains why with the same power setting, you can't go as fast as level flight in a climb, but can go faster in a descent. The climb thrust has to help out lift and overcome downward drag, while the descent thrust is being helped out by downward drag. That's the handwaving, non trigonometry way of describing it, but I know some of you went "greek-greek-geek" to yourselves, as you skipped over the trig.

To answer anoynmous, yes, the thrust often is acting other than directly along the flight path, giving a component equal to the sine of that angle times the total thrust, that acts in the same direction of lift, and reducing the forward thrust to total thrust times the cosine of that angle. For small angles, the sine is small enough, and the cosine is close enough to one, that people don't worry too much about it. As the angle of thrust theta above the flight path increases, as it does when the airplane flies more slowly, then the upward forces in level flight must be counted as Lift + Thrust(sinθ)). The authors of my Air Canada-recommended textbook are not especially concerned about the off-flight path component of thrust at normal speeds, so I'm leaving it out. Directed thrust like on the Harrier jet (did you see the movie True Lies?) steers us towards the realm of rocket science. I once saw a t-shirt that had equations all over it, and said "As a matter of fact I am a rocket scientist." I wish I had bought it.

Friday, February 03, 2006

One and One and One Make Four

I'm still reviewing my aerodynamics, paying far more attention to the jet performance side than I ever did before. I'm going to explain some background today. I will skip some parts though, so don't panic if I don't explain everything. If you want to know everything, you'll have to buy the textbook yourself.

An airplane in flight has three forces acting on it: weight, acting down as a result of gravity; thrust, acting forward in the direction the engines are pointing, as a result of the engines working; and the aerodynamic force, acting kinda up and kinda back, as a result of air whacking against the airframe. People who write aerodynamics textbooks like to be more precise than "kinda up and and kind back" but pause for a moment to envision a whole lot of air whacking against an airplane, and realize that reality isn't about neat lines. Aerodynamicists are clever, though, so they've defined all the air-whacking forces that act parallel to the direction of the onrushing air before the airplane disturbed it (equivalent to the direction of travel of the airplane) as drag and all the air-whacking forces that act perpendicular to that onrushing air as lift. This is easily achieved using a wand of trigonometry. That way they can draw a diagram of the airplane being acted upon by four forces: weight, thrust, lift and drag, depicted as nice straight lines. If you type "four forces airplane" (feel free to use your nationally approved spelling) into Google and select the image search, you will find hundreds of diagrams like this one. One is legally required to begin any discussion of aerodynamics with such a diagram.

In level flight, thrust counteracts drag. (At cruise speeds, it's generally considered okay to believe that all the thrust acts forward, and ignore the fact that the thrust line may be angled a little above the flight path.) Therefore the thrust required to maintain level flight at any given airspeed is equal to the total drag at that airspeed. Extra thrust over and above that matching the drag is used to accelerate or climb.

Drag is affected by all kinds of things: the airspeed (v), the air density (ρ), and the wing area (S), the number of bugs smashed on the windshield, the size and shape of the airplane, and the way the wings are positioned with respect to the onrushing air. Total drag is the sum of three sorts of drag. One is parasite drag, from air that impinges on, slides past, and swirls around the airplane, impeding its forward motion. You might guess that the faster the airplane is going, the more the air impedes forward motion, and you would be right. Parasite drag is proportional to the square of the airspeed. Another sort of drag, induced drag, results from downwash from the wingtip vortices changing the direction of the aerodynamic force, so that less of the force is lift and more is drag. The slower the airplane is going, the more the wingtip vortices interfere with the airflow, so induced drag is inversely proportional to the square of the airspeed. Plot both those sorts of drag against airspeed, on the same graph, and the sum of the two curves is vaguly U-shaped. The low-speed end of the U is capped at the drag, mostly induced, corresponding to the lowest speed the airplane can achieve without falling out of the sky. That means that at very low speeds, drag is high, because of the contribution of induced drag. Drag reaches a minimum at an intermediate speed where induced drag is reduced, but parasite drag has not yet become large. The high-speed end of the graph swoops upwards, showing that the parasite drag becomes large at high speeds. Add in the the third form, wave drag, attributable to compressibility effects as flight approaches the speed of sound, and the top end of the graph bends noticeably upwards. Before reading this book I would have said there were two sorts of drag, because the airplanes I fly have never even reached half the speed of sound, leaving compressibility effects negligible. If other factors are held constant, drag increases proportionate to air density or wing area.

Aerodynamicists have devised an equation that summarizes this: Drag = 1/2 CD ρ S v2. The size and shape of the airplane, and the number of smashed bugs are all incorporated into the coefficient of drag, CD. If you're paying attention, you might be complaining that that above equation does not take into account induced or wave drag. But you see it does, because the value of CD for any particular airplane changes with angle of attack. At low airspeeds (high angles of attack), CD is large, and at high airspeeds it is much less, but its rate of decrease slows as airspeed approaches the speed of sound, so that the result of the equation matches the actual drag experienced by the airplane. In other words, CD is a fudge factor. It's like the Russian judge in figure skating, adjusting everyone's marks so that the final scores come out the way she wants them. A constant of proportionality that is not constant may seem supremely useless, but trust me for a few minutes, while I produce a similarly dubious equation describing lift.

Seeing as lift and drag are perpendicularly resolved components of the same aerodynamic force, it stands to reason that they would be described by the same equations. Thus Lift = 1/2 CL ρ S v2. The only difference is that CL, the coefficient of lift, replaces CD. Naturally the coefficient of lift varies with angle of attack, too: it is low at high airspeeds, corresponding to small angles of attack, increases with angle of attack, up to the critical angle, and then falls off again. Swept wing aircraft, like the typical passenger jet, have a much gentler CL curve than straight wing aircraft: the value does not increase as rapidly with AoA, and it falls off much more gradually, with no definable critical angle.

Now for the payoff to all these equations and fudge factors. In level flight, lift is equal to weight, and as weight is constant, lift must be constant across the speed range of the graph described above. But we know what the weight of the airplane is, so, despite all the fudge factors, we can know how much lift is being generated.

Next take the equation of lift and divide it by the equation of drag. (Just like high school: write one over top the other and cross out everything that is the same on the top and the bottom.) You discover that L/D = CL/CD. Everything else cancels. The ratio of the whacky coefficients is the same as the ratio of the lift to drag itself. Why is this a payoff? Because lift is what is holding us up ("good") and drag is what is holding us back ("bad"), we want to maximize lift while minimizing drag. The ratio of lift to drag becomes a maximum at the speed where drag is a minimum. This is where the induced and parasite drag curves intersect, and is known as (L/D)max (I say "ell dee max"). The speed, and more precisely the angle of attack that produces the speed corresponding to (L/D)max is very important to understanding aircraft performance. And it's one of the places where the jet and the propeller case diverge.

So tomorrow I can get to the good stuff.

[Little edit, in response to a comment: in both places where I equate two things "in level flight," I mean level, unaccelerated flight. No unbalanced forces.]

Thursday, February 02, 2006

Not Yet Jet Performance

I've got a month left in my preparation for the job interview date I set for myself.

I have been busy rehearsing answers to all the standard airline interview questions, but the problem with imaginary interviewers and lateral thinking, is that my imagination comes up with some seriously untenable interview strategies. While it would be impressive if I were to happen to rescue the airline CEO's five-year-old daughter from terrorists while I was on the way to the interview, without arriving late or disheveled, there is a very small chance of my being able to employ that particular method of scoring points with the hiring panel. Therefore logically no more than a very small portion of my time and energy should focus on those details. It's less literally life-or-death, but my best answer to the question "tell me about yourself" deserves more attention.

I may practice some of my interview answers on you, but such posts will be rare and sparse, for two reasons. One is that this is an anonymous blog, and of course many of the answers to human resources questions will contain personal information. If I skip the personal details, and just tell stories, spinning them into perfectly tailored little sales pitches for hiring Aviatrix, you might be very interested. So would be a lot of people. And sadly, that's why I won't post them. Many hits on my blog are from people looking for interview "gouge" as it's called. I'm not going to risk having my stunning Have you ever had a supervisor who lost his or her temper? story plagiarised by a candidate who interviews the week before me. You might think that doesn't happen, but people run around looking for the "right" answers to questions instead of their own answers.

Another thing I've been doing lately is reviewing technical information. At least one airline hands candidates a written test which they must pass before they even get a chance at an interview. Boning up on formulae is of course less emotionally draining than reviewing one's experiences to decide how best to respond to What is the worst thing we could discover about you? Way back when I started this blog, I bought an aerodynamics textbook. I shoved it in my flight bag and have been reading it on and off, but not on the same days I've been blogging. I tend to read the same bits over and over again because although they make sense while I'm reading, the details of the information seems to drift back out through one of the orifices in my head, after I put the book down. Perhaps explaining some of it will cement it in my head, and as there is only one right answer to this sort of thing, I don't mind those ahead of me knowing it too. What started out as the introduction to this entry has become rather long (something I have to watch when answering interview questions) so I'll make this a post in itself, and tell you about jet performance later.