Ask the pilot

Stripping down in the cockpit. Plus, lessons on how to stall out at the speed of sound.

Published May 2, 2003 7:30PM (EDT)

I've been asked to comment on the recent but already infamous story of the Southwest pilots terminated for going au naturel in flight. But I'm withholding statement since these sorts of things have a way of becoming wildly distorted when stripped, if you will, of context.

I don't know what they were doing. And no, I've never taken my clothes off during flight. Except for once, in the summer of 1995, when a pavement-melting heat wave was sweeping across the Midwest and I was based at Chicago as a first officer on a 64-seat ATR-72.

The European-built ATR was a wonderful example of technology for the sake of itself -- a fragile, gratuitously sophisticated aircraft deployed on short-range commuter routes that require sturdy, low-tech dependability. And in all that fancy wiring and plumbing, they forgot the air conditioning. Tiny eyeball vents blew wisps of tepid air through the cockpit and cabin.

On this particular day, the temperature had hit 107, and a thick haze had dropped over O'Hare like a yellow blanket of steam. I was up front, finishing my preflight checks and waiting for the captain. I was so hot that I could hardly move. So I took my shirt and tie off. Pilot shirts, which look nice but are mostly polyester, are uncomfortable enough in a perfect climate. Crank the heat and it's like wearing chain mail. I also removed my shoes and socks.

The captain arrived -- a serious, overweight fellow in his 50s whom I'd never met before. He stepped into the cockpit and discovered Patrick Smith drenched in perspiration, dehydrated and wearing only blue pants and a Sony headset.

He didn't speak at first. Then he sat down, looked at me and said quietly: "You are going to put your clothes back on, aren't you?"

My answer was no. I told him I'd get dressed again as soon as the interior temperature dropped below 90 degrees, provided I was still conscious. I offered to put a T-shirt on, but the only one I had handy, from my luggage, was a Hüsker Dü tour shirt from 1983. It was almost as yellow and discolored as the putrid Chicago sky, and on the front it said "Metal Circus," which seemed only to further discombobulate the captain. "Arright, fine," he said. "Just don't let anyone see you."

And so I flew bare-chested, all the way to Lansing and back.

I wish I had a more racy -- or at least embarrassing -- story, but that's about as crazy as it gets. Years later, on another scorching hot flight, a pilot showed me how to make a squeeze bottle of cold air by filling an empty soda container with dry ice and poking a hole in the cap. Of course, I had more exciting uses for dry ice.

Anyway, on that note ...

I'll have you know it was an editorial decision, not the author's, to go with the word "geekfest" in our lecture about aerodynamics two weeks ago.

No harm done, it seemed, and, after all, I've used the word myself to characterize airliner buffs and other aviation obsessives. Back when this column made its debut, one of the first letters I received called me "an aeronautics geek," which I was tempted to accept as a compliment if only it hadn't been modified by "fucking."

But then I started thinking. Are pilots geeks by virtue of their profession, or only when they start elaborating on the technicalities of ailerons and airfoils? It doesn't seem fair. To most people, flying a plane isn't geeky; and you can't fly a plane without understanding how it works. Do we call doctors "medical geeks" when they explain brain surgery? (Though I guess their business tends to be a bit more philanthropic than flying, avoiding the tease.) Do we call painters and rock musicians "arts geeks"? (But then, artists and bass players meet a lot more women than pilots do.)

Where on a résumé, exactly, is geek status revealed? Is it found at the bottom under "personal," or is it inherent in the applicant's career objective? Geekiness has always been a sort of panoccupational attribute, I thought, a product of personality, not specific to a chosen field. You tell me. I'm a guy who barely knows the multiplication tables and got a D in 11th grade algebra. Am I a geek in spite of that, or exactly because of it? Or does it all depend how I dress?

A reader -- cynical, astute and maybe a bit paranoid -- comments: "I thought geeks were computer nerds. I am beginning to see that perhaps being a geek (or a nerd) means doing or knowing absolutely anything useful, and the term is now used to describe anyone who does not use charisma, manipulation, charm or forcefulness as their means of achieving a position in society."

You mean I got no charisma? Besides, how many geeks get to carry guns to work?

Dear Geek, why are the wings of jetliners angled rearward, while those of smaller planes tend to be straighter and more simple?

You're asking for it. Would you like a dissertation on things like mach tuck, shock stalls and Dutch roll? I'll try to keep it simple:

The faster a plane flies, the more lift a given wing will generate, as our old hand-out-the-car-window experiment will easily prove. But as lift increases, so does its ugly cousin, drag. And the more drag on hand, the more energy -- i.e., fuel -- it takes to keep moving. Simpler, more rectangular wings are very practical for flight at low speeds, but they produce copious amounts of drag when traveling fast.

With economy at stake, wings are designed to achieve optimum performance -- the perfect tradeoff between lift and drag -- not when taking off or landing, but during cruise flight. That's at very high altitudes, and often just shy of the sound barrier. Honing this efficiency is an art -- grist for the engineers, wind tunnels and computers. Airfoils are meticulously sculpted, even on smaller planes. That straight, blockish wing of a 30-seater is a lot more high tech than you think.

Meanwhile, as a wing cuts through the sky, the molecules of air are accelerated across its camber. This acceleration can reach critical levels where a shock wave builds along the wing's surface, killing lift. On a typical jetliner, this occurs at just about the speed of sound. Sweeping the wings backward induces a more spanwise flow and delays the formation of the shock wave. On a plane like the 747, the angle of sweep is over 40 degrees. Supersonic aircraft, like military fighters or the Concorde, have highly swept, virtually triangular delta wings.

One disadvantage of sweepback is an inherent instability contributing to adverse rolling and yawing. To combat this, swept wings are canted upward from the root, which counteracts the side-to-side swing. This tilt, which is most easily seen from a nose-on perspective, is called dihedral.

Because a wing -- especially one with a lot of sweep -- reaches its best performance at very high speeds, it needs a lot of support, literally, when it's slowed down (one of the reasons why Concorde's takeoff and landing speeds are so high). That's where all the flaps, slats, etc., come into play, as detailed two weeks ago. Straighter-winged planes also have these devices, as they too are designed to be most efficient at cruise speeds, but they're usually not as elaborate.

A friend of mine, obviously intoxicated, claims he once sat next to a deadheading pilot who tried to tell him that an airplane can be going too fast and too slow at the same time. Or maybe the pilot was intoxicated. Can you explain?

First of all, we should review the meaning of a stall. To aviators, stalls involve wings, not necessarily motors. To put it roughly, a plane stalls when the wing runs out of lift. In a jetliner, different areas of a wing will stall at different values, but the basic idea is easily demonstrated back on the highway. Tilt your hand a little too steeply, or brake the Honda beyond a certain point, and your arm ceases to fly. You've stalled.

Strictly speaking, stalls are a function of angle, not speed, but we needn't get so abstruse. It's easier for the layperson to fathom as speed.

However, it's not only a question of how fast the plane itself is moving, but how fast the air is flowing around the wing. An aerodynamicist cares more about the speed of the air, so to speak, than that of the plane. Getting back to the previous question, air speeds up as it passes over the wing. As a typical swept-wing airliner nears the speed of sound, this acceleration can form a shock wave atop the wing, which disrupts the normal airflow and destroys lift. A stall.

Thus, not only are there low-speed stalls, but high-speed "shock stalls" as well. At upper altitudes, where the air is very thin, a plane can find itself caught in an aerodynamic paradox: The higher it flies, the faster it needs to go to maintain lift and economy; but the faster it goes, the closer it comes to that critical point of airflow acceleration and formation of the shock wave. Cruising at 45,000 feet, margins of lift are already greatly reduced thanks to the lack of air density. Add the shock stall phenomenon, and yes, you have a situation where a plane is caught between going too fast and too slow at (almost) the same time. The speed that will result in a low-speed stall, and the one that will cause a high-speed stall, become progressively closer at extreme altitudes until they are virtually the same.

But relax. Much as it's interesting to consider, this is more of a theoretical illustration than anything passengers need to worry about during that eerie calm over the middle of the ocean. It only happens at the very edge of the performance envelope, which is not where airliners operate. Crews calculate buffer speeds to keep them at a safe distance from this proverbial razor's edge.

Oh yeah, well what about those strange-looking ridges, like rows of tiny spikes, you sometimes see on the wing?

We've now reached maximum geek.

Those are called vortex generators, and you sometimes see them on the tail too. Their purpose, counterintuitive as it might sound, is to sandpaper the air as it flows across that portion of the airfoil. At certain speeds and/or angles, air can begin to burble and diverge, disrupting lift and potentially causing a stall. By roughing the air, it tends to better adhere. Specific points are more prone to this than others, and that's where you'll find the vortex generators.

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Do you have questions for Salon's aviation expert? Send them to AskThePilot and look for answers in a future column.

By Patrick Smith

Patrick Smith is an airline pilot.

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