Ask the pilot

What makes a plane stay in the air, anyway? And what happened the night Flight 3407 fell from the sky near Buffalo, N.Y.?


Patrick Smith
February 27, 2009 4:36PM (UTC)

For a lay reader to understand what apparently befell Continental Express Flight 3407 outside Buffalo, N.Y., on the evening of Feb. 12, he or she needs to first understand how a plane flies. The latest media summaries, rich with technical jargon, are faulting the pilots, and perhaps rightly so, but before you can understand how the crew may have erred, and before you can make sense of terms like "stick pusher" and "stall," it's essential to grasp how a plane stays in the air and why Flight 3407 was unable to do so.

The best way I can explain flight is like this: The next time you're driving down the highway in your Prius, stick your hand out the window, parallel to the ground, and move it like a wing. Bend it upward slightly, and it rises. It's flying, no? Granted, your hand isn't going to get your Prius off the ground, but now imagine that your hand is really, really big, and the car has enough horsepower to go really, really fast. Becoming airborne is all about attaining the right surpluses among the four competing forces of flight: that is, procuring enough thrust over drag, and enough lift over weight. Or, as Orville Wright put it: "The airplane stays up because it doesn't have time to fall."

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There's also something in Flying 101 known as Bernoulli's principle, named for Daniel Bernoulli, an 18th-century Swiss mathematician who never saw an airplane. When forced through a constriction or across a curved surface, a fluid will accelerate and its pressure will simultaneously decrease. Our fluid is air, which moves faster over the top of the wing, which is curved (less pressure), than it does along the flatter surface below (higher pressure). The result is an upward push, the wing floating, if you will, on a high-pressure cushion of air. The lateral cross section of a wing, around which the air does its thing, is called the airfoil.

I'll be chided for a less than nuanced explanation, but truly that's the gist of it: Bernoulli's pressure differential, together with the simple, hands-out-the-window deflection of air molecules, provides the essential and indispensable component of flight: lift.

Loss of lift is called a stall. In aerodynamic terms, stalls have nothing to do with engines and everything to do with wings. The basic idea is easily demonstrated back on the highway: tilt your hand a little too steeply, or brake the Toyota below a certain point, and your arm ceases to fly. Technically, the point at which a wing stalls is a function of its angle against the oncoming air -- something called "angle of attack" -- and not speed, per se; but it's perfectly acceptable for the reader to understand it in terms of speed.

Thus it stands to reason that flight is a lot more precarious at lower speeds, such as during takeoffs and landings, than it is during cruise flight. It's true that wings are augmented with supplemental components -- namely, flaps and slats -- for extra help in slow flight, but lift margins remain thin. The wing is giving you all it's got.

Which brings us to Buffalo and Flight 3407.

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One of the most dangerous thing you can do to a wing, especially at lower speeds, is to coat it with ice. As I explained in last week's column, the devil here isn't the weight of the frozen material, but the way it disrupts airflow over and around the wing's contours, robbing it of precious lift. My Prius experiment notwithstanding, wings are designed to be as efficient as possible, and this requires them to be meticulously sculpted, not only in cross section, but span-wise too -- the curvature and thickness changing from leading edge to trailing edge, and from root to tip. Once ice begins messing with the mathematics of this design, all sorts of nastiness can result. In a worst-case scenario, heavy icing can induce a full-on aerodynamic stall, at a point well in advance of when a clean wing would stall. Indeed, that seems to be what happened to 3407.

The Dash-8 Q400 aircraft is outfitted with pneumatically inflated leading-edge boots that are designed to break away accumulated ice. This isn't as good a system as the heated surfaces used on larger planes, but it's common to most turboprop aircraft and adequately effective if operated properly. What went wrong isn't yet known. It's possible the devices were malfunctioning, were not operated properly, or were simply overwhelmed by an extremely severe icing event. One way or another, the wing iced over and stalled.

Or, I should say, was about to stall. The Q400 is equipped with a high-tech stall warning system, as well as something called a "stick pusher," a device that automatically nudges the plane's steering column, and thus its nose, downward to help avoid an incipient stall. In a small aircraft like a Cessna, stalls are docile and easily recovered from, but in a transport-category machine like the Q400, recovery can take several thousand feet, and the plane is smart enough to take matters into its own hands at such a critical moment.

Reportedly, the stick pusher on Flight 3407 was activated. And because of the ice, it would have done so at a higher speed than the pilots would normally expect.

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Were they caught off-guard? As suggested by investigators, recognition of impending trouble would have been made more difficult by the fact that the autopilot was engaged. Both the airline (Colgan Air) and the National Transportation Safety Board, drawing from past incidents, recommend flying the plane manually during icing conditions. This allows the crew to have a better feel for any adverse effects. Symptoms of dangerous icing -- sluggish response, buffeting, etc. -- can be masked by the automatic pilot, which makes small, often imperceptible adjustments to maintain control.

Be that as it may, pilots everywhere can sympathize with the crew's desire to have the autopilot on rather than off. Hand-flying at such a work-intensive juncture probably seemed counterintuitive, a poor technique. We imagine the scene in the cockpit: It's shortly before touchdown, and both pilots are busy with the pre-landing preparations. They are also concerned about the weather, frequently looking outside, to check for ice accumulation on the airframe. Perhaps they realize the situation is getting serious. But how serious? Suddenly, the stall warning is triggered. "At this airspeed? This can't be right." Except it is right, and now the stick pusher does its thing, abruptly forcing the nose down.

A crash, at this juncture, is still avoidable. Indeed, the technology has worked as it should, to save the aircraft from a deadly stall. The proper response would be to increase power to the maximum while resisting the urge to pull back on the wheel. The plane is only 1,600 feet above the town, about two minutes from touchdown, but short of actually striking the ground, the idea is to let the plane accelerate, as smoothly and quickly as possible, even at the expense of altitude.

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But instead, another mistake, this one fatal: The captain grabs the wheel and pulls the nose upward, against the force of the pusher. Maybe he sees the lights of the town rushing toward him?

We may never know whether this pull was the desperate reflex of a panicked pilot, or something more reasonable, such as an attempt to merely lessen the severity of the stick pusher's downward force. Whatever his intent, the effect of suddenly overcoming the pusher, together with the ice having degraded the normal feel of the controls, sent the plane flailing. What the pilot may have believed was a suitable upward tug turned out to be a highly unstable maneuver. Almost instantly, the nose went soaring upward past 30 degrees. (For reference, the nose-up angle during takeoff is typically around 15 degrees.)

Even in level flight the plane was on the verge of stalling. In this configuration, forget it. It promptly stalled, rolled violently to one side, and plummeted to the ground. There was not enough altitude even to begin a recovery.

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And now people are asking: Had the crew not been using the autopilot, would the result have been different?

It's hard to say.

Was the Q400 itself part of the problem? Are turboprop planes more susceptible to dangerous icing than jets?

Not necessarily. The pneumatic anti-icing boots used on turboprops are not as effective as the heated surfaces used on pure jets, but turboprops have been flying through snow and ice for 50 years, mostly without incident. I piloted four different turboprop models, all with pneumatic boots, through several heavy winters, and had no real complaints. (And as explained in this space a week ago, modern turboprops are by no means quaint or old-fashioned. All in all, the Q400 is more sophisticated than many large jets.)

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And what of pilot skill? Regional airline pilots tend to be younger (though not always) and less experienced (though not always) than those in the cockpits of mainline jets. Was this a factor?

Possibly, but it's difficult to know. It warrants mention that almost all of the fatal commercial airplane accidents in this country over the past seven years have involved regional carriers, and mistakes by the crew were a chief or contributing factor in at least three of them. But while the intangibles of experience are obviously valuable, hours in a logbook are not always the best predictor of performance under pressure. I'll point out that vastly more experienced pilots have made vastly more grievous errors than anything the Colgan pilots are charged with. On the whole, traveling aboard regional planes is probably, on some level, less safe than traveling on mainline jets. But less safe is by no means unsafe.

"Pilot error." When we see it in the newspaper or hear it on TV, it sounds so neat, so concise. Like running a red light, or turning left instead of right. But most of the time it's a nuanced, multilayered thing. What we saw in Buffalo was not some flagrant violation of the rules or reckless behavior. What we saw was imprudent technique and perhaps a flash of panic -- two pilots who let an unusually bad situation get the best of them.

Next time: Capt. Sully speaks to Congress. Tough times ahead for airlines and their workers

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Do you have questions for Salon's aviation expert? Contact Patrick Smith through his Web site and look for answers in a future column.


Patrick Smith

Patrick Smith is an airline pilot.

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