In yesterday's Wall Street Journal, J. Lynn Lunsford and Andy Pasztor reported that investigators looking into the Colgan commuter-plane crash in Buffalo were beginning to think that the pilots' handling of the situation, rather than the intrinsic perils of "airframe icing" conditions, may have been the immediate cause of the tragedy. (Previously here; also, valuable posts here by Miles O'Brien and here by Patrick "Ask the Pilot" Smith of Salon.) The WSJ article, titled "Pilot Action May Have Led to Crash," quoted unnamed "people familiar with the situation" to this effect:
The commuter plane slowed to an unsafe speed as it approached the airport, causing an automatic stall warning, these people said. The pilot pulled back sharply on the plane's controls and added power instead of following the proper procedure of pushing forward to lower the plane's nose to regain speed, they said. He held the controls there, locking the airplane into a deadly stall, they added.
With all the usual caveats -- that it can take months or years to find out the real cause of airplane disasters, that sometimes the real cause is never known, that these unnamed sources might prove to be wrong, etc -- here is why this information could be significant. What follows is an Aerodynamics 101 explanation that would be obvious to people in the flying world but perhaps not so evident to the general reading public:
"Stall" is a very important word in aviation, but it means something entirely different from what most readers (or passengers) would assume. It has nothing to do with the operation of the power plant. That is, an airplane stall has nothing in common with an automotive stall. A car stalls when something goes wrong with the engine. An airplane stalls when something goes wrong with the flow of air over the wings. When birds flew into both engines of a USAir jet last month, the engines lost power and stopped -- but the airplane didn't "stall."
The crucial point about aerodynamic stalls is that they occur when the wing's "angle of attack" into the air is too high. That is, the wing is angled so sharply into the oncoming wind that the air can no longer flow smoothly over the wing's top surface to generate lift. When the wings stop generating lift, the airplane becomes dead weight and falls right out of the sky.* A Wikipedia primer on the whole topic is here; a passage on "How a Wing is Flown" from Wolfgang Langewiesche's unsurpassed 1944 classic on airmanship, Stick and Rudder, can be found here, via Google Books. (Yes, Wolfgang L. was the father of William Langewiesche, now of Vanity Fair but for many years my Atlantic colleague and flying mentor.)
For the pilot of any airplane, large or small, the practical implications of a stall center on whether you are pulling the airplane's nose up (by pulling the control wheel or stick backwards, toward your body) or pushing the nose down (by pushing the stick forward, away from you). Everyone who has ever flown an airplane has gone through stall-recovery drills. These involve climbing to a safe altitude; pulling the stick back more and more until you raise the nose so high and make the angle of attack so great that the airplane stalls and begins falling toward the earth; and then immediately pushing the stick forward as the very first step in getting the airplane under control and flying again.
There are other parts of the recovery process, but "nose down," which means stick forward, is something you drill so many times that it's meant to become 100% reflexive. It is mildly unnatural -- when the plane is falling, the first thing you do is make sure the nose is pointing down -- but it is the only way to reduce the angle of attack, allow air to flow over the wings again, and turn what is a plummeting brick back into a flying machine. Think of the comparison with the "turn into the skid" advice that drivers get for handling a problem on icy roads. Everyone has heard this in driver's ed; a significant number of people have experienced it; virtually no one ever practices it. Even amateur pilots have practiced stall-recovery drills, starting with the all-important "nose down" step, many many times. The point is to make "nose down" second nature.
So if these reports stand up over time, and if the evidence ultimately shows that whoever was controlling the plane reacted in exactly the wrong way, it will be the rare case of a professional air crew, out of panic or for whatever reason, forgetting an elementary procedure that they certainly knew. After the USAir water-landing in the Hudson, many people observed that the casualty-free outcome was both an individual and a collective achievement. Individually, the air crew (pilot, copilot, attendants) reacted with supreme competence. Collectively, everyone involved did exactly what they had been trained to do. If what the WSJ says turns out to be what really happened, the Colgan-Buffalo crash will be a startling case of individual failure, which in turn will raise questions of how a professional air crew could have reacted this way. _____
* This Wikipedia graph gives an idea of how a stall develops -- and how it feels, when you're deliberately stalling an airplane in training drills.
As the angle of attack increases, the lift provided by the wings increases too. That is, as you pull the stick back, raising the nose of the plane and increasing the wings' angle of attack into the air, the airplane climbs. The further you pull the stick back, the greater the lift and the steeper the ascent -- up to a point.
For the airfoil charted here, that point is an angle of attack of about 18 degrees. Beyond that critical stall angle, if you pull back further on the stick, the wind no longer flows smoothly over the wings. They suddenly stop developing lift, and the airplane simply falls. I've done that many times in training. Apparently in Buffalo it happened for real.
Update: After posting this, I saw an item from the WSJ's Scott McCartney making a similar point about the contrast between the airmanship on display in the USAir and Colgan episodes.