I like James Fallows because he teaches me interesting things about China, beer, flying, and politics. Things I probably wouldn’t learn otherwise. Today Jim* wrote a fascinating post about what we’ve learned about the Air France crash from the recent black box recovery, what new questions it raises, and why most people have no clue what “stalling” means in the context of flying. Here’s an excerpt about what it means when a plane “stalls”:
3) The plane “stalled,” but not in the way you think. The great impediment to accurate coverage of many airplane crashes involves the world “stall.” Its normal meaning, to 99 percent of the reading public, is that an engine has stopped or failed. Engines do sometimes fail on airplanes, and in some cases can even stall in the normal sense. But the “stalls” and “stall warning” signals mentioned in the blackbox report mean something entirely different.
An “aerodynamic stall,” which maybe is the term we should always use, involves the angle of a wing as it moves through the air. The term of art here is “angle of attack,” and it measures how sharply the wing’s edge is angled up into the oncoming wind. Bear with me for some illustrations, from this excellent explanatory site. This one shows what angle of attack means.
The next sequence shows what a “stall” means, in aerodynamic terms. In the top illustration, the wing is almost horizontal to the oncoming wind, with a very low angle of attack. Let’s say it’s zero degrees. It produces no lift.
In the middle drawing, the angle of attack is higher — let’s call it eight degrees. At this angle, wind flows over and under the wing in a way that produces more lift than at a lower angle.
But then look at the third illustration:
There the angle of attack is higher still — let’s say, 15 degrees. But instead of producing more lift, it produces much less. The angle the wind would have to follow across the wing is too steep. Instead the airflow is disrupted and the wing (not the engine) “stalls.”
The transition from a high angle of attack, to a too-high angle, can be fairly abrupt. You pull back on the controls, raising the nose of the plane and increasing the angle of attack. You get more lift, and more lift — and then suddenly you get dramatically less. The wings start to shudder, as they are approaching a stall and losing lift; and then, in a fully developed stall, there’s a “break” as the plane stops flying and the nose drops to point straight down to the ground. There are lots of variations involving type of plane, whether you’re in a turn, and other factors. But the main point is, an airplane “stalls” not because its engines fail but because the pilots have increased the wings’ angle of attack too much. This also means that the plane’s airspeed is too low.
So when you read frequent references in the Air France report to “stall warnings” etc, they don’t mean that there was an engine problem of any sort.* They mean that, for whatever reason, all three members of a professional flight crew responded to warnings that the plane was flying too slowly/had too high an angle of attack — by deciding to pull back on the controls. Which leads to:
4) This report raises a new question. The new info from the black box concerns, among other things, the “control inputs” the pilots were applying during the last stages of the flight. What they were doing with the throttle to control power, with the ailerons (to roll right or left), with the rudder (to yaw the nose from side to side), and with the elevator (to pitch the nose up or down). Without the black box there would be no way to know those things for sure.
And the main puzzle, as several of the initial stories point out, is why a team of experienced pilots would kept pulling back on the controls, to increase the nose-up pitch, when the stall warnings were going off. This is a puzzle because being trained to do exactly the opposite is practically the foundation of learn-to-fly courses. If a plane is losing speed and threatening to stall, you recover by pointing the nose sharply down and adding power (plus other things). This reduces the angle of attack, builds air speed, and allows the wings to start providing lift once again.
Every pilot has done this in practice time and again through his or her flying career. “Stall recovery” drills are part of every basic flying curriculum, every recurrent competency drill, every bit of familiarization with a new airplane. I had not flown an airplane for several months because of my recent stay in China. So when I went out this past weekend for a recurrent-training flight, the instructor put me through a series of stall-recovery drills — exactly as I expected him to do.
Why this didn’t happen in the Air France cockpit is the next stage of the mystery to explain. There is more to say about this tragedy at some point — including the role of the pitot tubes, what auto pilots can and can’t do, and similarities to other airline disasters — but that is what I have time for now. And, of course, sympathies to all affected by the tragedy.
I’ve been stereotypically misinterpreting this for years (by imagining engines stopping, followed by a drastic nose dive). Now I won’t.
*I refer to James as Jim because I once sent him an email, and he responded and signed it “Jim”. It’s extremely impressive to me that someone with tens of thousands of blog subscribers, who probably receives dozens of emails from strangers each day, took the time to personally respond to my email (which didn’t even contain a question). Jim’s a classy guy.