Photo: curimedia via Flickr
The disappearance of Air France flight 447 over the Atlantic three years ago was as mysterious as it was terrifying.With no wreckage or black boxes to examine, investigators were forced to guess what had happened, and the guesses didn’t make much sense.
Then, two years later, when the wreckage was found and the black boxes miraculously recovered, we got a lot of answers, which were no less terrifying.
Basically, with the plane, an Airbus, flying normally at cruising altitude, one of two speed sensors momentarily cut out. This caused the autopilot to get two different speed readings, and this, as planned, caused the autopilot to shut off. And control of the plane was then handed back to the pilots.
The junior officer, who was flying the plane, then made some bizarre decisions, some of which are still mystifying.
Instead of keeping the plane flying straight and level, he pulled the nose up and started to climb. The climb became so steep that the plane’s airspeed slowed and the plane neared stall speed. The plane’s stall warning alarm sounded. And it sounded again, more than 70 times, as the plane eventually entered a full stall and plunged toward the ocean.
(An aerodynamic “stall” is not an engine stall. It’s a condition in which the plane’s “angle of attack” against the air moving over the wings becomes so steep that the wings no longer provide lift.)
The way to recover from a stall, as most pilots learn the first day in flight school, is to point the nose of the plane downward to increase the flow of the air over the wings.
Bizarrely, the junior pilot in control of the Airbus never appeared to realise that the plane was in a stall, despite the repeated stall warning alarms. Instead of pushing the plane’s nose downward, he kept trying to pull it upward–to make the plane climb. Neither of the other two pilots on the flight deck realised the the plane was in a stall, either–until the plane was 2,000 feet off the water and there was no time left to recover. The plane was operating normally all the way down, so the crash was caused–as most crashes are–by pilot error.
To trained pilots, the Air France pilots’ behaviour in this case is mystifying: With the stall warning alarms going off, how could none of the pilots have figured out what was happening?
Well, thanks to a forthcoming report on the crash, as well as this excellent article in the Telegraph, we now have a better explanation.
Two factors in the Airbus’s design may have contributed to the confusion and crash.
The first was the fact that, unlike those in Boeing planes, Airbus control “sticks” do not move in tandem. This means that, when one pilot does something, the other pilot may not know what he or she is doing. On a Boeing plane, when one pilot pulls the stick back, the other pilot’s stick also moves back, so it’s instantly clear what’s happening. This is not the case on an Airbus: When one pilot’s stick moves, the other remains in place.
This appears to have contributed to the Airbus crash: One of the two senior pilots finally asked the junior pilot what he was doing just before the plane hit the water. And when he said he was trying to climb, the other pilot realised the mistake, but by then it was too late.
The other Airbus design feature that may have contributed to the crash was even more frustrating. The plane’s “angle of attack” sensor is driven in part by a speed sensor on the fuselage. After the Airbus entered a full stall, the plane’s forward speed slowed to the point where the angle of attack sensor no longer functioned correctly. When this happened, the cockpit stall warning, which had been blaring “stall! stall!” shut off.
At one point during the descent, the junior pilot regained enough forward airspeed for the sensor to start working again. As soon as this happened, the stall warning sounded again. Here, again, the pilot’s reaction should have been to shove the stick forward and the nose down, but for whatever reason, he never did that. Instead, perhaps panicked by the alarm sounding again, he went back to what he had been doing–trying to climb. That put the plane back into a full stall, at which point the stall warning alarm again shut off.
In other words, when the plane’s situation became so serious that it was plunging toward the ocean in a full stall, the warning alarm that might have clued the pilots into what was happening shut off. And when the pilot, perhaps accidentally, finally started to do the right thing, the alarm started squawking again. So he stopped doing it. And none of the pilots realised that they were in a stall until the end.
The good news here is that the Air France crash alerted every pilot in the world to the hazards of a high-altitude stall. And that warning, combined with some new training procedures, make it less likely that the the errors that brought down the Air France plane will happen again.
But that is presumably small consolation to the relatives of those on Air France 447.
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