Last minutes of flight AF447
December 7, 2011 6 Comments
A transcript of the cockpit voice recorder in the final minutes of flight Air France 477 was leaked in the book Erreurs de Pilotage, and in turn a good article was created here. I’ve excerpted a few bits of dialogue below, but plenty more in the full version.
02:08:07 (Robert) Tu peux éventuellement prendre un peu à gauche. On est d’accord qu’on est en manuel, hein?
You can eventually pull it a little to the left. We’re agreed that we’re in manual, yeah?
02:10:06 (Bonin) J’ai les commandes.
I have the controls.
02:10:07 (Robert) D’accord.
Bonin reacts irrationally. He pulls back on the side stick to put the airplane into a steep climb, despite having recently discussed the fact that the plane could not safely ascend. The stall warning sounds, a synthesized human voice that repeatedly calls out, “Stall!”, followed by a loud and intentionally annoying sound called a “cricket.” For the duration of the flight, none of the pilots will mention it, or acknowledge the possibility that the plane has indeed stalled—even though the word “Stall!” will blare through the cockpit 75 times. Throughout, Bonin will keep pulling back on the stick, the exact opposite of what he must do to recover from the stall.
02:11:21 (Robert) On a pourtant les moteurs! Qu’est-ce qui se passe bordel? Je ne comprends pas ce que se passe.
We still have the engines! What the hell is happening? I don’t understand what’s happening.
“If the person in the right seat is pulling back on the joystick, the person in the left seat doesn’t feel it,” says Dr. David Esser, a professor of aeronautical science at Embry-Riddle Aeronautical University. “Their stick doesn’t move just because the other one does, unlike the old-fashioned mechanical systems like you find in small planes, where if you turn one, the [other] one turns the same way.” Robert has no idea that, despite their conversation about descending, Bonin has continued to pull back on the side stick.
02:13:40 (Robert) Remonte… remonte… remonte… remonte…
Climb… climb… climb… climb…
02:13:40 (Bonin) Mais je suis à fond à cabrer depuis tout à l’heure!
But I’ve had the stick back the whole time!
At last, Bonin tells the others the crucial fact
02:13:42 (Captain) Non, non, non… Ne remonte pas… non, non.
No, no, no… Don’t climb… no, no.
02:13:43 (Robert) Alors descends… Alors, donne-moi les commandes… À moi les commandes!
Descend, then… Give me the controls… Give me the controls!
Bonin yields the controls, and Robert finally puts the nose down. The plane begins to regain speed. But it is still descending at a precipitous angle. As they near 2000 feet, the aircraft’s sensors detect the fast-approaching surface and trigger a new alarm. There is no time left to build up speed by pushing the plane’s nose forward into a dive. At any rate, without warning his colleagues, Bonin once again takes back the controls and pulls his side stick all the way back.
02:14:23 (Robert) Putain, on va taper… C’est pas vrai!
Damn it, we’re going to crash… This can’t be happening!
Why did the pilot ignore the stall warning?
According to the article,
the reason may be that they believe it is impossible for them to stall the airplane. It’s not an entirely unreasonable idea: The Airbus is a fly-by-wire plane; the control inputs are not fed directly to the control surfaces, but to a computer, which then in turn commands actuators that move the ailerons, rudder, elevator, and flaps. The vast majority of the time, the computer operates within what’s known as normal law, which means that the computer will not enact any control movements that would cause the plane to leave its flight envelope.
But once the computer lost its airspeed data, it disconnected the autopilot and switched from normal law to “alternate law,” a regime with far fewer restrictions on what a pilot can do. “Once you’re in alternate law, you can stall the airplane,” Camilleri says.
According to Camilleri, not one of US Airway’s 17 Airbus 330s has ever been in alternate law. Therefore, Bonin may have assumed that the stall warning was spurious because he didn’t realize that the plane could remove its own restrictions against stalling and, indeed, had done so.
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