It is one of the most compelling mysteries of modern aviation. An airliner and 228 people vanished in the South Atlantic. This isn’t Agatha Christie stuff. The killer is not the Professor with a lead pipe in the conservatory of a 19th century Nile river schooner.
The technology on an Airbus A330 is so advanced that the airplane itself streamed digital messages to a maintenance center in France, a continent away. The airplane, in distress, was robotically reporting its death.
We have surveillance and weather satellites patrolling the sky, instant global communications and radar coverage of all but the most remote parts of the world. Airliners like the big A330 shouldn’t disappear into thin air. With all the digital dandruff in the ether, there is no ‘thin air’ anymore.
Actually, ‘thin air’ was part of the problem.
After two years the mystery seemed destined to stay unsolved forever. But in a stunning turn last May, the Flight Data and Cockpit Voice Recorders were found and recovered from the ocean floor. After two years on an abyssal plain four kilometers deep, the ‘black box’ data was intact.
The technical hairs are still being split by aviation voyeurs in PPRuNE and similar street forums. But the basic story is straightforward.
The airplane was cruising at 35,000 feet in mild turbulence, over 10 kilometers above the earth. A discrepancy among the three airspeed indicators occurred. The autopilot’s mechanical brain could not tell which airspeed was correct, so it automatically disconnected to prevent harm.
“Human Pilot, you have superior intelligence, you fly the airplane!”
The airplane continued to fly normally. The airplane did not care whether airspeed was correctly reported or not. Its wings were fat with lift and quite happy with what the actual airspeed was.
The pilot flying, the most junior member of the flight crew, took control and pulled back on his side stick, sending the airplane into a zoom climb at 7.000 feet per minute, twice the climb rate on takeoff.
We don’t know why the pilot climbed. (In Death by Ignorance, we had a diagram of a Spitfire climbing steeply to attack a Messerschmitt. But that was war.)
As it climbed, the wings’ angle of attack (shown at 11 degrees above) neared the critical angle where the wing stops flying as drag overcomes lift. Also, at those altitudes the air is very thin. The A330 was surfing a thinning airstream.
But an airplane does not care how close it is to stalling. It just either flies or does not fly.
At 38,000 feet the airplane ran out of momentum, the airflow across the wings diminished, and the airplane wanted to descend. If the pilots had let it, the airplane would have simply nosed down and flown downward. This is not a problem. Flying downward is not the same as falling downward.
But the pilots kept the nose up. As the airplane mushed down nose high, the angle of attack between the steeply inclined wings and the oncoming airflow instantly became 35 degrees or more. As we have seen, the wings cannot fly like this. The wings stalled, and the airplane fell.
They were not doomed. They just had to lower the nose, let the wings fly again, and then recover.
With the stall horn blaring for over 50 seconds, the pilots never verbally noted or acknowledged the stall warning horn.
Later, the pilots’ union attacked the airspeed indication fault that caused the autopilot to disconnect in the first place. As the union, Air France and Airbus hardened their positions, no one noticed the union was whining about pilots having to fly the airplane.
I thought that is what pilots do. Fly the airplane.
The original airspeed ‘disagree’ fault lasted just 54 seconds. But the pilots kept the airplane stalled all the way to the ocean, three and a half minutes and 38,000 feet down. The angle of attack was always 35 to 60 degrees. Except for brief, tentative stabs, the pilots held the controls full nose-up.
But all they had to do was lower the nose.
They hit the Atlantic ocean at nearly 11,000 feet per minute. Normally airliners touch down on a runway at less than 600 feet per minute. The impact was catastrophic and unsurvivable.
Peter Garrison, a respected technical writer in FLYING magazine, provided insight even as he asked a question, in last month’s issue.
Reactions ran a predictable gamut from incredulity about the crew’s actions (for stalling the airplane in the first place and then failing to recognize the stall and recover) to sympathy with the crew (darkness, turbulence, bafflement, a cacophony of incomprehensible and contradictory warnings), and then to the controversy over automation, human-machine interfaces and the devaluation of basic airmanship among line pilots. Pending the release of the full CVR transcript, however, the one great question that loomed over the discussion could not be answered: “What were they thinking?”
The Cockpit Voice Recorder transcript has since been released.* The crew’s final minutes are distressing to dwell on now, since I am writing this aboard an airliner over Germany at that very same altitude of 38,000 feet.
In the next and last article on this series on three accidents, we will see what they were thinking.
Posted from Bruxelles, Belgium.
September 17, 2011
References – fascinating reading
The “New Findings” update which summarized first findings from the recovered ‘black boxes.’ An eye-opener, it was the first real data from on board the airplane. It’s only 4 pages, and the key points are on page 3, repeated as a comment below this post. Released May 27, 2011.
The full Third Interim Report of the BEA, the official body investigating the accident. Released July 29, 2011.
The press conference for the release of the Third Interim Report of the BEA. This is interesting — the BEA talked about what happened in the cockpit.
*The transcript of the Cockpit Voice Recorder (last few minutes of the flight) are posted as a comment below this article.