Who can forget the horror of learning, a few years back, that a modern Airbus A330-200 aircraft simply vanished over mid-Atlantic, initially without a trace?
Over the weekend, the Telegraph published a chilling analysis of the June 1, 2009 crash of Air France Flight 447 from Rio de Janeiro to Paris. After the autopilot disconnected in a thunderstorm--a fail-safe response to faulty airspeed data from frozen pitot tubes--the pilot flying AF447 commanded excessive nose-up trim, entering a deep stall. Incredibly, the pilot held this attitude throughout the plane's descent.
For most of its four-minute, 7-mile plunge, AF 447 could have resumed controlled flight if the pilot flying had lowered the nose and held the attitude long enough to regain flying speed. This basic stall recovery procedure is taught to every pilot in initial training and drilled repeatedly in flight checks.
What went wrong? In a word: technology. Too much technology in between the pilots and the aircraft.
But there is another, worrying implication that the Telegraph can disclose for the first time: that the errors committed by the pilot doing the flying were not corrected by his more experienced colleagues because they did not know he was behaving in a manner bound to induce a stall. And the reason for that fatal lack of awareness lies partly in the design of the control stick – the “side stick” – used in all Airbus cockpits.
Cockpit controls in most modern airliners are connected to computers, not directly to engines and flight surfaces. Both Boeing and Airbus make extensive use of such "fly-by-wire" systems. Where Airbus goes further--a step too far in the case of AF447--is in eliminating the tactile signaling provided by the older controls. In a conventional aircraft, the pilot flying commands a nose-up attitude by pulling the stick back and the stick on the other side of the cockpit likewise moves aft, providing feedback to the PNF (pilot not flying). Boeing emulates this control behavior in its fly-by-wire aircraft, Airbus does not. Relax back-pressure in a new Boeing airliner and the nose will lower to its previously trimmed position, just like an old-style cable-and-hydraulics plane. In an Airbus, the computer carries on with the last command input until receiving a new one, even though the stick is visibly in the neutral position.
Under normal circumstances, the Airbus approach provides a lower pilot workload--a better "user experience" in Web 2.0-speak. However, in the corner case of an emergency with multiple instrument failures, overloaded pilots lack the physical cues that would likely have saved 228 souls on board AF447.
And the problem of too much well-meaning-but-intrusive technology goes beyond the flight controls: even the design of the computer-automated stall warning contributed to the disaster.
Bonin’s insistent efforts to climb soon deprived even the computers of the vital angle-of-attack information. An A330’s angle of attack is measured by a fin projecting from the fuselage. When forward speed fell to 60 knots there was insufficient airflow to make the mechanism work. The computers, which are programmed not to feed pilots misleading information, could no longer make sense of the data they were receiving and blanked out some of the instruments. Also, the stall warnings ceased. It was up to the pilots to do some old-fashioned flying.
With no knowledge of airspeed or angle of attack, the safest thing at high altitude is to descend gently to avoid a stall. This is what David urged Bonin to do, but something bewildering happened when Bonin put the nose down. As the aircraft picked up speed, the input data became valid again and the computers could now make sense of things. Once again they began to shout: “Stall, stall, stall.” Tragically, as Bonin did the right thing to pick up speed, the aircraft seemed to tell him he was making matters worse. If he had continued to descend the warnings would eventually have ceased. But, panicked by the renewed stall alerts, he chose to resume his fatal climb.