21 December 2011

How Bad Decisions Happen

Popular Mechanics has a chilling report on the cause of the crash of Air France 447, which was the result of a sequence of bad decisions by the crew. The fight recorders recovered from the bottom of the ocean reveal confusion and poor judgment among the pilots.  Despite being warned by the planes automatic systems more than 75 times that the plane was in a stall, the pilots never acknowledged the warning.

The article introduces a distinction between "normal law" which are the restrictions placed upon the plane by a computer to disallow any decision that takes it out of its flight envelope. However, Flight 447 was operating not in "normal law" but in "alternate law" -- PM explains:
Again, the stall alarm begins to sound.

Still, the pilots continue to ignore it, and 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. "You can't stall the airplane in normal law," says Godfrey Camilleri, a flight instructor who teaches Airbus 330 systems to US Airways pilots.

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.

It's quite possible that Bonin had never flown an airplane in alternate law, or understood its lack of restrictions. 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.
The tragic sequence of decisions on board Air France 447 will no doubt help to make flying even safer than it is today, and should also serve as a case study in decision making more generally. Many poor decisions can be attributed to a confusion between an expectation that one is operating under some conditions akin to "normal law" when the actual decision context is better characterized in terms of "alternate law."