Another thing that wants clarification is what happens to a wing when it stalls. Because unintentional stalls can have dire consequences, pilots are trained to regard the stall as a sort of cliff beyond which all support is lost. This is inaccurate. Wings continue to lift even when stalled. Typically, a wing may abruptly lose 30 percent or more of its lift at the stall; but plenty still remains. The problem is not so much that lift disappears as that drag increases enormously, and airplanes with normal power-to-weight ratios simply can’t overcome the extra drag in order to maintain altitude. With sufficient power, however, an airplane can continue to fly, and can even climb, with a fully stalled wing. Modern fighters and exhibition aerobatic airplanes do so all the time.
So a stalled airplane is not just a brick. It can continue to fly, and that is why the long descent of AF447 should be thought of as a mushing glide, not as a parachute-like “plunge” — a term that appears all too often in newspaper accounts.
Airplanes with swept wings of fairly high aspect ratio tend to pitch up at the stall. Wings are twisted, and vortex generators and other aerodynamic tricks are used to prevent this pitch-up; but when a swept-wing airplane is driven deep into a stall and held there, its likely reaction is a stable, nose-high rocking oscillation. This is the behavior that Flight 447 exhibited.
But this is not a deep stall, because the horizontal stabilizer, even though its apparent angle of attack with respect to the flight path may be very large, is most likely not reduced to utter impotence. The center section of the swept wing continues to produce a downwash in its wake, and the actual angle of attack of the stabilizer is influenced by this downwash. The stabilizer is immersed in energetic airflow. Response to the stick may be sluggish, but it is there.
The extreme angles of attack experienced by AF447 are outside the realm of certification flight testing and beyond what can be accurately duplicated in a simulator. The behavior of different types of airplanes in extreme attitudes and with largely separated airflows is idiosyncratic and impossible to predict. For what expert opinion is worth, however, I polled a number of aerodynamicists about whether AF447 would likely have responded to a nose-down command, and all of them, while saying that this was no more than a feeling or a hunch, thought that it would.
The BEA’s May report unleashed a frenzy of activity on Internet discussion sites (notably the extremely instructive Professional Pilots Rumour Network, www.pprune.org). 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, “there but for the grace”), and then to 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?”
