The accident raises many questions about automation, systems design and pilot training that will be debated for a long time — not that these very subjects were not already controversial before the accident occurred. In all the philosophical discussion of the competing demands of pilot and airplane autonomy, however, a practical detail tends to get short shrift: the nature of the stall itself.
The BEA, or Office of Investigations and Analyses, which is the French equivalent of our NTSB, released on May 27 an interim report summarizing data extracted from the recovered flight data recorder (FDR), together with a few snippets from the cockpit voice recorder (CVR). The FDR data that have been released as I write this suggest a mushing glide at 200 to 250 ktas with the nose 15 degrees above the horizon and an angle of attack of between 35 and 60 degrees.
Apart from wing rock and a gradual right turn of nearly 270 degrees, the airplane’s attitude was stable. Since a fully developed stall is widely regarded as an out-of-control condition, it has been suggested that the airplane was in a “deep stall.”
The term “deep stall” came into use in the 1960s when it was found that T-tailed jet transports could get into a condition in which the turbulent wake of the stalled wing enveloped the horizontal stabilizer. Deprived of high-speed airflow, the elevator became ineffective and the airplane could not recover from the stall. The defining characteristic of a deep stall, then, apart from the absence of a yawing component, is that you cannot get out of it by normal use of the controls.
Since the term came into general use, we have seen various cases of deep stalls in airplanes other than T-tailed transports. There have been several instances where the wings of canard airplanes have stalled when the forward surface was already stalled; the airplanes settled vertically to the ground like overloaded parachutes. Burt Rutan used a deep stall of sorts to achieve re-entry deceleration in his SpaceShipOne. In the case of SpaceShipOne, the stable stall was made possible by rotating the horizontal stabilizers to an extremely negative angle, like the dethermalizers that free-flight modelers use to keep their airplanes from disappearing over the horizon.
An important difference between SpaceShipOne and a deep-stalled canard or T-tailed transport is that SpaceShipOne’s stabilizers never stopped working, and the spaceplane immediately recovered when the stabilizers returned to their normal position. The “feathered” descent can really be seen as an extreme case of a normal stall, where the horizontal tail, itself unstalled, drives the wing past its stalling angle of attack and holds it there. There’s no law against the imprecise use of terminology — unfortunately — but a term is most useful when it is clearly defined, and “deep stall” ought to be used only to describe situations in which both wings and longitudinal control surfaces are stalled and/or ineffective for other reasons.