Deep stall

Deep stall is a dangerous condition that effects certain aircraft designs, notably those with a T-tail configuration. When entering slow flight with a high angle of attack, it is possible for the wing to "blank" the airflow over the tail control surfaces. In this situation it becomes impossible to control the pitch of the aircraft, and the plane will continue flying in its current attitude until it hits the ground. Throughout this period the wing is actually not stalled at all, although the effects are similar enough to warrant the name.

Although effects similar to deep stall had been known to occur on many aircraft designs, the name first came into widespread use after a deep stall caused the prototype BAC 1-11 to crash, killing its crew. This led to changes to the aircraft, including the installation of a stick shaker in order to clearly warn the pilot of the problem before it occured. Stick shakers are now a part of all commercial airliners. Nevertheless the problem continues to periodically haunt new designs, in the 1980s a prototype of the latest model of the Canadair Challeger bizjet entered deep stall during testing, killing one of the test pilots who was unable to jump from the plane in time.