Deep stall is a dangerous condition that effects certain aircraft designs, notably those with a T-tail configuration. In these designs, the turbulent wake of a stalled main wing "blanks" the horizontal stabilizer, rendering the elevators ineffective and preventing the aircraft from recovering from the stall.
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 occurred. 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 Challenger business jet entered deep stall during testing, killing one of the test pilots who was unable to jump from the plane in time.