Stall (flight) - Deep Stall

Normal flight Deep stall condition – T-tail in "shadow" of wing The deep stall affects aircraft with a T-tail configuration.

A deep stall (or super-stall) is a dangerous type of stall that affects certain aircraft designs, notably those with a T-tail configuration. In these designs, the turbulent wake of a stalled main wing "blankets" the horizontal stabilizer, rendering the elevators ineffective and preventing the aircraft from recovering from the stall.

Effects similar to deep stall had long been known to occur on many aircraft designs before the term was coined. Gloster Javelin WD808 was lost in a crash on June 11, 1953 to a "locked in" stall and Handley Page Victor XL159 was lost to a "stable stall" on March 23, 1962. The name "deep stall" first came into widespread use after the crash of the prototype BAC 1-11 G-ASHG on October 22, 1963, killing its crew. This led to changes to the aircraft, including the installation of a stick shaker (see below) to clearly warn the pilot of the problem before it occurred. Stick shakers are now a standard part of commercial airliners. Nevertheless, the problem continues to cause accidents; on June 3, 1966, a Hawker Siddeley Trident (G-ARPY) was lost to deep stall; deep stall is suspected to be cause of another Trident (G-ARPI) crash on June 18, 1972; on April 3, 1980, a prototype of the Canadair Challenger business jet entered deep stall during testing, killing one of the test pilots who was unable to leave the plane in time; and on July 26, 1993, a Canadair CRJ-100 was lost in flight test due to a deep stall. It has been reported that a Boeing 727 entered a deep stall in flight test, but the pilot was able to rock the airplane to increasingly higher bank angles until the nose finally fell through and normal control response was recovered. A 727 accident on December 1, 1974 has also been attributed to a deep stall.

Reports on the crash of Air France Flight 447 have stated that the accident involved a deep stall entered at 38,000 ft (11,582 m) and continued for more than three minutes until impact, but this was a steady state conventional stall because this aircraft does not have a T-tail.

Canard-configured aircraft are also at risk of getting into a deep stall. Two Velocity aircraft crashed due to locked-in deep stalls. Testing revealed that the addition of leading edge cuffs to the outboard wing prevented the aircraft from getting into a deep stall. The Piper Advanced Technologies PAT-1, N15PT, another canard-configured aircraft, also crashed in an accident attributed to a deep stall. Wind tunnel testing of the design at the NASA Langley Research Center showed that it was vulnerable to a deep stall.

In the early 1980s, a Schweizer SGS 1-36 sailplane was modified for NASA's controlled deep-stall flight program.