What dangerous phenomenon did the One-Eleven prototype crash reveal?

Hunting Aircraft had begun design studies for a jet-powered replacement for the successful Vickers Viscount, which led to the 30-seat Hunting 107. The One-Eleven prototype, flown by test pilot Mike Lithgow, crashed on 22 October 1963 during stall entry tests, with the loss of all on board. It was on its fifth test flight, intended to assess stability and behaviour during the approach to the stall and recovery from it, with the centre of gravity in different positions.

The cause of the crash was that the aircraft entered a stable stall condition from which it was impossible to recover, because the wings blocked airflow over the tailplane's elevators. It was the first accident attributed to the phenomenon known as deep stall, characteristic of rear-engined, T-tailed aircraft. The investigation led to the discovery of what came to be called deep stall or superstall, caused by the reduction of airflow to the tailplane due to the combined shielding effect of the wing and the rear engine nacelles at high angles of attack, preventing recovery to normal nose-down flight.

Once the deep stall condition was recognised, relatively simple preventive measures were introduced, including stick-shakers, which warn of an imminent stall, and stick-pushers, which automatically actuate the elevators and physically lower the nose before the stall is reached, while the tailplane and pitch controls remain effective.