Deep-stall protection
Warmtoast quotes [re the fatal BAC 1-11 accident]:
"Investigation showed the cause to be a deep stall, resulting from the T-tail and rear-mounted engine configuration, and remedial action included the installation of powered elevators, a stick-pusher, and modification of the wing leading edges. These changes were adequate to prevent the aircraft from assuming an inadvertent and dangerous angle of attack, a condition peculiar to this configuration, in which the wing loses lift and the horizontal tail surfaces are unable to restore longitudinal stability.
"...the detailed investigation of the cause and remedy of the deep-stall phenomenon was to prove of considerable value to aircraft designers and manufacturers worldwide."
[Unquote]
If memory serves, the total package of stall protection installed in the One-Eleven and its big stable-mate, the VC10, was three-fold. At a given Alpha, the engine ignition system is activated (both A and B igniters). [This anti-flameout protection is because of the intakes, like the much-higher tailplane, being affected by turbulent air from behind the wing.] At a higher angle, the stick-shaker starts. Eventually, the "Stall Ident", consisting of a powerful pneumatic stick-pusher, operates.
During in-service flight tests, the calibration of the system has to be checked. The AoA probes on the 1-11 and VC10 do not send readings to any cockpit gauge, so special probes are fitted for test purposes. This enables the crew to verify the thresholds for the three phases of stall protection. Waiting for the stick-push can be an interesting experience... Having said that, the VC10 pre-stall ignition system is frequently activated during routine acceleration/flap-retraction out of hot-high airfields.
The remaiing prototype 1-11 was fitted with a deep-stall recovery parachute after the accident. On one test flight, this was deployed. The parachute jettison mechanism subsequently failed, leading to a forced landing on Salisbury Plain. The aircraft was recovered by road, and later entered airline service.