Investigation Summary
The accident occurred during the development phase of the BAC One-Eleven, a short-range jet airliner featuring a T-tail and rear-mounted engines. The aircraft involved, G-ASHG, was the very first BAC One-Eleven airframe produced. During the test flight, the crew was investigating the aircraft's behavior at the edge of the flight envelope. As the aircraft reached a high angle of attack, the turbulent wake from the stalled main wings blanketed the T-tail, rendering the elevators ineffective. This aerodynamic shielding prevented the pilots from pushing the nose down to break the stall. The aircraft became locked in a stable, high-sink-rate descent known as a deep stall (or super stall). The investigation was significantly aided by the recovery of one of the aircraft's two flight recorders, which provided critical data on elevator positions and incidence angles. This was the first major accident attributed to the deep stall phenomenon, which is a specific risk for aircraft with T-tail configurations.
Final Conclusions
The investigation concluded that the aircraft entered a stable stalled condition during a stalling test. Recovery was impossible because the wings blocked the airflow over the elevators on the tail. The crash was the first to identify the deep stall phenomenon peculiar to rear-engine T-tailed aircraft.
Video Analysis
These Pilots Sacrificed Themselves So That We Could Fly ... — A detailed look at the 1963 BAC One-Eleven crash and how the sacrifice of its test crew led to the discovery of the deep stall phenomenon.
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Airframe & Maintenance
Systems & Failure Modes
T-tail Configuration
A design where the horizontal stabilizer is mounted on top of the vertical fin. In high angle-of-attack conditions, the tail can be blanketed by the wing's wake.
Elevators
Flight control surfaces on the tail used to control pitch. During the deep stall, these became ineffective due to lack of clean airflow.
Interesting Facts
- 01This was the first accident attributed to the phenomenon known as deep stall.
- 02The aircraft was the first prototype of the BAC One-Eleven and was on its 53rd flight.
- 03The crash occurred only 23 minutes after takeoff.
- 04The aircraft had completed 81 airframe hours prior to the accident.
- 05The incidence meter on the flight deck only read up to 25 degrees, while the aircraft reached 50 degrees during the stall.
Safety Actions & Advisories
Stick-Shakers
Introduction of devices that vibrate the control column to provide a tactile warning to pilots as the aircraft approaches a stall.
Stick-Pushers
Implementation of systems that automatically apply nose-down elevator pressure to prevent the aircraft from reaching a critical angle of attack.
Incidence Meter Range Extension
Requirement for incidence meters to be capable of registering higher angles of attack to ensure pilots have accurate data during extreme flight conditions.
Emergency Evacuation Procedures
Establishment and practice of emergency abandonment drills for test flight crews to improve survival chances during experimental flight hazards.