British European Airways Flight 548 - Operational Background

Operational Background

The aircraft operating Flight BE 548 was a Hawker Siddeley Trident Series 1 short- to medium-range three-engined airliner. This particular Trident was one of twenty-four de Havilland DH.121s ordered by BEA in 1959, and with the constructor's number 2109 it was registered to the corporation as G-ARPI in 1961. By the time of the aircraft's first flight on the 14 April 1964 the company had become Hawker Siddeley Aviation, and Papa India was delivered to BEA on 2 May 1964. The Trident I was equipped with Krueger flaps on its wing leading edge.

While technically advanced, the Trident (and other aircraft with a T-tail arrangement) had potentially dangerous stalling characteristics. If its airspeed was insufficient, and particularly if its high-lift devices were not extended at the low speeds typical of climbing away after take-off or of approaching to land, it could enter a deep stall (or "superstall") condition, in which the tail control surfaces become ineffective (as they are in the turbulence zone of the stalled main wing) from which recovery was practically impossible.

The danger first came to light in a near-crash during a 1962 test flight when de Havilland pilots Peter Bugge and Ron Clear were testing the Trident's stalling characteristics by pitching its nose progressively higher, thus reducing its airspeed: "After a critical angle of attack was reached, the Trident began to sink tail-down in a deep stall." Eventually it entered a flat spin, and a crash "looked inevitable", but luck saved the test crew. The incident resulted in the Trident being fitted with an automatic stall warning system known as a "stick shaker", and a stall recovery system known as a "stick pusher" which automatically pitched the aircraft down in order to build up speed if the crew failed to respond to the warning.

These systems were the subject of "one of the most comprehensive stall programmes on record", involving some 3,500 stalls being performed by Hawker Siddeley before the matter "was squared off to the satisfaction of ... the ARB" (Air Registration Board). The stall warning and recovery systems tended to overreact: of ten activations between the Trident entering service and June 1972, only half were genuine, although there had been no false in-flight activations. BEA Trident pilots were questioned informally by one captain, over half of the pilots said that they would disable the protection systems on activation, rather than let them recover the aircraft to a safe attitude. Random checks carried out by the airline after the accident showed that this was not the case; 21 captains stated that they had witnessed their co-pilots react correctly to any stall warnings.