BOAC, unable to find mention on the French BEA site but Flight Global has the following.
1994 | 1630 | Flight Archive
BY DAVID LEARMOUNT
The Airbus Industrie A330 which crashed at Toulouse during a test flight, lost directional control when its speed dropped dramatically after it was pitched up under autopilot control, according to data sent by Airbus to A330operators. Airbus says that "...analysis has not revealed any aircraft, engine, flight-control and autopilot system malfunction".
The consortium has told A330 operators that pilots who experience an engine failure at low speed should disconnect the autopilot and limit pitch-attitude. Airbus defines low speed as the minimum selectable autopilot speed (VLS), or 125kt (230km/h), whichever is the higher.
The accident happened during a simulated engine failure, when airspeed fell from the planned 150kt to only 100kt — then dropped to 90kt, while the pilot was taking over manual control — taking the aircraft well below VMCA [minimum control speed on a single engine at full power]. Airbus says that the floor" system did trigger, but "...was inhibited by the crew". Alpha-floor provides automatic full power on both engines when the angle of attack reaches a safe maximum.
The purpose of the flight was to test "experimental autopilot control-laws", the bulletin tells operators.
For the test, the left engine's thrust was selected to idle and its associated hydraulic system shut down. When directional control had been lost, the aircraft banked 110° to the left. The pilot had already taken manual control and begun to pull out of the dive, with wings level, but the height was insufficient for recovery.
The test intention, says the bulletin, was: "...to evaluate an experimental autoflight system developed for the A330 Pratt & Whitney version, at the limits of the flight envelope — maximum aft centre of gravity [CG], minimum [go-around] speed, with high rate of climb and engine failure — associated with the development of the autoland all-weather Category III operation".
Take-off conditions revealed in the bulletin were: configuration 2, weight 145t (very light), full aft [flight] CG (42%), fuel 23t and maximum take-off thrust selected. With these conditions the reference speeds were: V,, 126kt; V2, 126kt; VR, 135kt.
The flight history as notified to A330 operators included the following details:
• altitude set on autopilot flight control unit was 2,000ft (600m);
• unstick occurred at 136kt, 25s after power application;
• pitch attitude was "driven up to 28°" to maintain 150kt;
• at 150kt, 6s after unstick, the autopilot was engaged, number one engine selected to idle and the blue-system hydraulic circuit switched off;
• "...as expected the aircraft's pitch angle started to decrease from 29° to 25°";
• then, however, because of the low selected altitude, the autopilot reverted to the "altstar" mode (altitude acquire mode), "...thus preventing the intended task completion". At that point the aircraft began to pitch up again. Airbus believes that the unwanted pitch-up was because of "alt-star" mode demands when the aircraft's rate of climb began to decrease following loss of power on the engine. "The crew immediately identified the subsequent speed and pitch evolution and let the situation develop, most probably to enable proper recording of the event.";
• "...the aircraft reached a pitch of 33° and the speed reduced rapidly to lOOkt, which is 18kt below Vmca";
• the pilot took manual control: "Speed continued to decrease and the aircraft suffered an uncontrollable lateral departure at about 90kt." The aircraft hit the ground 35s after unstick.
ASN says PROBABLE CAUSES: "At the present stage of its work, the commission estimates that the accident can be explained by a combination of several factors none of which, taken separately, would have led to an accident.
The initial causes are primarily related to the type of the test and its execution by the crew during the last takeoff:
1) choice of maximum power (TOGA) instead of Flex 49; 2) very aft CG for the last takeoff; 3} trim set in the takeoff range, but in too high a nose-up position; 4) selected altitude of 2000 feet; 5) imprecise and late definition of the test to be conducted and the tasks to be performed by the captain and first officer, respectively; 6) positive and very rapid rotation executed by the first officer; 7) the captain was busy with the test operations to be performed immediately after take off (engagement of the autopilot, reduce thrust on the engine and cut off the blue hydraulic system) which temporarily placed him outside the control loop; 8) in addition the absence of pitch attitude protection in the autopilot altitude acquisition mode played a significant role.
The following is also contributed to the accident: 1) The inability of the crew to identify the mode in which the autopilot was placed; 2) the confidence of the crew in the expected reactions of the aircraft; 3) the late reaction from the flight test engineer when faced with a potentially hazardous change in parameters (speed in particular); 4) the time taken by the captain to react to an abnormal situation."