Investigators have determined that an Airbus A340-600 collided with a test-pen wall in Toulouse after it was powered up while unchocked, and an attempt to steer the aircraft out of danger reduced the effect of the brakes.
The test crew failed to reduce the engine thrust as the jet surged forward, attempting instead to stop the aircraft by pressing the brake pedals and turning the nose-wheel sharply to the right. It smashed into the pen wall, seriously injuring four of the nine people on board.
A division of the France's BEA investigation agency, which conducted the inquiry, has released its final report into the accident, stating that "lack of detection and correction" of test-procedure violations helped lead to the accident.
It points out that procedures for the test, aimed at checking for oil leaks, were not correctly followed, noting that the engines were all operating at high power and the aircraft was left unchocked.
Detailing the events leading to the collision, it says the aircraft arrived in the test-pen at 14:19 on 15 November 2007 and underwent routine engine tests for about 1h 40min.
Just before 16:00 the power of the Rolls-Royce Trent 500 engines was increased to an engine pressure ratio of 1.25 - with the thrust levers corresponding to a position between maximum continuous thrust and maximum take-off thrust.
All four engines were operating. While the parking brake was on, registering 179 bar (2,600lb/in²), the inquiry says the applied thrust was around the limit of the parking-brake capacity.
At the time of the accident an Airbus employee was occupying the right-hand seat of the jet while an Abu Dhabi Aircraft Technologies technician was in the left-hand seat.
Flight-recorder data shows that, shortly after 16:02, the person in the left-hand seat warned that the aircraft was moving. The ground speed began gradually increasing to 4kt (7.4km/h) over the next few seconds and, after a second call that the aircraft was moving, the recorder registered pedal-braking and the deactivation of the parking brake.
Brakes on the A340-600 are linked to two hydraulic circuits: the "green" normal circuit and the "blue" alternate. The parking brake is on the blue circuit and only applies to the left- and right-hand main undercarriage bogies, not the centre bogie.
If the parking brake is released and the brake pedals applied, the "green" circuit comes into play. The pedals act on all three main bogies.
Recorder data shows that "green" circuit brake pressure on the A340 rapidly rose to 173 bar while the "blue" circuit pressure dropped.
About 7s after the first movement warning, the nose-wheel was turned sharply right. Activating the nose-wheel steering inhibits braking on the central bogie, becoming completely ineffective past 20° of steering.
The aircraft swung 37° to the right, but continued to accelerate, its speed increasing from 4kt to 31kt in 7s, before the aircraft struck the test-pen wall, demolishing its forward fuselage. The jet was written off.
In its report into the accident, the BEA says the aircraft and its braking system functioned correctly, but says that the nose-wheel steering "limited the effectiveness" of the brakes.
"Surprise led the ground-test technician to focus on the braking system, so he did not think about reducing the engines' thrust," it adds.
Only after the collision with the wall were the throttle levers retarded to the idle position.
Source: Flight International
