Investigators are trying to establish why engine runs being undertaken on the Etihad Airways Airbus A340-600 involved in the Toulouse ground-test accident on 15 November were not being carried out in line with the manufacturer's guidelines for such tests.
France's BEA investigation agency has determined that the A340-600, which was undergoing pre-delivery engine and brake tests, was being held at standstill with the parking brake on and all four Rolls-Royce Trent 500 powerplants running with a relatively high engine pressure ratio of 1.24-1.26. Wheel chocks were not inserted under the aircraft at the time of the accident.
In an information telex to operators published last week, Airbus says that high-thrust engine ground runs are normally only performed on one powerplant at a time, with the corresponding engine on the opposite wing running at limited thrust to counterbalance. It adds that wheel chocks should be installed during such tests.
While the parking brake is set, the A340's alternate brake system provides the brake pressure. This pressure was normal during the Toulouse A340's engine run.
Investigators say that the aircraft, for as-yet undetermined reasons, began to move forward after the engines had been running for about 3min.
The Airbus communication says the crew applied brake-pedal input within 1-2s of the initial movement and switched off the parking brake the regular braking system's pressure rose to its normal level.
But Airbus adds that all four thrust levers were only retarded to their "idle" setting about 2s before the aircraft collided with the test-pen wall at a speed of around 30kt (55km/h). By that point the aircraft had been in motion for around 11s.
"There is no evidence of any aircraft system or engine malfunction," says Airbus vice-president for flight safety Yannick Malinge. "Airbus reminds all operators to strictly adhere to [aircraft maintenance manual] procedures when performing engine ground runs."
The BEA, which says that no technical malfunction has been found in either the engines or the A340's brakes, continues to investigate the accident, which destroyed the aircraft and injured several of the nine Airbus and Abu Dhabi Aircraft Technologies personnel on board. The airliner had been scheduled for delivery to Etihad Airways on 21 November.