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Jet-blast damaged elevator cables on crashed Moorea Twin Otter

Investigators have concluded that on-ground jet-blast contributed to the failure of elevator cables on the Air Moorea de Havilland Canada Twin Otter which crashed in French Polynesia last year.

France's Bureau d'Enquetes et d'Analyses had already determined that the Twin Otter crashed, killing all 20 occupants, after the pitch-up control cable failed shortly after departure from Moorea on 9 August 2007.

But in a final report the inquiry details the reasons for the failure, stating that three phenomena featured in a "chain of events" leading to the crash.

Outer strands on the cable had already suffered significant wear. But the BEA has also determined that several strands broke while the aircraft was on the ground, and believes that this on-ground failure was due to exposure to jet-blast.

It points out that the Twin Otter operated to Papeete, the airport on Tahiti served by Airbus A340s. By considering the airport layout the inquiry has constructed a theoretical scenario showing the blast from an A340 during start-up - even at a distance of 80m (260ft) behind - could have exposed the parked Twin Otter's empennage to exhaust velocities of 20-90kt.

Tahiti jet-blast scenario
 © Bureau d'Enquetes et d'Analyses

Such forces could have overstressed the elevator cables. The scenario has precedent: in August 2005 an Air France Regional Saab 2000's left aileron control-rod buckled after exposure to jet-blast. After it took off from Marseille, the crew had to perform an emergency landing when the aircraft became difficult to control.

Wear and damage to the Air Moorea Twin Otter's elevator cable eventually rendered it too weak to handle load forces after the lone pilot retracted the flaps after take-off.

Air Moorea Twin Otter fin recovery
 © Bureau d'Enquetes et d'Analyses

Six new safety recommendations are included in the final report, including a ban on stainless-steel cables on the type. The BEA also says authorities should raise awareness of the potential blast risk.

It states that the operator omitted special inspections of the aircraft, that crews lacked information and training on loss of pitch control, and that the manufacturer and aviation authorities failed to take full account of wear phenomena.

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