Newly-released details about August’s fatal Air Moorea de Havilland Canada Twin Otter accident in French Polynesia show the aircraft suffered a sudden and rapid loss of control, causing it to crash into the sea just 11s later.
A fractured elevator cable discovered in the wreckage remains a focus of attention for French investigators but the inquiry, which has just completed its initial phase, has not yet formally linked the damage to the 9 August accident.
Elevator pitch-up and pitch-down in the Twin Otter are achieved using two cables, each about 12m (39ft) long, running along the right-hand side of the fuselage from the pilot’s control column.
Investigators from Bureau d’Enquetes et d’Analyses (BEA) found that both of these cables, as well as the two rudder cables, were severed at the forward section of the aircraft, and that all the damage was consistent with tensile overload.
But crucially they also discovered that the pitch-up elevator cable had fractured towards the rear and that this damage had a “different appearance”. BEA says the elevator cables showed evidence of localised wear and broken filaments from contact with structures in the rear fuselage.
Recovery personnel retrieved the entire pitch-down cable from the sunken wreckage. But an 8.8m (29ft) section of the pitch-up cable, between its forward and rear fracture points, was missing.
Investigators have already recommended inspection of Twin Otter control cables “without any relation” having yet been established between the cable failure and the accident.
BEA says the aircraft departed from Moorea Airport’s runway 12 at just after 22:00 for the short flight to Papeete, which typically took seven minutes at a height of 600ft.
For the first few seconds of flight the aircraft climbed normally. Sound analysis indicates that flaps were retracted during this period and then the propeller speed was reduced from 2,100rpm to 1,900rpm, in accordance with company procedures. The aircraft had reached a height of around 300-400ft at this point.
Two seconds after reducing the propeller speed, the lone pilot suddenly exclaimed surprise. Three seconds later, the enhanced ground-proximity warning system sounded, indicating the aircraft was descending at around 1,000ft/min. A second proximity warning sounded and the pilot increased the propeller speed to 2,100rpm again, but the warnings continued.
Just 68s after commencing its take-off run, and 11s after the pilot’s initial expression, the aircraft struck the water killing all 20 on board. There was no distress call. BEA says the aircraft was within weight and balance limits and the two Pratt & Whitney Canada PT6A engines functioned normally during the event.
Flight-surface control cables normally have a life-span of five years but the investigators say this is limited to one year in regions with a high salt concentration in the air.
While still in operation with its previous owner, a US company, the aircraft had its cables replaced in March 2005. As part of the transfer to Air Moorea last year the aircraft also underwent maintenance in Canada where its cables were removed, inspected and reassembled, with the exception of the aileron cables which were changed.
Source: FlightGlobal.com
