Jet blast damaged controls of Air Moorea Twin Otter

 - December 30, 2008, 5:43 AM

The Air Moorea DHC-6-300 Twin Otter turboprop that crashed near Moorea Island on Aug. 9, 2007, took off with an almost failed pitch-control cable, due to recent exposure to jet blast, according to French Bureau d’Enquêtes et d’Analyses (BEA) chief Paul-Louis Arslanian. Other causes include the omission of cable inspections and the use of stainless-steel cables, which are more prone to wear. The crash killed the pilot and all 19 passengers shortly after takeoff for a seven-minute flight between Moorea and Tahiti, French Polynesia.

The cable failed immediately after the pilot commanded flaps up and exerted force on the control wheel to offset the pitch associated with the flaps’ retraction. Investigators determined that the pilot applied a force close to 50 daN (110 pounds of force). The new cable had a resistance of 782.5 daN or 1,760 pounds of force, so investigators concluded that the cable was weakened before takeoff.

They found significant wear, due to normal rubbing on the cable. Of the 133 wires the cable was made of, 72 were either broken or worn at more than 90 percent. However, this was not enough to explain the in-flight failure. Moreover, investigators found no evidence of fatigue.

To break several strands, including the central one, and leave just a few wires untouched required a force of approximately 500 daN (1,100 pounds of force). Such force cannot be exerted while the controls are free in movement, as stops prevent it. But it can be exerted while the controls are locked. At night, Moorea’s Twin Otters are parked on Tahiti Faa’a airport with the control wheel locked in the pitch-down position.

Because there was no evidence of shock on the horizontal empennage and winds recorded during the months before the fatal event were far short of certification levels, investigators considered the possibility of jet blast.

The layout of the airport makes it possible for an Airbus A340 beginning to taxi to blast a parked DHC-6. According to the BEA, a 15-degree deviation from the A340’s normal position subjects the Twin Otter’s empennage to jet blast, causing a force between 0.2 and 2.8 times the limit load on the pitch-up cable. According to investigators, one exposure was enough to break most wires.

Contributing to the accident was the cable’s excessive wear. The BEA noted that replacement intervals are calendar-driven, rather than based on flight cycles. The accident aircraft had logged 5,150 cycles in only 841 hours since it entered service with Air Moorea. In addition, the operator had not performed the special inspections for operations in a maritime environment. Also, the cable was made of stainless steel, which while corrosion-proof is prone to wear. The BEA thus recommends the use of carbon steel.

Another contributing factor was the lack of pilot training for loss-of-pitch-control recovery. Such recovery is possible using trim, but the BEA acknowledges that the pilot, trained or not, had only a few seconds to react. Eleven seconds elapsed between the pilot’s exclamation and the airplane’s hitting the water.