Ramon Lopez/WASHINGTON DC
Mechanical or software failures have been ruled out as causes of the fatal crash on 8 April of a Bell Boeing MV-22B Osprey tiltrotor. As a result, the US Marine Corps has cleared the aircraft to resume flying.
The USMC's eight Ospreys were grounded after an MV-22B in a 200ft (60m) hover pitched nosedown, yawed to the right and hit the ground, killing 19 on board.
Lt Gen Fred McCorkle, USMC deputy chief of staff, aviation, says four development aircraft will resume flying immediately, with the four remaining low-rate initial production MV-22Bs resuming operational evaluation within two weeks.
McCorkle says part of the investigation is on crew performance in the seconds before aerodynamic lift was lost, causing the dive. He says the aircraft descended too rapidly, in excess of1,000ft/min (5.08m/s). The maximum recommended rate is 800ft/min at speeds below 40kt (74km/h). The Osprey had slowed to 37kt.
McCorkle believes the MV-22 suffered "power settling" or a "vortex ring state" - in which it is difficult to stop a descent. This occurs when the rotor downwash velocity is about equal to the rate of the descent, which causes air to recirculate back through the rotors. Recovery may be possible by reducing power and pushing the nose down, but the MV-22 was too close to the ground.
Source: Flight International
