The helicopter was engaged on a post-maintenance test-flight following the fitment of a newly overhauled main rotor gearbox and combining gearbox. Eyewitnesses heard unusual noises coming from the helicopter before the tail boom apparently folded forward around the cabin. The helicopter then fell to the ground, catching fire on impact. All three occupants received fatal injuries. Examination showed that the two gearboxes and the main rotor had detached before impact. Subsequent investigation showed that the left freewheel showed clear evidence of slippage under load; the right freewheel also showed signs of slippage but not to the same extent. It is concluded that a series of freewheel slippages followed by aggressive re-engagements led to the structural failure. The reasons for the slippage however, cannot be proven conclusively. Although it was found that the rollers forming part of the freewheel mechanism had come from a manufactured batch that had been coated using an incorrect process, no laboratory testing could reproduce any greater tendency for such a coating to cause slippage. The helicopter manufacturer recorded five incidents of slippage under load, coinciding with the introduction of rollers from this batch. Satisfactory performance of the freewheels resumed following the removal from service of the incorrectly coated batch of rollers.
AS355F1, G-XCEL 07-06.pdf (888.58 kb)
Published 10 December 2014