Safety Topics: Severing of Drive Train

THIS accident concerns a Sikorsky S‐61N helicopter which took off after a 25 minute period of ground running with rotors turning. Around five minutes into the flight, as transmission torque was increased to 80% to initiate a cruise/ climb from 250–300 ft agl, the crew heard a whine. This completely disappeared on lowering the collective to 60% torque, but when torque was again increased to 75–80%, the whine reappeared and rapidly became worse. The crew identified the noise as coming from the left side of the transmission and selected the main gearbox (MGB) emergency lubrication on. Around one minute after the crew first heard the whine there was a loud bang, power was lost and the aircraft yawed. After the bang, the No 1 Engine shut down automatically. The crew established the aircraft in autorotation and achieved a gentle landing on sloping ground five nm east of the airfield, using No 2 Engine power to cushion the touchdown. With the exception of holing of the tail rotor drive shaft fairing as a result of contact from a main rotor blade, no damage resulted from the landing. After landing, oil was seen pouring down the left side of the aircraft internally and externally.