An errant washer is being blamed for the emergency landing of a Bristow Sikorsky S-92A. The Norwegian Safety Investigation Authority (NSIA) has concluded that the washer was probably introduced into the aircraft’s main gearbox (MGB) during maintenance and eventually became lodged standing on edge in the oil passageway, reducing the flow of oil in the return oil channel.
According to the NSIA, the washer likely had been present for nearly 500 flight hours before the event occurred on Sept. 25, 2020, triggering cockpit warnings for MGB loss of oil pressure and rising oil temperature. The crew declared an emergency but continued the flight for another 25 to 30 nm to the airport at Sola, the intended destination. Upon landing, visual evidence revealed a significant oil leak on the left side of the MGB.
NSIA believes that the washer entered the MGB when the generator was removed from the flange on the accessory module—a procedure done 487.35 flight hours before the incident. Post-incident investigation located the washer in the oil passageway after the input module had been disassembled. The safety agency recommended that the Norwegian Civil Aviation Authority follow up on the operator’s procedures and routines related to foreign object debris in connection to gearbox maintenance.