Incident
Approximately five minutes before landing at a hospital, the main rotor gearbox (MGB) oil pressure warning light illuminated in a helicopter. The pilot continued to the hospital, landed, and performed an immediate engine shutdown. A mechanic then disconnected the wiring to the MGB oil pressure switch, which extinguished the warning light. Believing the switch had failed, the mechanic asked the pilot to run the helicopter, perform a hover, and if everything appeared normal, to fly it back to the base hospital. The pilot conducted the run and hover, then departed from the hospital. About one minute later, the helicopter crashed.
Witness Observations
Witnesses reported hearing the helicopter approach the crash site at a low altitude, accompanied by a slow thumping noise.
Examination Findings
Examination of the MGB revealed that the oil pump idler gear had seized in the oil pump due to undetermined reasons, and the oil pump drive shaft had failed due to overstress. The teeth on the engine input gears, intermediate gears, and MGB drive gear in the MGB combining gearbox had failed due to high-temperature overstress, which was the result of oil starvation. The helicopter was not equipped with a MGB oil pressure indicator.
Maintenance Context
The maintenance procedure for troubleshooting an illuminated MGB oil pressure warning light requires first checking the electrical circuit; if that does not correct the problem, the oil pressure switch should be changed. The mechanic stated he did not have the maintenance manuals with him while working on the helicopter. The MGB had been installed in the helicopter after overhaul, three days and four flight hours before the accident.
Cause
Oil starvation due to a seized oil pump idler gear and failed drive shaft led to high-temperature overstress and gear failure, resulting in the crash.