Accident Overview
Shortly after lifting off from a hospital helipad to pick up a patient, the helicopter's engine lost power. The aircraft descended into a parking lot, landed hard, and rolled over, coming to rest on its right side.
Engine Disassembly Findings
Disassembly of the engine revealed that both the N1 (gas producer turbine) and N2 (power turbine) shafts had separated. Coke deposits and rub marks were observed on the outside diameter of the N1 shaft, and coke deposits were present on the inside diameter of the N2 shaft.
Metallurgical examination determined that the coke buildup was consistent with reduced oil flow, which led to rubbing between the shafts. The rubbing generated frictional heat, resulting in softening and subsequent failure of the shafts.
Oil Flow Analysis
An o-ring was found lodged in the main oil passage of the accessory gearbox housing. Oil flow testing and calculations showed that this o-ring reduced the oil flow to the turbine shafting from 0.82 pounds per minute to between 0.00 and 0.10 pounds per minute.
Maintenance Records
Maintenance records indicated that the engine had accumulated 531 hours since a 3,500-hour gear train inspection. That inspection included disassembly, cleaning, and reassembly of all accessory gearbox components.