Airbus AS 350 BA FX1 N6094H accident near Wailuku, Hawaii

Probable cause

The failure of the engine’s pneumatic fuel control line fitting, resulting in a loss of engine power. Contributing to the failure was the repeated loosening and re-torqueing of the fitting during required maintenance and inspection.

NTSB analysis

The commercial pilot of the helicopter was conducting the third local tour flight of the day when, about 10 minutes after departure, the main rotor speed (rpm) started to decrease and the low rotor warning alarm sounded. The pilot lowered the collective pitch control to increase rotor rpm, and the helicopter started to descend. He then raised the collective pitch control, and the main rotor rpm started to decrease again. The pilot entered an autorotation, and the helicopter landed hard in tall grass, resulting in substantial damage to the fuselage and tailboom.

A test run of the engine revealed that, when power was applied, the engine would not achieve a speed above about 78% Ng (engine gas generator speed). Fuel flow indications during the test run were between 125 and 140 pounds per hour (pph); the required value was 300 pph. A leak test of the pneumatic controls revealed that the fitting for the input line to the pneumatic pressure (Pc) filter housing was fractured. A second test run of the engine, following replacement of the Pc filter assembly, was successful. Thus, the fractured fitting rendered the control system incapable of modulating Pc pressure to the fuel control, which resulted in a loss of engine power.

Further examination of the fitting revealed fracture features consistent with fatigue and overload. The Pc filter requires continuous recurring inspections. The filter housing is hard-mounted to the side of the engine plenum and connected to the system via hard pneumatic lines with torqued fittings. Each time that the filter is inspected, the filter housing fittings are subjected to mechanical loosening and re-torqueing. It is likely that the repeated loosening and re-torqueing of the fitting during required maintenance and inspection contributed to the fitting's failure. Following the accident, the manufacturer issued a service bulletin regarding the service and inspection of Pc filter fittings.

Text from the NTSB. Open the NTSB docket for WPR14LA251 for the full record.