WPR10FA085 · Eurocopter AS350BA (FX2)

As part of the required normal 6-month competency check for the commercial pilot, a Federal Aviation Administration (FAA) inspector was performing an examination of the pilot's competency in responding to a total loss of engine power event while in cruise flight. To perform this examination, the FAA inspector announced the beginning of the simulated power loss procedure and moved the fuel flow control lever out of the flight detent (the full forward, full open position) and back just enough to keep the lever from springing back into the detent; this was done to ensure that the engine was not supplying power to the rotor system during the autorotation. However, when the fuel flow control lever was moved, the helicopter yawed right and the generator out warning light illuminated, indicating that the engine had flamed out. The pilot briefly attempted a restart, but the engine exceeded the temperature limit and he discontinued the start attempt. Because of their close proximity to the ground, there was inadequate time to attempt another restart of the engine. The pilot-in-command (PIC) identified and proceeded toward a forced landing site. Due to obstructing trees in the touchdown zone below the area where the simulated engine out was conducted, the PIC overflew the trees with up collective input, leading to a decay of the main rotor rpm. Thereafter, insufficient rotor rpm remained to cushion the touchdown. Also, because of down-sloping terrain, the distance between the helicopter and ground level increased seconds before landing, thereby increasing the helicopter's absolute altitude and contributing to a hard impact with the ground.

The fuel control unit and the power turbine governor were removed from the engine and taken to a test facility where they were installed on calibrated test benches and tested in accordance with the manufacturer’s test procedures. The results of the fuel control unit test showed that the internal parts all worked properly; however, the fuel flow at every test point was below the specified minimum limit. The results of the power turbine governor test revealed that the unit was out of specified limits at each test point. The discrepancies noted would affect the fuel flow at the high end of the schedule and is indicative of an improper rigging procedure. Examination of the unit showed that the maximum stop setting had been adjusted in the field and that the travel was set at 80 degrees instead of the required 86 degrees.

While some of the discrepancies found during the tests of the fuel control unit and the power turbine governor are not serious, those at the low end of the fuel schedule are of particular concern. The minimum fuel flow, idle, and the cut-off settings were found to be below the manufacturer’s specified minimum limits; when combined with hysteresis, or the lagging of a physical effect on a body behind its cause, the chances of insufficient fuel flow being delivered to the engine during any engine deceleration maneuver (i.e., moving the throttle out of the flight detent) increases dramatically. When the FAA inspector moved the fuel flow control lever as the pilot was manipulating the collective during the beginning of the autorotation, it is likely that the unloading of the engine sent a signal to the fuel control unit to rapidly decrease the fuel flow at the same time the fuel control lever was being brought out of the flight detent and moved aft, which helped induce the flameout.

Review of the operator’s flight and maintenance records found a pilot write-up that noted that about 9 months prior to the accident the engine had flamed out when the throttle was manipulated during the start sequence. The operator’s maintenance department was unable to find a reason for the event and released the helicopter back to service. The company pilots were aware that this particular helicopter had a “touchy throttle.” The pilot noted that, when operating the throttle lever in this particular helicopter, “you have to be gentle and slow with it as you retard the lever…if you pull it back to far or fast, it will shut off the fuel.” At least four prior instances of flameouts as a result of minor throttle movements were uncovered in deposition testimony of company pilots. All occurred on the ground and three of the flameouts happened as pilots were bringing the fuel flow control lever back toward ground idle during the post-flight engine cool down period. These instances were not documented in the maintenance records, and no records of attempted remediation were found.

Verbatim NTSB analysis and probable cause from the NTSB dataset