CEN11FA118 · Eurocopter AS350-B2

The emergency medical services flight crew was departing from their home base helipad on the fourth flight of the day. The pilot reported that he began to lift straight up in a slow climb to about a 100- to 125-foot-high hover. As he completed a left pedal turn to depart into the wind, he confirmed that he was clear vertically of a tree that was directly east of the helipad. As he began a slow transition to forward flight, he heard a loud "bang." The pilot immediately reacted to the sound by slightly lowering the collective and turning to the right to get back to the helipad. He lowered the nose briefly and then began to flare the helicopter as he pulled up on the collective. The helicopter hit hard on the helipad, which spread the helicopter's landing skids so that the helicopter was lying on its belly, and the fuel tank ruptured during the impact. There was no postimpact fire. The pilot turned off the main electrical push button and began to yell "get out" to the flight crew. The pilot exited the helicopter unassisted and crawled about 15 to 20 feet away; medical crewmembers on board were assisted out of the helicopter by first responders.

Postaccident examination revealed that damage to the helicopter was consistent with a high vertical velocity impact. Examination of the main rotor and tail rotor drive systems revealed evidence of low rpm on impact. Recorded nonvolatile data indicated that the engine experienced a flameout before ground impact. During the on-site examination, all fuel, oil, and air lines and connections were found to be tight. Additionally, the fuel flow lever was found in the flight detent, and the fuel shutoff lever was found lock-wired in the open position. There was no indication that the fuel filter annunciator light had illuminated during the accident flight, which would indicate that the fuel filter was clogged or that the filter bypass had opened. The on-site examination revealed that the fuel filter cartridge exhibited no anomalies, and the bypass open indicator was not extended. The bypass open indicator operated normally during subsequent tests.

The engine was run on a test cell and operated within the original equipment manufacturer's specifications for all tests performed. Tests performed on the fuel control unit, starting drain valve, overspeed drain valve, pressurizing valve, bleed valve, start electrovalve, and the fuel injection manifold, as well as a three-dimensional check of the axial compressor, revealed no malfunctions or failures that would have precluded normal operation.

The fuel was examined and no contamination was present. There was no evidence of ingestion of water, snow, or ice that would lead to an engine flameout. During the accident flight, the weather conditions required for ice buildup in the air intake filter were not present, and the temperatures in the intake plenum and air filter before engine start were not conducive to ice formation.

Although examination of the helicopter's fuel circuit revealed that it operated correctly, during testing air was drawn into the fuel circuit between the fuel filter assembly and the fuel pumps when the fuel filter assembly drain valve was depressed. Numerous tests were conducted to determine if the air in the fuel lines and the fuel filter assembly could be purged during the engine start sequence and/or by normal engine operation. The tests revealed that some air would still remain trapped in the upper cavity of the fuel filter assembly (the volume of the cavity measured 33 milliliters). The additional tests, which simulated the fuel flow rates at flight idle, maximum takeoff power, and maximum engine generator speed, revealed that the volume of trapped air was about 1 to 2 cubic centimeters. An exemplar engine was subjected to 16 tests on a test cell to determine the effects on the engine if the trapped air was released through the fuel bypass valve located on top of the fuel filter assembly. The engine did not flame out during any of the tests, although one of the tests using a pre-clogged cartridge resulted in a fuel flow interruption lasting 5 milliseconds, a transient disturbance of the engine parameters, and a transient slower rate of power increase for about 5 seconds. The tests did not account for variables such as a "cold soaked" aircraft, cold fuel, air flow disturbances due to rotor wash, the effects of air filters certified under supplemental type certificates and the possible degraded condition of those air filters, no use of compressor outlet pressure bleed air, and no electrical load on the engine.

Analysis of the faults that resulted in the uncommanded in-flight shutdown of the engine indicated that the most likely reason for the flameout was an interruption of fuel flow to the engine. Although the investigation found that air could be introduced into the fuel system through the fuel filter assembly, the mechanism that would allow air to disrupt the fuel flow to the engine and result in an engine flameout could not be determined.

Verbatim NTSB analysis and probable cause from the NTSB dataset