ERA14TA096 · Eurocopter AS350B3

After about 1 hour of flight, the helicopter crew began conducting landing refresher training with the use of night vision goggles. The copilot completed a single approach and three landings to the 5,015-ft runway, with one landing at the beginning, one near the middle, and one near the end. The copilot then lifted the helicopter off the ground once more, hovered, and began a transition to forward flight with forward cyclic and a slight amount of increased collective. Both crewmembers then heard and felt a loud explosion from the vicinity of the engine compartment. Engine sound increased rapidly, and the pilot mentioned a possible low rotor speed condition. He believed that the copilot then lowered the collective slightly in response to his statement, and, while doing so, the noise increased and helicopter oscillations began. The helicopter became almost uncontrollable, the pilot took the controls (which the copilot positively relinquished), and the copilot made two "mayday" calls. The pilot regained sufficient control to land the helicopter beyond the departure end of the runway. As the helicopter cleared the runway, the pilot observed an orange glow on the snow-covered ground, and, after landing, the copilot attempted to put out the engine compartment fire with a small onboard extinguisher, without success. The local fire department arrived about 10 minutes later and extinguished the fire, but much of the engine compartment had been consumed, and the helicopter structure below the engine compartment had been thermally compromised and substantially damaged.

The were no cockpit image or flight data recorders onboard the helicopter; however, there was a vehicle and engine multifunction display (VEMD) unit. As recorded by the VEMD and confirmed by the pilots, about the time of the explosion, a red "GOV" light illuminated in the cockpit, which would have indicated a "stepper motor or resolver failure" in the fuel control.

Subsequent examinations of the engine, components, and wiring did not reveal the source of the explosion, but did note charring and evidence of high heat. The engine's free turbine blades were also found shed from the turbine disk at a manufactured overspeed notch in the blade roots as designed to prevent a subsequent turbine disk rupture. Of the fuel control items that could be examined, no preexisting anomalies could be found. However, their proximity to extreme heat could have triggered the red GOV light and the loss of automatic fuel metering.

The helicopter's collective twist grip mechanical stop had been upgraded with one that automatically freed the grip once the red GOV light illuminated. The helicopter did not have an automatic fuel control backup system; thus, fuel flow and, therefore, engine output had to be manually controlled by the pilot once the red GOV light illuminated.

Although not the cause of the accident, the crew's inability to recognize that they were operating in a manual fuel control regime likely exacerbated the situation. Changes in collective position would have resulted in changes of engine and rotor rpm, and the inflight excursions and later engine overspeed upon landing were the likely result of collective movement without fuel control compensation.

Further examination of the engine also revealed impact deposits on the axial compressor blades and gas generator turbine blades. The material was likely from the engine's sand filter and cowling, indicating that the engine compartment fire was ongoing while the engine was still rotating, with a flame established in the combustion chamber. The fact that the fire continued after engine shutdown until the fire department arrived and put it out indicates that, regardless of the crew's actions, the structural damage to the helicopter still would have occurred.

Verbatim NTSB analysis and probable cause from the NTSB dataset