What happened
On January 17, 2014, a Eurocopter AS350B3 helicopter, registration N578AE, was conducting night flight and night vision goggle (NVG) training at Houlton International Airport (HUL) in Houlton, Maine. The aircraft, operated by U.S. Customs and Border Protection, had recently completed a search and rescue mission.
During a transition to forward flight following several landing practices, the crew heard and felt a loud explosion originating from the rear of the aircraft. This was immediately followed by severe vertical vibrations and an increase in engine noise. The pilot in command took control as the aircraft experienced significant yaw instability and controllability issues. The crew observed a red "GOV" warning light, indicating a failure in the automatic engine governing system.
As the aircraft experienced severe pitch and roll oscillations, the pilot managed to maintain enough control to land the helicopter beyond the end of runway 23, near some snow banks. Upon landing, the crew observed an orange glow in the snow and flames emerging from the engine compartment. Although the crew attempted to use a portable fire extinguisher, the fire continued until the local fire department arrived approximately 10 minutes later to extinguish the flames.
There were no injuries to the two pilots.
The investigation
Investigators examined the engine and various components to determine the cause of the explosion. While the source of the initial explosion could not be identified due to fire damage, several key findings were noted:
- The engine's free turbine blades had shed from the turbine disk at a designed overspeed notch, a feature intended to prevent a turbine disk rupture.
- Foreign object debris (FOD) was present on the inlet compressor blades.
- The engine's sand filter and parts of the intake duct were missing or heavily damaged.
- Impact deposits on the compressor and turbine blades contained elements like antimony, suggesting the engine compartment fire was already active while the engine was still rotating.
- The Vehicle and Engine Multifunction Display (VEMD) recorded several over-limits, including high engine gas generator speed (NG), high free turbine speed (NF), and high rotor speed (NR).
- The VEMD also recorded a "stepper motor or resolver failure," which correlates with the red "GOV" light seen by the crew.
Findings
- The engine compartment explosion caused substantial damage to the engine and the helicopter structure.
- The crew's inability to recognize they were operating in a manual fuel control regime—necessitated by the loss of automatic governing—likely exacerbated the flight oscillations. Changes in the collective position without corresponding fuel flow compensation led to extreme attitude excursions and rotor overspeed.
- The fire was intense enough to cause thermal structural damage to the area below the engine "bathtub."