History of Flight
On January 17, 2014, at about 2040 eastern standard time, a Eurocopter AS350B3, registration N578AE, operated by U.S. Customs and Border Protection (CBP), experienced an engine compartment explosion during a night training flight at Houlton International Airport (HUL), Houlton, Maine. The two commercial pilots were not injured. Night visual meteorological conditions prevailed. The helicopter was operating under a company flight plan for local public use.
According to the copilot's written statement, the helicopter had just returned to HUL after a search and rescue mission, with the copilot flying. The crew then began takeoff and landing practice for night flight and night vision goggle (NVG) recurrency training. The copilot stated that after completing a single approach and three landings to runway 23, he took off again, hovered, and began transitioning to forward flight. As he applied cyclic, he heard and felt a loud explosion from the rear of the helicopter, followed by severe vertical vibration and very loud engine noise. The helicopter experienced cyclic controllability problems and yaw instability. The pilot in command (PIC) called out the rotor rpm warning horn. Helicopter control decayed rapidly, and the PIC took control after 5 to 10 seconds.
The copilot made two mayday calls via the footswitch, and the helicopter began severe pitch and roll oscillations. During some oscillations, the left side door opened but was closed again. The copilot noticed the red "GOV" light (manual mode engaged, loss of automatic governing) illuminated. After 10 to 20 seconds, the PIC regained some control, engine and rotor noise decreased, and he landed the helicopter beyond snow banks at the end of the runway. After emergency shutdown, the PIC saw an orange glow reflected in the snow, though the FIRE light was not lit. The copilot saw flames from the engine compartment and attempted to extinguish them with a portable fire extinguisher without success. The local fire department arrived about 10 minutes later and extinguished the flames.
The PIC reported that after the explosion, the main rotor (NR) overspeed warning sounded and vertical vibration developed. He noted that tail rotor thrust seemed present but yaw oscillations soon began. He observed two amber caution lights and what he believed were two red warning lights. Severe vertical vibrations and uncontrollable yaw oscillations continued. The PIC focused on keeping the helicopter level and avoiding ground contact. He could not ascertain airspeed; the helicopter entered 30- to 40-degree banks on three occasions. After about 30 seconds, attitude excursions calmed, and he landed beyond the snow bank. The PIC later identified the lights as an amber ENG CHIP light and red GOV light, with amber FUEL P and DOOR lights. He stated NVGs did not hamper his vision.
Aircraft Information
The 2003 Eurocopter AS350B3 was powered by a single Turbomeca Arriel 2B engine driving a three-blade main rotor and conventional tail rotor. It had skid landing gear, dual controls, and NVG-compatible cockpit lighting. Engine speed was governed by a Digital Engine Control Unit (DECU), a single-channel fuel governor. The collective twist grip had been modified with an automatic release allowing manual fuel flow when the GOV light illuminated. No flight recorders were installed; the Vehicle and Engine Multifunction Display (VEMD) and DECU stored maintenance parameters.
Meteorological Information
HUL weather recorded at 2053: clear skies, wind from 200 degrees true at 5 knots, visibility 10 statute miles, temperature 1°C, dew point 0°C, altimeter 29.83 inHg.
Wreckage and Impact Information
The helicopter came to rest about 250 feet beyond the end of runway 23. Engine cowling pieces not consumed by fire were removed. Most structure was undamaged; landing forces were normal. No rotor blade strikes occurred. Main and tail rotor drive continuity was confirmed. The emergency fuel handle and rotor brake handle were pulled; emergency power switch up; twist grip in MIN. Right side belly panel latches were undone except one.
Fire damage was mostly confined to the engine bathtub area, with thermal structural damage below. The engine's sand filter was missing. Free turbine blades were separated from the disk, consistent with blade shedding. Foreign object debris (FOD) damage was found on inlet compressor blades. The intake funnel, adjusted valve assembly, bleed valve, and exciter box were melted. The engine was examined at Turbomeca USA with FAA and NTSB oversight. Debris was found inside but its origin (ingestion or firefighting) could not be determined.
Tests and Research
The VEMD recorded 16 failures on the last flight (Flight 2791, duration ~1 hour 10 minutes). Key over-limits included T4 MED and HI exceedances, NG overspeed, NF over-limit (maximum 510 rpm), and NR over-limits. Test Reference 129 indicated a stepper motor or resolver failure, triggering the red GOV light. Test References 131 and 133 indicated NF sensor failures at >141% NF. The DECU was operational; out of 32 fault slots, 4 blocks matched the VEMD data, including a stepper motor failure. However, the stepper motor failure could not be duplicated in tests. The Hydro-Mechanical Unit (HMU) showed thermal damage but passed electrical and functional checks; components were in very good condition. The electrical harness had severe fire damage but no faults were found. The twist grip rod was out of neutral position, consistent with manual fuel control.
Material analysis found impact deposits of nickel-base alloy on axial compressor blades, and debris consistent with charred engine cowling (including antimony, a fire retardant) throughout the engine. Deposits on turbine blades showed high aluminum and oxygen, consistent with oxidized sand filter or inlet duct remnants.
Despite extensive investigation, the exact cause of the engine compartment explosion and subsequent fire could not be conclusively determined based on available evidence.