Accident Details
On July 21, 2020, at 0558 Pacific daylight time, a McDonnell Douglas 600N helicopter, registration N208MP, sustained substantial damage in an accident near Thompson Falls, Montana. The pilot, who was the sole occupant, reported no injuries. The aircraft was operating under Title 14 Code of Federal Regulations Part 133 as a rotorcraft external load operation.
According to the pilot, while on approach to the landing zone, he lowered the collective to initiate a vertical descent. At that moment, engine and rotor speeds dropped suddenly, the helicopter yawed left and began to settle. As the ground approached rapidly, the pilot increased collective to cushion the landing using remaining engine power. The helicopter impacted the ground, and the pilot rolled the throttle to idle, but observed that engine speed and rotor rpm gauges indicated below normal idle parameters. When he moved the throttle to off, a grinding metal-to-metal noise was heard, and the rotor blades stopped almost immediately.
Post-Accident Examination
A postaccident inspection showed that the N1 system rotated smoothly and was continuous from the compressor to the starter/generator and first-stage turbine wheel. The N2 system also turned smoothly and remained connected to the aircraft power train. No foreign object damage was observed on the compressor impeller blades or inlet, and the inlet barrier filter system was intact and debris-free. The fourth-stage turbine wheel appeared normal when viewed from the exhaust collector. Borescope inspection of the first-stage turbine nozzle and blades revealed no abnormalities. The engine-mounted fuel filter bowl contained about one-third fuel, which appeared and smelled normal; the fuel filter element showed no obvious contamination. All fuel lines from the airframe to the engine were flow-checked and found unobstructed.
The engine electronic control unit (ECU), which retains data in nonvolatile memory, was downloaded. The ECU incident recorder function captures parameters every 1.2 seconds upon trigger actuation. The initial trigger was "low rotor RPM" (Nr Droop), falling below 92%. Within three seconds, the recorder captured "Flameout" and "Ng Low" triggers. Data indicated that the FADEC system attempted to relight the engine for 53 seconds until the throttle was moved to cut-off, but the engine did not restart.
Engine Testing
The engine was transported to an inspection facility and installed on a calibrated test stand. After a warm-up of about five minutes, power was increased to the takeoff setting, where 680 shaft horsepower (shp) was observed. No unusual vibration was noted. Several accelerations and decelerations were performed, followed by testing of the anti-ice system and bleed valve; all results were within specification. A full power calibration protocol yielded a maximum predicted take-off rating of 664 shp at standard day conditions, with no anomalies.
The fuel nozzle was removed and inspected; the inlet screen appeared normal and unobstructed. The fuel hose was tested on a bench with 600 pounds per hour of fuel flow to check for leaks; none were observed.