N208MPMcdonnell Douglas Helicopter 600N · 1998

On July 21, 2020, at 0558 Pacific daylight time, a McDonnell Douglas 600N helicopter, N208MP, was substantially damaged when it was involved in an accident near Thompson Falls, Montana. The pilot was not injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 133 rotorcraft external load operation.

The pilot stated that, as he was on an approach to the landing zone, he lowered the collective to start a vertical descent to land. At that time, the engine and rotor speeds suddenly decreased, the helicopter yawed to the left and started to settle. As the helicopter was approaching the ground fast, the pilot increased the collective to use the remaining engine power to cushion the landing. The helicopter impacted the ground, and the pilot rolled the throttle to idle, but noticed that engine speed and rotor rpm gauges were below the normal idle parameters. When he rolled the throttle to off, he heard a grinding "metal-to-metal" noise, and the rotor blades stopped spinning almost immediately.

A postaccident examination revealed the N1 system turned smoothly and was continuous from the compressor to the starter/generator and first-stage turbine wheel. The N2 system turned smoothly and was connected to the aircraft power train. Foreign object damage (FOD) was not observed on the compressor impeller blades or compressor inlet and the inlet barrier filter system was intact and free of debris. The fourth-stage turbine wheel was normal in appearance when viewed from the exhaust collector. The first-stage turbine nozzle and blades were viewed via borescope with no abnormalities observed. The engine mounted fuel filter bowl retained about 1/3 bowl of fuel, which was normal in smell and appearance. The fuel filter element presented no obvious contamination. All the fuel lines from the airframe to the engine were flow-checked with no obstructions noted.

The engine electronic control unit (ECU), which retains data in nonvolatile memory, was downloaded during the wreckage examination. The ECU includes an incident recorder function, which begins capturing engine and input parameters upon trigger actuation and records a line of data (“record”) every 1.2 seconds. The initial incident recorder triggering event was “low rotor RPM” (Nr Droop), which fell below the 92% threshold. Then, within 3 seconds, the incident recorder captured the “Flameout” and “Ng Low” triggers. According to the data, the FADEC system attempted to re-light the engine for the next 53 seconds until the throttle was rolled to cut-off; however, the engine did not restart.

The engine was transported to an inspection facility and installed on a calibrated test stand. After the engine was warmed up for about 5 minutes, the power was gradually increased to the takeoff setting, where the observed power was 680 shaft horsepower (shp). No unusual vibration was noted. Several accelerations and decelerations were achieved followed by testing of the anti-ice system and bleed valve, all of which were within specification. A full power calibration protocol was achieved resulting in a maximum predicted take-off rating of 664 shp at standard day conditions, with no anomalies noted.

The fuel nozzle was removed and inspected. The inlet screen was normal in appearance and not obstructed. The fuel hose was installed in a test bench and 600 pounds per hour of fuel was flowed through the hose to check for leaks. No leaks were observed.