Robinson R44 N225JM

The pilot rented the helicopter to make a local personal flight. Radar data indicated that, after maneuvering around the area of a nearby national conservation, the helicopter flew along a road back toward the airport. The helicopter continued at an altitude between 500 to 700 ft above ground level (agl) for about 30 seconds then radar data ended 1 nautical mile from the accident site. A witness observed the helicopter impact the ravine adjacent to the road and break apart on impact. Ground scar analysis and wreckage fragmentation revealed that the helicopter collided with terrain in a tail-low attitude, consistent with the pilot performing an autorotation before impact. Postaccident examination revealed evidence that the engine was running at the time of impact. The white exhaust signatures were consistent with lean operation. Examination of the engine compartment revealed that one magnet in the main rotor tachometer indicating system was separated from its housing from the yoke assembly; the other magnet assembly remained secured to the yoke assembly. The yoke assembly was not damaged, but there was evidence of slight damage to both senders, consistent with the magnet contacting the senders. With only one magnet installed, the main rotor tachometer rpm would indicate about 50% of the actual rotor rpm and the low rotor rpm horn would sound. It could not be determined if the magnet came free from the housing prior to impact. Examination of the magnet assembly revealed signatures consistent with compliance with a manufacturer service bulletin requiring the use of adhesive to secure the magnets. The helicopter was refueled the morning of the accident, and the fuel was sumped by a pilot receiving instruction and a flight instructor. The fuel samples from the main fuel tank were dirty and they opted to cancel the flight and have a mechanic flush the tanks. The mechanic never flushed the tanks, but the flight instructor reportedly took more samples later in the day until they were clean. When the accident pilot arrived before the flight, he was told that the helicopter had completed maintenance and was informed that an earlier flight was canceled because a pilot had found sediment in the fuel tanks. The accident pilot’s preflight actions could not be determined, but it is likely he would have sumped the tanks and found them to be clean, given that he was aware of an issue earlier in the day. Investigators took samples of the liquid in the gascolator while at the accident site. The color was an orange-yellow and there was some debris in the bowl that displayed a gelatinous consistency; the screen was clear. The remainder of the fuel found in the system was free of contamination. An analysis of the fuel was consistent with that of normal aviation fuel with the presence of iron and brass consistent with that of corrosion. A further examination of the remaining fuel showed the presence of fillers in a polymeric material. The origin of the material found is unknown and the effect, if any, on the helicopter performance could not be determined. The reason the pilot conducted an auto-rotation could not be determined. It is feasible that if the magnet from the rotor tachometer separated inflight, the pilot would hear the magnet contact the airframe, have the low-rotor warning horn sound, the main rotor tachometer rpm would display 50%, and in response he would perform an autorotation. This scenario could not be determined because of the damage incurred to the airframe during the accident sequence. Because there was evidence that the engine was running at the time of impact, it could also not be determined if the fuel contributed to an inflight event that resulted in the pilot’s decision to make an autorotation.

Quoted verbatim from the NTSB record.