Robinson Helicopter Company R44 II N61NM accident near El Dorado Hills, California

Probable cause

Maintenance personnel's inadequate routine maintenance on the helicopter engine, which led to low cylinder compression, improper magneto timing, and camshaft lobe wear, which subsequently resulted in degraded engine performance.

NTSB analysis

The commercial pilot and passenger departed in the helicopter on a personal flight to a nearby lake, but decided to change course during the flight to meet a friend at his home. They approached from the north with the intention of landing in the backyard. After entering an out of ground effect hover, the pilot received a low rotor rpm indication and the helicopter began to descend. The pilot was able to restore the rotor rpm to normal momentarily, but after completing a 360° right pedal turn to search for a suitable landing site, the helicopter started to sink again. The helicopter's right skid then impacted the ground, which resulted in a dynamic rollover and substantial damage to the fuselage. The property owner stated that he saw intermittent puffs of dark gray smoke coming from the tailpipe that turned into a steady stream after the helicopter completed the pedal turn.

According to maintenance logs, verification of engine-to-magneto timing, a differential compression check, and confirmation of valve movement using a relevant service bulletin were completed during the most recent 100-hour inspection (about 104 flight hours before the accident); however, postaccident examination found that the magnetos were improperly timed by 10o, cylinder compression was low, and a camshaft lobe was severely worn and spalled, which would have inhibited two intake valves from operating normally during the accident flight and resulted in a reduction in the effectiveness of two cylinders. These anomalies resulted in a reduction in the engine's available power, and the absence of this power precluded the helicopter from maintaining an out of ground effect hover. The presence of dark smoke from an operating engine can be the result of an excessively rich mixture; however, the origin of the dark gray smoke in this case could not be determined.

A flight instructor who regularly flew the helicopter stated that, despite his repeated reports of degraded engine performance, one of the multiple mechanics responsible for servicing the helicopter justified the performance issues as the result of age, as the engine was nearing its next major overhaul. This likely encouraged the instructors who flew the helicopter to dismiss the problem as well and to "manage" the lack of power instead. Additionally, the mechanic responsible for the helicopter's most recent inspection stated that he did not comply with the manufacturer's service bulletin to inspect the valves for adequate movement, which likely would have prompted an inspection of the camshaft and discovery of the lobe wear.

Text from the NTSB. Open the NTSB docket for WPR17LA188 for the full record.