WPR20LA189 · Aerospatiale SA342J 1708

On the day of the accident, the pilot made a series of personal flights that totaled about 1 hour 15 minutes of flying time. During the last flight, he noticed that the engine was operating about 50° higher than it normally did. Immediately after the pilot lowered the collective to descend for a landing, the engine lost total power. When the helicopter was about 25 ft above ground level and at a speed of 10 to 15 knots, the pilot entered an autorotation to land. The combination of altitude and airspeed did not allow the pilot to establish a stabilized autorotation, and the helicopter landed hard, which resulted in a postaccident fire that consumed most of the helicopter.

Postaccident examination of the engine and transmission revealed no mechanical malfunctions or anomalies that would have precluded normal operation. The airframe was thermally destroyed and could not be thoroughly examined, and the engine did not undergo a full tear down. A witness described hearing “three or four pops” and a loud noise, indicating that the engine had surged before it lost total power.

The pilot purchased the helicopter in 2009. A mechanic maintained the helicopter in accordance with an annual maintenance schedule that was similar to the manufacturer’s periodic maintenance schedule and addressed all the manufacturer’s maintenance requirements, including criteria for engine cleaning. The mechanic performed an engine compressor wash and rinse once a year as part of the annual inspection and not according to the number of hours that the helicopter had been flown. He used a non-approved cleaner and non-de-mineralized drinking water and did not run the engine after rinsing it. According to the manufacturer both the infrequent engine cleaning and the use of non-demineralized water could result in higher exhaust temperatures. The last engine wash occurred in July 1019.

According to the maintenance logbook, the pilot had accrued about 167.8 flight hours in the helicopter from 2011 to 2019. This amount of time throughout the 9-year timeframe likely resulted in multiple downtime periods that exceeded 72 hours and would have required an engine cleaning. The helicopter manufacturer published a service letter about engine condition follow-up checks for aging engines, which included the accident engine. According to the mechanic, the owner had performed at least two of the engine aging follow-up checks in the service letter, but there was no documentation of the results of the checks. The absence of these surveillance data precluded a determination of the engine’s condition before the accident. The manufacturer provided other causes for engine performance degradation including erosion of the air ducts, ingestion of foreign objects, vibrations with rubs leading to an abnormal increase of functional clearances, and unbalance caused by the accumulation of foreign objects. None of these examples were cited in the engine examination report. The cause of the engine surge could not be determined with the available evidence from this investigation.

Verbatim NTSB analysis and probable cause from the NTSB dataset