Aerospatiale SA315B N57948 accident near Hildale, Utah

Probable cause

Maintenance personnel’s failure to properly secure the EGT thermocouples, which resulted in catastrophic overtemperature of the engine and a total loss of engine power.

NTSB analysis

The pilot was conducting a local flight with passengers when, shortly after takeoff, the engine “felt sluggish” and lost power. He subsequently performed an autorotation to a road. The helicopter landed hard; the main rotor blades impacted the tail boom and the right skid splayed outward. A witness-provided video of the flight showed flames emanating from the exhaust tailpipe within a few seconds of the helicopter taking off and continuing until the helicopter impacted the ground.

The engine uses two thermocouples to provide a signal to the Exhaust Gas Temperature (EGT) gauge in the cockpit. During a postaccident examination of the engine, the lower thermocouple was found to be hanging loose from its boss on the exhaust section and not exposed to the engine exhaust. The upper thermocouple was found tight but not safety wired. The upper probe was found with thermal damage that melted the probe wire in the gas path. The lower probe did not exhibit any damage. Severe thermal damage was observed on all three stages of turbines and the 2nd and 3rd nozzle guide vanes. No additional pre-impact anomalies were noted with the engine or airframe.

The engine turbine section had been replaced 3.4 flight hours before the accident. The replacement would have required the removal and reinstallation of the engine exhaust section and both thermocouples. A periodic inspection of the engine was also completed at that time. The mechanic who performed the turbine replacement and periodic inspection stated he checked the installation of the thermocouples as part of his post-maintenance procedures. Postflight maintenance checks and pilot preflight checks specified checks of the engine, including the exhaust section where the thermocouples are located. According to the pilot, the EGT during startup was at least 100° cooler than before the turbine replacement and temperatures during hover and flight remained about 50° below the limit for continuous operation.

Although the mechanic stated that he checked the thermocouples after maintenance, the accident is consistent with maintenance personnel not securing the thermocouples properly. This resulted in lower EGT temperatures being presented to the pilot than what were being generated by the engine during operation. It is therefore likely that the pilot unknowingly exceeded the engine’s operating temperature limits, resulting in a catastrophic overtemperature event and total loss of engine power.

Text from the NTSB. Open the NTSB docket for WPR24LA161 for the full record.