Accident Overview
On May 27, 2024, at 0800 mountain daylight time, an Aerospatiale SA-315B helicopter, registration N57948, was substantially damaged during an accident near Hildale, Utah. The helicopter was operated as a personal flight under Title 14 Code of Federal Regulations Part 91. The pilot and two passengers received minor injuries, and one passenger was uninjured.
Pilot Account
The pilot stated he was flying family members and friends around a relative’s residence. He completed one local flight of 15 to 20 minutes with four passengers, landed, and shut down the engine to clean his windscreen. Three passengers then boarded for a second local flight. The pilot reported that the second takeoff was normal, but shortly after takeoff, the engine “felt sluggish.” At about 50 feet above the ground and an airspeed of about 50 knots, he observed diminishing rotor speed and nearly complete loss of engine rpm. He performed an autorotation to a road. The helicopter landed hard; the main rotor blades struck the tail boom, and the right skid splayed outward.
Witness Observation
A witness-provided video of the flight showed flames emanating from the exhaust tailpipe within seconds of the helicopter taking off and continuing until impact with the ground.
Postaccident Examination
Examination of the helicopter revealed external thermal damage corresponding to the exhaust pipe inlets and piping; the exhaust pipe was crushed consistent with impact from the accident sequence. The exhaust pipe was removed to inspect the three stages of turbines. Severe thermal damage was observed on all three turbine stages and on the second and third nozzle guide vanes.
The engine uses two thermocouples to provide a signal to the exhaust gas temperature (EGT) gauge in the cockpit. The lower thermocouple was found hanging loose from its boss on the exhaust section. A review of photographs taken immediately after the accident confirmed the lower thermocouple was in that loose condition. The upper thermocouple was found tight but not secured with safety wire, as required by the engine maintenance manual. The upper probe exhibited thermal damage that melted the probe wire in the gas path; the lower probe showed no damage. No additional preimpact anomalies were noted with the engine or airframe.
Maintenance History
A review of the engine logbook showed that the turbine section was replaced on May 9, 2024, which would have necessitated removal and reinstallation of the engine exhaust section and thermocouples. A periodic inspection was also completed on that date. The engine and airframe accumulated 3.4 hours between the turbine section replacement and the accident. The pilot recalled flying about six flights after the replacement. He reported that after maintenance, EGT ranged from 350 to 400°C during startup and was about 450°C at hover or in flight, noting that startup temperature was at least 100°C cooler than it had been for the previous 1.5 years. The helicopter flight manual specifies a maximum operating tailpipe temperature of 550°C for five minutes and 500°C for continuous operation.
The mechanic who replaced the turbine section stated he has been a full-time mechanic since 2017, employing one non-mechanic assistant and often relying on friends who are qualified mechanics for quality control. He noted that the pilot, who is not a certified mechanic, assisted with some work on the engine. The mechanic recalled checking the security of the thermocouples following maintenance.
Additional Information
The EGT indicator in the cockpit receives input from the two thermocouples in the exhaust section. They deliver an electromotive force proportional to the exhaust temperature, which is summed and subjected to preset resistance before being displayed. If one thermocouple experiences a lower temperature, its electromotive force is lower, resulting in a lower displayed EGT.
Chapter 5 of the engine maintenance manual includes a procedure for a check after the last flight of the day that involves inspecting the exhaust section. The pilot’s operating handbook notes that the first step in preflight external checks is to confirm that the inspection of the last flight of the day has been completed, and that a walkaround includes inspection of the exposed engine.
The National Transportation Safety Board Pilot/Operator Aircraft Accident/Incident Report Form 6120.1 was not received from the pilot.