N57948

On May 27, 2024, at 0800 mountain daylight time, an Aerospatiale SA-315B, N57948, was substantially damaged when it was involved in an accident near Hildale, Utah. The pilot and two passengers received minor injuries and one passenger was uninjured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.

The pilot stated he was flying family members and friends around a relative’s residence. He completed one local flight of 15-20 minutes with four passengers, landed, and shut down to clean his windscreen. Three passengers then boarded the helicopter for a second local flight. The pilot said that the second takeoff was normal, but shortly after takeoff the engine “felt sluggish.” He estimated the helicopter was about 50 ft above the ground at an airspeed of about 50 knots when he saw the rotor speed was diminishing and engine rpm was nearly gone, so he proceeded to perform 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.

Postaccident examination revealed external thermal damage corresponding to the exhaust pipe inlets and piping; the exhaust pipe had been crushed consistent with impact from the accident sequence. The exhaust pipe was removed to examine the three stages of turbines. Severe thermal damage was observed on all 3 stages of turbines and the 2nd and 3rd nozzle guide vanes.

The engine uses two thermocouples to provide a signal to the EGT gauge in the cockpit. Upon external examination of the engine, the lower thermocouple was found to be hanging loose from its boss on the exhaust section (Figure 1). A review of photographs taken onsite showed that the lower thermocouple was in the same loose condition immediately following the accident (Figure 2). The upper thermocouple was found tight but not safety wired. The engine maintenance manual states that the thermocouple retention nut should be secured using stainless steel wire during installation. 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. No additional preimpact anomalies were noted with the engine or airframe.

Figure 1 – Loose Thermocouple

Figure 1 – Engine and Loose Thermocouple (Courtesy of Hildale Police Dept.)

A review of the engine logbook revealed the turbine section was replaced on May 9, 2024, which would have required the removal and reinstallation of the engine exhaust section and thermocouples. A periodic inspection of the engine was also completed on that date. The engine and airframe accumulated 3.4 hrs between the time of the turbine section replacement and the accident.

The pilot told investigators that he thought he flew about 6 flights after the turbine section replacement. He stated that following the maintenance, the EGT ranged from 350-400°C during startup and was about 450°C at hover/flight. He said, “startup is at least 100° cooler than it had been running for the previous 1.5 years.” The helicopter flight manual states that the maximum operating tailpipe temperature (EGT) is 550°C for 5 minutes and 500°C for continuous operation.

The mechanic who replaced the turbine section of the engine stated he is a full-time mechanic and has operated his current business since 2017. He employs one non-mechanic employee to assist him with manual labor and often relies on friends who are qualified mechanics to perform quality control and check his work. He stated the pilot and owner of the helicopter, who is not a certified mechanic, assisted him with some of the work on the engine. He recalled that he checked the security of the thermocouples following maintenance checks.

The EGT value presented at the EGT indicator (gauge) in the cockpit initiates from the two thermocouples located in the engine exhaust (T4) section. The thermocouples deliver an electromotive force proportional to the T4 temperature at each thermocouple to an automatic control box, where they are summed together and subjected to a preset resistance at a fixed level before being received at the EGT indicator. It operates such that if one thermocouple is subjected to a lower temperature than the other thermocouple, its electromotive force will be lower and the sum of the electromotive forces from both thermocouples will be lower. Subsequently, the EGT indicator in the cockpit will show a lower engine exhaust temperature.

Chapter 5 of the engine maintenance manual, “Inspection – TBO and Life Limits – Special Checks,” contains the procedure for the “Check after last flight of the day.” That procedure includes steps to check the exhaust section of the engine.

The pilot’s operating handbook normal procedures states that the first pilot check concerning the preflight external checks is to ascertain that the inspection of the last flight of the day has been completed. The POH also demonstrates the proper methodology for conducting a walkaround before flight, which includes inspection of the exposed engine.

A National Transportation Safety Board Pilot/Operator Aircraft Accident/Incident Report Form 6120.1 was not received from the pilot.