WPR24LA265

The pilot’s misdiagnosis of an erroneous engine instrument indication and his subsequent decision to enter an autorotation, which resulted in a hard landing due to degraded visual cues and low ambient light conditions. Contributing to the accident were the erroneous engine indications likely caused by an undetermined electrical supply disruption; time pressure, pilot fatigue, and plan continuation bias as daylight diminished and the crew aimed to complete the flight; and cognitive overload from multiple airframe and engine caution indications.

The pilot of the helicopter departed on the final stage of a multileg public use contract flight to reposition a helitack crew. The U.S. Forest Service (USFS) contract specified that operations were restricted to day visual meteorological conditions; the planned arrival time at the destination was about 30 minutes after sunset and just short of the 14-hour limit of the pilot’s duty day. Review of onboard video recordings revealed that, about 18 minutes after takeoff while in level flight, the measured gas temperature (MGT) exceeded the limits allowed for cruise flight. The overtemperature indication triggered a “CHECK INSTR” annunciation and an exceedance recorded by the MGT gauge, which could only be reset by a mechanic after the flight. Following this, the pilot then appeared to reduce engine power and continued the flight. About 30 minutes later, the barrier inlet filter caution light illuminated. The pilot found this indication unusual, because the filter had been serviced the week prior. He activated the filter bypass system and continued with the flight. Due to arrival time constraints, the crew discussed the option of diverting to an alternate airport and staying there overnight; however, they ultimately decided to continue to the destination. Shortly thereafter, the MGT gauge began to indicate a temperature increase into the yellow range and then the red range, accompanied by another “CHECK INSTR” warning. The pilot perceived this indication, and the helicopter’s response as he began to troubleshoot, as evidence of an engine overspeed condition, and he chose to initiate a precautionary landing to an open field nearby. Review of the cockpit video, however, revealed that none of the other engine gauges corroborated the high MGT reading, consistent with an erroneous MGT indication. When his control inputs failed to arrest the perceived overspeed or restore normal engine response, the pilot entered an autorotation to a closer cornfield. The autorotation was conducted during the diminishing ambient light conditions of dusk. The lighting conditions, combined with the height of the corn, obscured the pilot’s depth perception and limited his ability to accurately judge the timing of the landing flare, resulting in a hard landing. Impact forces were sufficient to cause separation of the main transmission, fragmentation of drive system components, and extensive secondary damage to the engine, including fragmentation of the turbine wheels after hard-body ingestion. Postaccident examination revealed no evidence of pre-impact mechanical failure, fatigue, or thermal distress of the engine, and review of the engine control unit’s (ECU) non-volatile memory did not reveal any event indicative of an engine overspeed, overtemperature, or associated malfunction. Bench testing of the MGT gauge did not reveal any anomalies, and despite the in-flight display irregularities, the gauge recorded only one exceedance, which appeared to match the initial overtemperature indication observed earlier in the flight. However, the erroneous displays observed during the flight could be simulated during bench testing by lowering and then restoring the unit’s electrical supply voltage. Therefore, the condition was likely caused by an undetermined disruption in the electrical supply to the MGT gauge. The accident sequence was initiated by compounding operational stressors. The accident occurred at the end of a long duty day for the pilot, who likely was beginning to feel the effects of fatigue, and the flight was conducted at a time of day that would have created significant time pressure. The pilot then allowed the engine to operate above normal MGT levels, possibly to expedite arrival. Although power was reduced and the flight continued, the overtemperature indication further increased the pilot’s cognitive workload, and the need for a mechanic to intervene later would have been an additional stressor. The unexpected barrier inlet filter annunciation that followed likely increased the pilot’s anxiety, and although a diversion was discussed, the prospect of an overnight stop and the resulting logistical impacts contributed to plan continuation bias. Subsequent anomalous MGT indications became the triggering event that overwhelmed the pilot, who misdiagnosed the symptoms as a developing engine problem that he likely attributed to the earlier issues.

The pilot reported 3,855 total hours of flight experience, including 69 hours in the 90 days preceding the accident, and 26.8 hours of flight time during the previous 7 days. He had accumulated 908 hours in the Bell 407 and was issued an interagency pilot qualification card by the USFS on June 10, 2024. On the day of the accident, the pilot was dispatched from the Pacific time zone, where his duty day began at 0700. Crew duty days are limited to 14 hours; a 14-hour duty day starting at 0700 PDT would have concluded by 2200 MDT. On August 2, 2024, about 2107 mountain daylight time, a Bell 407 helicopter, N20BH, was substantially damaged when it was involved in an accident near Raft River, Idaho. The pilot and three passengers sustained minor injuries. The helicopter was operated as a public aircraft in support of the USFS. The helicopter was privately owned, and the flight originated under the Bureau of Land Management’s use of a USFS Exclusive Use flight service contract. The purpose of the flight was to transport three USFS helitack crew members from Gerlach, Nevada, to their base at Pocatello, Idaho, with intermediate stops at Winnemucca and Elko, Nevada. The pilot reported that the first two legs were uneventful, and that the accident occurred on the final leg. On the day of the accident, sunset at Pocatello was 2049. The terms of the contract limited operations to day VFR only. The crew planned to arrive at 2120; however, according to the pilot, they were given permission to arrive as late as 2125, which was possible as long as he maintained a groundspeed of 120 knots while enroute. The pilot stated that, about 55 minutes after departing Elko, the barrier inlet air filter light started to flicker. The filter had been serviced the week prior, and although the helicopter had been flying in a dusty and smoky environment since then, he thought such an activation was premature. A short time later, the filter light transitioned from flickering to on, and the pilot activated the filter bypass system by pressing the “FILTER” light switch. Due to arrival time restraints, the crew then discussed the option of diverting and completing the flight the following morning but chose to continue to Pocatello. The pilot reported that, a few minutes later, he noticed the engine MGT starting to rise into the yellow band of the gauge. In response, he reduced power and began to descend. However, the MGT continued to rise, and at an altitude between 1,000 and 1,500 ft, it continued to climb past the gauge redline. The pilot decided to make a precautionary power-on landing and began looking for a suitable landing area. As he was maneuvering the helicopter toward a grass field, he determined that the engine was starting to overspeed. The pilot raised the collective control twice to try to arrest the overspeed, but the engine did not respond, so he decided to perform an autorotation into an adjacent corn field. Due to the corn's unknown height, the pilot was unable to determine the optimal time to initiate the flare, and the helicopter landed hard after dropping from about 5 ft above the ground. The pilot reported that immediately after landing, the engine came back on for about 10 seconds and was shaking the aircraft and crew violently. The helicopter came to rest in a cornfield about 41 miles southwest of Pocatello. At the accident site, the surrounding corn stalks were about 6 ft tall. The helicopter was in a level attitude, with the main skids splayed outward such that the cabin belly was resting on the ground. The main rotor transmission assembly detached from the airframe and had rotated left and dropped down into the cabin compartment. The engine, which sat behind the main transmission, remained attached to the airframe. The engine output drive shaft and associated couplings had failed forward of the engine, resulting in fragments of the surrounding structure and drive components being propelled up into the inlet plenum. Three of the four main rotor blades remained attached at the hub, and the fourth was partially attached and bent opposite the direction of rotation. Two blades were shattered about midspan, with their remaining fragments located within the immediate vicinity of the accident site. The tail rotor and associated gearbox had detached and were located about 30 ft from the main wreckage. MGT Indicator Gauge The helicopter’s MGT gauge screen was composed of a segmented radial temperature indicator with color-coded bands, along with a series of seven-segment displays that indicated the temperature numerically along with the exceedance status. (see figure 2.) On power-up or under conditions where the unit sensed a software or hardware anomaly, it was designed to perform a BIT, which included an illumination of all display segments. Figure 2 - MGT Gauge The gauge, in correspondence with the flight manual’s engine operating guidance, indicated a continuous operating temperature of 100 to 727° C with a green band. The 5-minute takeoff range temperature was between 727° and 779° C and denoted by a yellow band. The maximum takeoff temperature was 779° C and indicated by a red bar. The transient, 12-second range was between 780° and 843° C with the upper limit bound with a red triangle, and the maximum start and shutdown range, which was not to exceed 10 seconds above 843° C or 1 second at 927° C, was indicated by a red circle. The design of the unit was such that if the temperature was in the yellow takeoff region (727° to 779° C) for 4 ½ minutes the segments of the display would flash as a warning. If this temperature condition continued for an additional 30 seconds, the numeric display would show an “E” indicating that an exceedance had been logged in non-volatile memory (NVM). An amber CHECK INSTR caution light would also illuminate on the annunciator panel if an MGT exceedance was about to or had occurred. An exceedance would also be logged into if the temperature climbed into the transient range (780° to 843° C) for more than 10 seconds, or if it ever entered the transient upper limit. Once the exceedance flag was triggered, the “E” indication remained on the screen until a mechanic reset it using specialized software. MGT Indicator Gauge Examination The unit was examined and tested at an FAA-certified overhaul facility, in accordance with an approved test protocol, and no anomalies were noted. It was found that by lowering the input supply voltage to around 6 volts direct current (VDC), the unit would exhibit the increasing MGT behavior observed during the accident sequence. Once it had dropped below 6VDC, the unit would display random characters in a manner almost identical to that observed before the accident. With the restoration of supply power to the nominal 28VDC, the unit restarted and performed a BIT check, consistent with that observed during the accident flight. (see figure 3) Figure 3 – Increasing MGT (left), increasing with random segments (center), BIT check (right). The unit’s memory was accessed using the standard maintenance software, and one exceedance had been recorded (the unit can record 50 exceedances). This exceedance was for 737° C and lasted 5 seconds. It was recorded on the day of the accident and appeared to be consistent with when the CHECK INSTR light came on and stayed on after the overtemperature indication during the initial cruise phase of the flight. MGT Gauge Airframe Installation The MGT indication system comprised the indicator mounted to the center instrument panel, which was supplied with electrical power via a 3-amp circuit breaker mounted to the overhead panel. The MGT signal was captured by 3 thermocouples connected in parallel and mounted to the engine exhaust section. The thermocouples provided data to both the MGT instrument and the ECU. On the airframe, the MGT circuit breaker was found in the clo