WPR24LA265 · Bell 407

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.

Verbatim NTSB analysis and probable cause from the NTSB dataset