American Eurocopter Llc AS350B3 N617GC accident near Potrero, California

Probable cause

The pilot’s loss of control during cruise flight due to incapacitation for reasons that could not be determined.

NTSB analysis

The pilot was scheduled to conduct a routine Customs and Border Protection (CBP) air-support mission after several days of sick leave. Flight track data indicated that, after about 1.5 hours of flight, the helicopter entered a series of turns and a rapid descent. The final flight track point was about 820 ft above ground level and about 280 ft from the accident site. No further flight track data were available, and no eyewitnesses or surveillance video captured the accident flight.

A signal from an emergency locator transmitter (ELT) was received by a central rescue coordination center operated by the United States military, a typical routing procedure for ELT signals. The United States military subsequently communicated the detection to a division within the helicopter operator responsible for monitoring aircraft and receiving ELT signals. Without delays, the earliest a helicopter could have reached the accident site was about 10 minutes. The response was delayed by the time required to relay the ELT information among the responding organizations and by uncertainty concerning the coordinates supplied to the search crews, who located the wreckage about 53 minutes after the ELT began transmitting. A CBP emergency medical technician began CPR about 4 minutes later. The pilot’s autopsy identified a complete transection of the high cervical spinal cord, which would have prevented spontaneous breathing. The pilot also sustained multiple full-thickness heart lacerations and numerous other severe injuries. An NTSB medical analysis determined that the pilot’s injuries likely would have been rapidly fatal and that survival would have been highly unlikely even with an optimal rescue response. Therefore, the response time likely did not affect the pilot’s survivability.

Postaccident examination of the helicopter revealed no preimpact mechanical anomalies or malfunctions that would have precluded normal operation. The weather was clear with light wind, and the pilot made no radio transmissions reporting a problem or distress. For about the first 1.5 hours of the accident flight, flight track data indicated the pilot was actively and effectively controlling the helicopter while flying alone. The helicopter then entered a series of turns and a rapid descent that did not appear consistent with the preceding flight path or a normal approach to landing. The helicopter’s flight track, descent rate, and accident-site signatures indicated that it impacted the ground at a high rate of descent with substantial left-side loading in visual flight conditions without adverse wind. The available flight track data did not show a recovery attempt before the track ended. Given the absence of a preimpact mechanical malfunction, adverse weather, an external collision, or evidence of an intentional maneuver, pilot incapacitation was the most plausible explanation for the loss of control.

The Vehicle and Engine Multifunction Display (VEMD) recorded a main-rotor-speed exceedance of 466 rpm. The manufacturer determined that the exceedance was not associated with an engine power-turbine overspeed and was likely associated with main-rotor desynchronization (i.e., autorotation) during descent. Because no flight-control or cockpit recordings were available, the investigation could not determine the pilot’s control inputs or the circumstances that produced the exceedance.

An NTSB medical analysis identified severe multivessel coronary artery disease and evidence of prior heart-muscle damage, which significantly increased the pilot’s risk of a sudden impairing or incapacitating cardiovascular event. Although the autopsy did not identify evidence that such an event occurred, an autopsy cannot exclude a sudden cardiovascular event. Heart-rhythm disturbances generally leave no identifiable autopsy evidence, and a heart attack or stroke may leave no such evidence if it occurs shortly before death.

Alternatively, the pilot may have fallen asleep. Before the crash, the helicopter had been operating at a relatively stable altitude and a generally consistent track. The pilot had recently used diphenhydramine, a sedating antihistamine that can cause drowsiness. His recent respiratory illness also may have increased fatigue or sleepiness. However, available evidence was insufficient to determine whether the pilot experienced a cardiovascular event, fell asleep, or became incapacitated for another reason.

In accordance with CBP procedures, a clearance authority stationed at another base approved the flight. The clearance authority had no aviation background and would not have been familiar with the pilot’s normal demeanor. The required risk assessment did not address illness, medication use, or other human factors challenges, and the clearance authority did not know the pilot well enough to compare his demeanor with his usual behavior. However, the pilot reported that he had recovered from a cold, felt well, and had no symptoms other than a hoarse voice. The investigation could not determine when he used diphenhydramine or whether the clearance process could have identified or mitigated the condition that led to the loss of control. Therefore, the evidence was insufficient to identify the clearance decision as a contributing factor.

Based on the available evidence, the pilot likely lost control of the helicopter after becoming incapacitated. The pilot’s cardiovascular disease increased his risk of a sudden incapacitating cardiovascular event, while his use of diphenhydramine and recent illness may have increased his risk of falling asleep. The available evidence was insufficient to determine the specific cause of his incapacitation.

Text from the NTSB. Open the NTSB docket for WPR25FA062 for the full record.