Pilot Induced B407 HEC Power Loss (Guardian Helicopters N999GH at PG&E Training site, Livermore, CA)

On 11 May 2022 Bell 407 / Eagle 407HP conversion N999GH of Guardian Helicopters was involved in an accident near Livermore, California during a Part 133 rotorcraft external-load flight. The pilot was seriously injured.  remarkably, the lineman, suspended on a long line below the helicopter, escaped with only minor injuries.

Wreckage of Guardian Helicopters Bell 407 / Eagle 407HP N999GH at the PG&E Livermore Training Center (Credit: FAA via NTSB)

The Accident Flight

The US National Transportation Safety Board (NTSB) safety investigation report was issued on 14 May 2024.  The NTSB explain that:

The pilot was performing a HEC [Human External Cargo] long-line qualification exam flight at the Livermore Electric Safety Academy, a training operations facility owned by the Pacific Gas and Electric Company (PG&E). The pilot was flying the helicopter solo from the right seat with the door removed so that he could lean outside and observe below.

According to the helicopter operator, this was the second time he had taken the check ride, having not passed on the first attempt.

The maneuvers were observed and monitored on the ground by a group of examiners.

The pilot had about 3,500 hours of total flight time, including about 1,000 hours as a flight instructor and agricultural pilot and almost 175 hours on type. He started working for the helicopter operator about 15 months earlier.  The pilot had logged c 87 hours of FAR Part 133 external load time before the accident (most in the AS350) and c 10 in the B407.

After performing the initial maneuvers, the pilot transitioned to the HEC phase of the exam by carrying a lineman on a 60-ft long line. While maneuvering the helicopter at an altitude of about 175 ft above ground level (agl), the helicopter lost engine power.

Multiple witnesses recounted observations that matched the pilot’s statement. All stated that the helicopter appeared to be operating without issue throughout the maneuvers until they heard a change in engine tone, with some then observing the main rotor blades slowing as the helicopter began to rapidly descend

In response, the pilot selected what he thought was the “emergency” throttle detent, but the engine did not respond. The pilot maneuvered the lineman away from the landing helicopter and performed an autorotation. The helicopter landed hard and sustained substantial damage.

The Safety Investigation

[E]xamination of the airframe, engine, and engine control systems did not reveal any evidence of preimpact mechanical malfunction or failures. The helicopter contained fuel, was loaded within its envelope, and weather was not a factor.

Onboard video recorded a section of the annunciator panel that showed some, but not all, engine warning lights, none of which illuminated at any point during the flight. The recording did capture an audio tone as the descent began that was the same frequency as the engine out and low rotor warning indicators.

The video recording appeared to show the pilot was anxious throughout the flight; however, considering the nature of the work he was performing and the stress he would have been experiencing, this is understandable. It could not be determined if this contributed to the accident.

The engine was test run in a test cell and it performed nominally.


Review of data recorded by the engine control unit (ECU) revealed that at the time of the loss of engine power, the throttle was in the “IDLE” rather than the “FLY” position as would have been required during flight. A few seconds later, the throttle position moved back into the “FLY” range.

The NTSB opine that:

The collective control, which housed the engine throttle twist grip at its end, was mounted on the pilot’s left side. The direction of throttle rotation meant that if the pilot maintained a tight grip on the throttle as he leaned right and out of the helicopter, the direction of throttle rotation was such that his hand could have inadvertently rolled the throttle out of FLY and into IDLE.

Under this scenario, the engine would lose power immediately; both audio and engine diagnostic data confirmed this had likely happened.

The pilot almost immediately rolled the throttle back into the FLY position but the design of the engine control system was such that it assumed the helicopter was on the ground and therefore limited power application to prevent an over-torque condition.

So, even though the pilot immediately remedied the problem, the engine would not have regained power.


The pilot stated as soon as he noticed a loss of power he moved the throttle beyond “FLY” and to the “emergency” position. However, although this position could be selected, it had no function on this type of helicopter; it was a hold out from the previous version of the helicopter that the pilot had flown.

This mistake had no bearing on the accident and would not have affected the outcome.

The NTSB note that:

The HEC operation required that the helicopter be flown at a low height and speed such that it was operating within the “AVOID” range of its Height-Velocity Envelope. In this range, a safe landing could not be guaranteed following an engine failure.

Additionally, the nature of the training facility, with multiple training towers and obstacles in the immediate vicinity, further reduced the likelihood of a successful outcome in the event of an emergency.

Guardian Helicopters Bell 407 / Eagle 407HP N999GH Accident Site at the PG&E Livermore Training Center (Credit: PG&E via NTSB)

Therefore, although the pilot made the mistake of inadvertently shutting down the engine in flight, he was still able to demonstrate the presence of mind and helicopter control to maneuver the helicopter away from the lineman and into an open area.

NTSB Probable Cause

Loss of engine power due to the pilot inadvertently selecting engine IDLE mode while in flight, which reduced rotor and engine speed to a level that could not sustain flight.

Safety Resources

The European Safety Promotion Network Rotorcraft (ESPN-R) has a helicopter safety discussion group on LinkedIn.  You may also find these Aerossurance articles of interest:

See: UK CAA‘s CAP 426: Helicopter External Load Operations and Launching a human external cargo (HEC) helicopter program too.

Aerossurance has extensive air safety, flight operations, HESLO, HEMS, SAR, airworthiness, human factors, helidecks, aviation regulation and safety analysis experience.  For practical aviation advice you can trust, contact us at: enquiries@aerossurance.com