Hughes 369 near Pylesville, MD — 2020-04-25

Probable cause

The loss of engine power due to fuel starvation as a result of unporting of the fuel tank supply pickup while the helicopter was maneuvered to move a conductor wire. Contributing to the accident was the helicopter’s inappropriate configuration for the type of operation being conducted, which impeded the pilot’s ability to release the long line and perform a successful emergency landing.

Contributing factors

Contributing to the accident was the helicopter’s inappropriate configuration for the type of operation being conducted, which impeded the pilot’s ability to release the long line and perform a successful emergency landing.

NTSB narrative

The commercial helicopter pilot was attempting, via a long line, to move a conductor wire while it remained in contact with the ground, which is classified as a Class C rotorcraft-load combination (RLC) operation. According to the pilot, while he maneuvered the helicopter about 150 ft above ground level, he pitched the helicopter nose up about 5° to 10°, with no lateral banking, for about 10 to 15 seconds. The engine then experienced a total loss of engine power. In the autorotation, which the pilot estimated to be about 4 to 5 seconds, the pilot was only able to release one of two mechanisms that secured the long line to the helicopter. As a result, just before touchdown, the long line became taut and caused the helicopter to roll over onto its left side. The tailboom, main rotor, and tail rotor sustained substantial damage, and the pilot was uninjured. Postaccident examination of the helicopter found 146 lbs of fuel onboard. The pilot later reported that the helicopter had about 200 lbs of fuel (slightly less than half of a full load) when he began the flight about 1.5 hours before the accident. An engine test run found no evidence of mechanical malfunctions that would have precluded normal operation of the engine. The investigation identified four previous accident investigation reports that extensively documented loss of engine power due to fuel starvation on MD369 series helicopters while they were maneuvered in Class C RLC long line operations. In these past accidents, the remaining fuel on board ranged between 93 to 151 lbs. The investigations of these accidents found varying levels of pitch up and/or lateral banking (common maneuvers during Class C RLC operations) could interrupt normal fuel flow to the engine (that is, unport) at fuel levels well above the standard fuel minimums required for visual flight rules operation. Based on information provided by the helicopter manufacturer, with 146 lbs of fuel onboard, a 28.5° positive pitch attitude, with no lateral banking, could unport the fuel supply to the engine in static conditions. Therefore, in dynamic conditions, such as maneuvering, unporting could occur at lower pitch attitudes. The operator’s operating limitations at the time of the accident stated that for any Class C RLC operation, the flight must begin with a full fuel load and last no more than 1 hour and explains the policy by citing the risk of uncovering the fuel port due to lateral banking during these operations. The accident pilot believed that, similar to the operation he had completed earlier in the flight, moving the conductor wire was a Class B operation because it would not require any lateral banking of the helicopter. Because the pilot misconstrued the RLC class of operation he was performing, he erroneously believed that he only needed a minimum of 100 lbs of fuel at landing, which is the fuel minimum he selected on the operator’s job hazard analysis form before beginning the accident flight; the form contained no references to RLC classes. As a result of the accident, the operator updated its minimum fuel policies on its job hazard analysis form and in its RLC flight manual. The policies now provide specific references to Class B and C long line operations and detailed examples to help pilots’ understanding of which fuel minimums apply for specific operations. Thus, without evidence of malfunctions that would preclude the engine from producing or maintaining power and given the occurrence of fuel starvation during other Class C RLC long line operations with similar levels of fuel onboard, it is likely that the accident helicopter's maneuvering and nose-up attitude during the pilot’s attempt to move the conductor wire led to unporting of the remaining fuel, which resulted in fuel starvation and the loss of engine power. Additionally, it is possible that the pilot could have successfully landed the helicopter following the loss of engine power had the long line been released. The pilot had to pull two separate release mechanisms to detach the long line because the helicopter was previously configured for human external cargo (HEC) long line operations, although the specific operation being performed when the accident occurred did not involve HEC and redundancy to secure the long line was not needed (HEC operations were being performed earlier in the flight). As a result, the pilot did not have sufficient time to activate both release mechanisms, and the helicopter was substantially damaged during the attempted landing.

Analysis

Primary failure mode: Fuel management
First missed decision gate: Pilot could have ensured adequate fuel before conducting long line operations.

NTSB coding

Fuel starvation · Maneuvering-hover

Evidence available

Photos
20 docket documents

View NTSB final report View NTSB docket