History of Flight
On April 25, 2020, at 1240 eastern daylight time, a Hughes 369D helicopter, registration N9159F, sustained substantial damage during an accident near Pylesville, Maryland. The pilot was not injured. The flight was conducted under 14 Code of Federal Regulations Part 133 as a rotorcraft external load operation.
According to the pilot, while performing human external cargo (HEC) long line operations, he heard on the radio that ground personnel were having difficulty moving a conductor power line. He proceeded to a landing zone about 300 to 400 feet from the area, dropped off the HEC, and then picked up a conductor hook via the long line with assistance from ground personnel. After attaching the hook to the conductor wire, he maneuvered for approximately 10 to 15 seconds to move the wire laterally while a crane supported its weight. The pilot reported applying slight aft and up pressure without lateral banking, maintaining a pitch attitude of about 5° to 10° nose up. After moving the wire to the desired location, he attempted to remove the hook, but before it was free, the helicopter entered a left yaw and the engine began spooling down. The pilot heard the engine out alarm, entered an autorotation at about 150 feet above ground level, and pulled the belly band release lever (one of two levers needed to release the long line) but did not have time to pull the second lever. During the flare, the long line, still attached to the helicopter and conductor wire, became taut, causing the helicopter to roll onto its left side. Main rotor blades struck the ground. Multiple witnesses heard the engine lose power and observed a rapid descent; one witness reported that the long line tightened and tipped the aircraft when it was about 3 feet from the ground.
Aircraft and Fuel System
The helicopter’s fuel system comprised two interconnected fuel tanks beneath the passenger seats, with fuel delivered from a pick-up port on the left side of the left tank. The rotorcraft flight manual listed total usable fuel as 421.9 lbs.
Manufacturer guidance (MD Helicopters Operational Safety Notice OSN2015-002) warned that long line operations placing side loads on the helicopter can create high fuselage pitch and roll angles and uncoordinated flight, increasing unusable fuel and risk of fuel starvation due to unporting of the fuel supply pick-up. The notice stated that MDHI helicopters are not certified for such operations and recommended increasing minimum fuel levels.
Wreckage and Examination
Post-accident examination revealed the helicopter resting on its left side with the long line still attached to the main hook and the power line. The tail boom and main/tail rotors were substantially damaged. No fuel spillage was observed. Flight controls (cyclic, collective, throttle) showed continuity through full range. The main hook release lever functioned normally. No obstructions were found in the turbine air inlet; oil and fuel filters were clean. Pressure and leak tests on the pneumatic and fuel systems revealed no leaks. The electrical fuel pump did not activate when power was applied; a replacement pump functioned normally. With the new pump, 146 lbs (21.5 gallons) of fuel were pumped, consistent with the gauge reading of about 150 lbs. The engine was test run and produced idle through takeoff power without anomalies; all parameters remained within tolerances. When positive pressure fuel supply was eliminated to simulate an electrical fuel pump failure, the engine maintained takeoff thrust.
Additional tests by the manufacturer provided a computer model indicating that with 21.5 gallons of fuel and 0° lateral banking, a pitch up attitude of 28.5° would unport the fuel pick-up. The model could not account for dynamic flight conditions. The NTSB database search found three similar fuel starvation events involving MD369 helicopters in Class C operations, with fuel quantities between 93 and 117 lbs and unporting at certain attitudes. An Australian investigation of an MD369ER also found possible air introduction at fuel levels below 151 lbs with 20° nose up and 20° right roll.
Organizational Information
The operator’s FAA-approved Rotorcraft-Load Combination Flight Manual (RLCFM) required landing with no less than 100 lbs of fuel. For Class C operations, it required starting with a full tank and limiting flight to one hour. A pre-flight Job Hazard Analysis form offered options for 100 lbs landing minimum and for wire/rope pull operations (max 1 hr with max fuel load). The form for the accident flight indicated the 100 lbs landing minimum option. The pilot reported that he understood more restrictive fuel minimums applied only to side pull operations involving right lateral banking. He considered this accident flight more like Class B operations. After the accident, the operator updated its forms and RLCFM to tie fuel minimums to specific RLC classes and provided examples of long line operations. Safety briefings were conducted with operational staff.