N9159F

HISTORY OF FLIGHTOn April 25, 2020, at 1240 eastern daylight time, a Hughes 369D helicopter, N9159F, was substantially damaged when it was involved in an accident near Pylesville, Maryland. The pilot was not injured. The helicopter was operated under the provisions of Title 14 Code of Federal Regulations (CFR) Part 133 as a rotorcraft external load operation.

The pilot reported that while he was performing human external cargo (HEC) long line operations, he heard on the radio that ground personnel were having difficulty moving a conductor power line (wire) nearby. He proceeded to the landing zone, which was about 300 to 400 ft from the area requiring assistance, and dropped off the HEC. Then, while hovering, he picked up a conductor hook via the long line (with assistance from ground personnel) and continued to the area that needed support.

He reported that after the hook was attached to the conductor wire, he began maneuvering for about 10 to 15 seconds to move the wire a short distance laterally, as a crane was supporting the weight of the wire. According to the pilot, while maneuvering, he applied "slight aft and up pressure" to move the conductor wire and there was no lateral banking. He believed the pitch attitude during the maneuvering was about 5° to 10° nose up. After the conductor wire was moved to the desired area, the pilot maneuvered to remove the hook from the wire, but before the hook was free, the helicopter entered a left yaw and the engine began "spooling down."

The pilot reported that he subsequently heard the "engine out alarm" and entered an autorotation by "slamming the collective down." The pilot reported that the loss of engine power occurred about 150 ft above ground level (agl) and that he immediately pulled the belly band release lever—one of two levers needed to release the long line (the belly band was a secondary cable support system the operator used for HEC operations to provide redundancy in the event of an inadvertent release of the cargo hook; see figure). The pilot stated that he did not have sufficient time to pull the second (mechanical release) lever on the cyclic control to release the long line.

As the helicopter entered the flare, the pilot pulled the collective up to complete the autorotative landing, but the long line, which remained attached to the helicopter and conductor wire, became taut and caused the helicopter to roll onto its left side. The main rotor blades impacted the ground.

Multiple witnesses on the ground reported that they heard the helicopter's engine lose power while the pilot was maneuvering, and they subsequently observed the helicopter begin a rapid descent. One witness stated that when the helicopter was about 3 ft from the ground, “the long line got tight and started to tip the aircraft over.”

The following figure shows the belly band around the fuselage, the main hook, and long line.

Figure. View of the helicopter at the accident site

AIRCRAFT INFORMATIONThe accident helicopter’s fuel system was composed of two interconnected fuel tanks installed beneath the passenger seats. Fuel was delivered to the engine from a fuel pick-up port on the left side of the left tank. The rotorcraft flight manual stated that the total usable fuel was 421.9 lbs.

Manufacturer Guidance

In November 2015, MD Helicopters, the type certificate holder at the time, published Operational Safety Notice OSN2015-002, “Fuel Starvation Due to Unporting of Fuel Supply Pick-Up.”

The notice warned operators that when the helicopters are used to conduct operations with a “long line” attached to pull or tow objects on the ground, a significant side load can be placed on the helicopter. These side loads can create high fuselage pitch and roll angles as well as uncoordinated flight, which in turn can increase the amount of unusable fuel and result in fuel starvation due to unporting of the fuel supply pick-up.

The notice further stated in part: MDHI Helicopters are not specifically certified for operations with the potential for sustained high fuselage pitch and roll angles in uncoordinated flight, such as powerline stringing operations. To help mitigate the possibility of fuel starvation and the potential safety risk, consider modifying fuel management procedures for such operations. Instead of allowing such operations with minimum fuel safety margins associated with normal flight attitudes during coordinated flight, consider increasing minimum fuel level requirements when operations will involve high deck angles in pitch and roll during uncoordinated flight.

AIRPORT INFORMATIONThe accident helicopter’s fuel system was composed of two interconnected fuel tanks installed beneath the passenger seats. Fuel was delivered to the engine from a fuel pick-up port on the left side of the left tank. The rotorcraft flight manual stated that the total usable fuel was 421.9 lbs.

Manufacturer Guidance

In November 2015, MD Helicopters, the type certificate holder at the time, published Operational Safety Notice OSN2015-002, “Fuel Starvation Due to Unporting of Fuel Supply Pick-Up.”

The notice warned operators that when the helicopters are used to conduct operations with a “long line” attached to pull or tow objects on the ground, a significant side load can be placed on the helicopter. These side loads can create high fuselage pitch and roll angles as well as uncoordinated flight, which in turn can increase the amount of unusable fuel and result in fuel starvation due to unporting of the fuel supply pick-up.

The notice further stated in part: MDHI Helicopters are not specifically certified for operations with the potential for sustained high fuselage pitch and roll angles in uncoordinated flight, such as powerline stringing operations. To help mitigate the possibility of fuel starvation and the potential safety risk, consider modifying fuel management procedures for such operations. Instead of allowing such operations with minimum fuel safety margins associated with normal flight attitudes during coordinated flight, consider increasing minimum fuel level requirements when operations will involve high deck angles in pitch and roll during uncoordinated flight.

WRECKAGE AND IMPACT INFORMATIONPhotographs provided by a Federal Aviation Administration (FAA) inspector who examined the helicopter at the accident site found that the helicopter had rolled over and come to rest on its left side, and the long line remained attached from the main hook on the helicopter to the power line. The tail boom and main/tail rotors sustained substantial damage. There was no evidence of fuel spillage at the accident site.

Additional examination of the helicopter supervised by the NTSB investigator-in-charge found that the cyclic, collective, and throttle each had continuity through the full range of motion. The main hook release lever opened the hook normally when activated.

There were no obstructions observed in the turbine air inlet. The oil filter and fuel filters were clear of any remarkable debris. Pressure and leak tests were performed on the engine’s pneumatic and fuel system; no leaks were observed on either system. The electrical fuel pump (start pump) would not activate when electrical power was supplied to the helicopter. A replacement electrical fuel pump was installed on the helicopter and functioned normally. With the new electrical fuel pump installed, a total of 146 lbs (21.5 gallons) of fuel was pumped from the helicopter. This volume was consistent with the fuel gauge, which displayed about 150 lbs.

The engine was subsequently removed and test run under the supervision of the NTSB investigator-in-charge. The engine produced idle through takeoff power, with no anomalies observed, and all engine parameters remained within tolerances throughout the test run.

For a portion of the test run, the positive pressure fuel supply was eliminated to simulate conditions similar to an electrical fuel pump failure. The engine continued to produce takeoff thrust consistent with the previous data when positive fuel pressure was available.

ADDITIONAL INFORMATIONFAA Regulations and Guidance

Advisory Circular (AC) 133-1B, Rotorcraft External-Load Operations, provided the following two definitions for Class B and C Rotorcraft-load combinations (RLC):

Class B RLC. The external load is jettisonable, carried above or below the skids, and lifted free of land or water during the rotorcraft operation. An air conditioner unit being lifted onto the roof of a tall building is an example of a Class B load (§ 1.1).

Class C RLC. The external load is jettisonable and remains in contact with land or water during the rotorcraft operation. Wire stringing, dragging a long pole, and boat towing are some examples of Class C loads (§ 1.1).

AC 133-1B does not contain minimum fuel standards based on the specific type of RLC class to be flown.

Part 133 requires no additional fuel minimums beyond that required in 14 CFR 91.151, Fuel Requirements for Flight in VFR Conditions.

TESTS AND RESEARCHAs part of this investigation, MD Helicopter provided the NTSB a computer model that outlined a combination of static pitch and roll angles and corresponding fuel levels at which the fuel pick-up point may become unported. According to this information, with about 21.5 gallons of fuel onboard, at 0° lateral banking, the pitch up attitude required to unport the fuel pick up was 28.5°. The computer model could not account for dynamic flight operations that may affect the movement of fuel in the tanks (for example, maneuvering, turbulence, or uncoordinated flight, which would allow for fuel to move freely within the fuel tank).

A search of the NTSB’s aviation accident database for 14 CFR Part 133 fuel starvation events involving any rotorcraft type performing Class C RLC operations found three reports relevant to this investigation.

In 1990, the NTSB investigated a helicopter accident involving a MD369D that was conducting external long lin...