Accident Details
On February 23, 2024, at about 0740 Pacific standard time, a Bell 47G-3B-1 helicopter, registered N2618W, was substantially damaged when it struck terrain near Garey, California. The pilot sustained minor injuries. The helicopter was being operated as a Title 14 Code of Federal Regulations Part 137 aerial application flight.
According to the pilot, he was flying about 5 ft above ground level at an airspeed of 40 knots while applying fungicide to a field. He initiated a climb to clear a set of power lines. As he increased power, he heard an engine out/low rotor audible warning and the engine simultaneously lost power. He performed an autorotation to an off-airport site. During the autorotation, the helicopter entered an uncommanded left roll. Despite control inputs, the pilot could not arrest the roll, and the helicopter impacted terrain in a nose-down, left-side-low attitude. The helicopter came to rest on its right side, with substantial damage to the tailboom and main rotor system.
Postaccident Examination
Examination of the airframe revealed impact damage to the right side. The right skid fractured at the struts and separated from the fuselage. The bubble canopy fractured and separated. The tailboom exhibited a fracture and separation consistent with impact from the main rotor blades.
The engine remained secured to the airframe. Manual rotation of the main engine driveshaft turned the stage four turbine wheel, confirming N2 drivetrain continuity. Motoring the engine via the starter generator smoothly rotated the compressor rotor, confirming N1 drivetrain continuity. Engine control continuity was confirmed from the throttle to the fuel control unit (FCU). All engine oil and fuel lines were secured, with B-nuts at least hand tight and no evidence of leakage.
A check of the pneumatic system by applying about 25 psi of air pressure at the Pc filter revealed minor air leakage at both B-nuts on both ends of the accumulator and a large leak at the PTG (power turbine governor) to FCU Pc input line. Closer examination showed the B-nut had no thread engagement and had slid aft down the Pc line. The input union on the FCU exhibited green torque paint similar in color to other B-nuts and inputs on the FCU.
According to Rolls-Royce, the engine manufacturer, the Pc line connecting the PTG to the FCU is the third of three Pc lines in the system. Pc air provides the power supply for the fuel control to modulate fuel delivery. With this line disconnected, the engine would reduce power to an unpowered static-state or sub-idle condition.
Further examination of the fuel control, fuel governor, and fuel nozzle showed normal operation. No other preaccident mechanical malfunctions or failures with the engine or airframe were found that would have precluded normal operation.
Maintenance History
According to maintenance records, on June 23, 2023, the fuel control was installed on the accident engine along with several other components, including the fuel pump, governor, fuel nozzle, bleed control assembly, and a tachometer generator.
On February 8, 2024—about 15 days before the accident—at a Hobbs time of 902.5, a maintenance facility completed the 150-hour/300-hour engine inspections, as well as several airframe inspections. The Rolls-Royce Alternate Inspection Schedule for the 150/300-hour inspection includes the requirement: “Inspect all B-nuts for application and alignment of torque paint. If missing, loosen the B-nut, retighten, and apply torque paint.” The Hobbs meter at the time of the accident read 931.7. The maintenance technician who signed off the work did not recall specifics of the inspection but reported using checklists.
Rolls-Royce released Commercial Service Letter (CSL) 2373, which addressed a special tool to access the subject B-nut. The CSL provides details on how to manufacture the tool locally, and the tool is also available for purchase.