History of Flight
On April 26, 2024, at 0659 mountain daylight time, a Bell 206-L4 helicopter, registration N988B, was destroyed in an accident near Anaconda, Montana. The pilot was fatally injured. The helicopter was operating under 14 Code of Federal Regulations Part 137 as an aerial application flight.
The pilot had been distributing fertilizer from a staging area for several days. The staging area was surrounded by hilly terrain. An employee performing ground support duties met the pilot at Deer Lodge-City-Municipal Airport (38S) in Deer Lodge, Montana, about 0545. The employee then drove a support vehicle to the staging area about 1 mile southeast of Anaconda.
Onboard GPS data showed the helicopter departed 38S about 0613, overflew the intended application area, and proceeded to the staging area. Security camera video from about 0.5 miles north captured the helicopter landing at 0633. The pilot and ground crewmember conducted a safety briefing with a customer representative. According to the representative, wind was calm during the briefing and remained so for several hours.
After the briefing, the helicopter lifted off at 0643 and maneuvered over the load truck. It departed with the first load of fertilizer about 0644, flew west, and exited camera view. It returned from the west, flew past the south side of the staging area, turned northwest, and completed a second onload. The helicopter departed again about 0650, flew west, and returned at 0656.
The helicopter approached the load truck a third time heading northwest, similar to previous approaches. It hovered over or near the truck for 2 to 3 seconds, then flew east. It reapproached and completed a third onload. The helicopter then departed and climbed to the west, reaching about 150 ft above ground and 40 kts groundspeed when it rotated 180° left and descended rapidly out of camera view behind terrain. No witnesses to the accident sequence are known.
The ground crewmember attempted to contact the pilot via radio when he did not return as expected, but received no response. The customer representative, working from his truck southwest of the load truck, noticed the helicopter's absence and drove to observe. He saw the helicopter in a nearby gully, returned to pick up the ground crewmember, responded to the accident site, and initiated a 911 call.
Aircraft Information
The helicopter was equipped with a bubble window on the left door. The pilot operated from the left seat. A spreader was suspended beneath the helicopter using a 25-ft long line attached to a load hook on the belly. The pilot could release the load in flight in an emergency.
A review of engine logbooks revealed that the spur adapter gearshaft was last accessible during an engine overhaul in March 2019 at 6,328.1 hours total time, 1,414.1 hours before the accident.
Wreckage and Impact Information
The helicopter came to rest mostly upright adjacent to a flat, dry pond area in a gully about 855 ft southwest of the load truck. The fuselage rested on a heading of about 060°. The bottom of the fuselage showed crushing deformation, and the left skid was splayed outward. One main rotor blade was fracture-separated about 3 ft from the rotor attachment point, with downward deformation. The remainder of that blade was about 15 ft forward. The second rotor blade remained attached to the mast with some bending opposite rotation. The spreader and 25-ft long line were about 30 ft aft, separated prior to impact; the long-line attachment shackle was unmarred. Fuel leaked from the wreckage after the accident.
All cockpit flight controls were present. The left collective was completely fractured at the elbow. The cyclic yoke was fractured in two places. Both cyclics displayed control continuity to the yoke. Collective levers showed continuity to the elbow break on the left collective. Control tubes in the control closet exhibited overload fractures. Cyclic and collective control continuity was established (with breaks) to the hydraulic actuators. Tail rotor continuity was established throughout, with breaks consistent with overload. No airframe or flight control anomalies were noted that would preclude normal operation.
The engine was displaced vertically in the engine compartment, with all mount struts damaged; all but two were fractured. Cockpit control continuity was not continuous from the collective lever and throttle twist grip to the power turbine governor and fuel control unit due to impact damage. The pilot throttle was in the ground idle position and immovable. Throttle control linkage in the engine bay was continuous to the fuel control unit (FCU) input lever. The FCU pointer indicator was at 0° (OFF).
Clean, clear liquid consistent with Jet A fuel was observed from the airframe fuel filter to the engine fuel pump, FCU, and fuel spray nozzle.
The N1 rotor was continuous from compressor impeller through engine gearbox, starter generator, fuel pump, and FCU. The gas producer turbine rotor did not turn when the N1 rotor was rotated. The N2 rotor was continuous from the 4th-stage power turbine rotor to the output driveshaft, but an audible rubbing or scraping sound was heard when rotated.
The engine was removed and transported to a manufacturer facility for further examination. Disassembly revealed that the spur adapter gearshaft (SAG) O-rings were not present in either the forward or aft O-ring grooves.
The turbine-to-compressor coupling shaft was found fractured into three pieces. Heavy coking was observed in the forward and aft spline locations of the coupling shaft, and between the coupling shaft and the power turbine outer shaft. Extensive coking was noted upon removal of the power-turbine-to-pinion-gear coupling shaft. No evidence of coked oil was observed in the aft O-ring groove. Analysis of coked material revealed fluorocarbon rubber signatures consistent with O-ring material. A CT scan of the piccolo tube revealed coked material sufficient to restrict oil flow in one orifice and block the other orifice of the oil jet to the No. 3 bearing.
Medical and Pathological Information
The State of Montana, Department of Justice, Forensic Science Division performed the pilot's autopsy. According to the autopsy report, the cause of death was multiple blunt force injuries.
The FAA Forensic Sciences Laboratory performed toxicological testing. Diphenhydramine was detected at 118 ng/mL in subclavian blood and 675 ng/mL in urine. Acetaminophen was detected in heart blood and urine.
Diphenhydramine is a sedating antihistamine found in sleep aids and cold/allergy products. It can cause cognitive and psychomotor slowing and drowsiness, with warnings about driving and operating machinery. One driving simulator study showed a single dose impaired driving more than a blood alcohol level of 0.1 g/dL. The FAA states pilots should not fly within 60 hours of using diphenhydramine.
Additional Information
Allison Engines Commercial Engine Bulletin (CEB) A-72-3108, Rev. 3, September 15, 1985, states: Leakage through the spur adapter gearshaft and turbine-to-compressor coupling joint reduces oil flow for turbine shafting lubrication and cooling. Proper sealing allows sufficient oil flow to keep shafting temperature below carboning temperature. A damaged or wrong part number O-ring may allow significant cooling oil flow to leak back into the gearbox, reducing flow between concentric shafts. This reduced flow is not sufficient to cool shafting below oil carboning temperatures. Carbon deposits can build up until rub occurs, potentially causing coupling or shaft failure.
The engine maintenance manual current at the last overhaul in March 2019 contained a caution: Failure to replace missing or damaged forward and aft spur adapter gearshaft seal ring and packing can cause too much carbon build-up and can cause sudden engine stoppage.
According to the FAA Rotorcraft Flying Handbook, a turning autorotation should be established at recommended airspeed at 700 ft AGL parallel to the touchdown area. FAA pamphlet P-8740-71 states a minimum altitude of 700 ft AGL with entry on downwind abeam the touchdown point for a 180° autorotation.
The helicopter's Height-Velocity performance chart indicated that, in general, pilots should avoid operations below 600 ft agl and below 65 knots when above 4,150 lbs gross weight, and below 500 ft agl and below 45 knots when below 4,150 lbs.