N988BBell 206-L4 · 2005

On April 26, 2024, at 0659 mountain daylight time, a Bell 206-L4 helicopter, N988B, was destroyed when it was involved in an accident near Anaconda, Montana. The pilot was fatally injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 137 aerial application flight.

The pilot had been conducting flights to distribute fertilizer from the same staging location for several days. The staging area was surrounded by hilly terrain. An employee of the operator, who was performing ground support duties, met the pilot at their base at Deer Lodge-City-Municipal Airport (38S), Deer Lodge, Montana, about 0545 on the day of the accident. The employee then drove a support vehicle to the staging area about 1 mile southeast of Anaconda, Montana.

Onboard GPS data showed that the helicopter departed 38S about 0613 and proceeded to overfly the intended application area before proceeding to the staging area. Security camera video, taken from cameras located about 0.5 miles north of the staging area, captured the helicopter arriving and landing at the staging area at 0633. The pilot and ground crewmember then conducted a safety briefing with a customer representative. According to the representative, the wind was calm during the briefing and remained calm for several hours that morning.

After the briefing, the helicopter lifted off at 0643 and maneuvered over the load truck. The helicopter departed the staging area with the first load of fertilizer about 0644, flew to the west and exited the view of the security camera. The helicopter re-entered the camera’s field of view from the west, flew past the south side of the staging area, and turned to a northwest heading as it approached the load truck before completing a second onload of fertilizer. The helicopter departed the staging area the second time about 0650, flew out of the field of view to the west, and returned into view from the west at 0656.

The helicopter approached the load truck a third time heading to the northwest, flying an approach similar to the previous approach. The helicopter appeared to hover over or near the load truck for 2 to 3 seconds, then turned and flew away in an easterly direction. The helicopter reapproached the load truck and completed a third onload of fertilizer. The helicopter then departed and climbed to the west. The helicopter reached about 150 ft above the ground and 40 kts groundspeed when the helicopter rotated about 180° to the left and descended rapidly until it went out of view of the security camera behind terrain. There were no known witnesses to the accident sequence.

The ground crewmember, who was in regular communication with the pilot, attempted to contact the pilot via radio when he did not return when expected; however, he received no response. The customer representative was conducting other work duties from his truck, which was positioned southwest of the load truck. He also thought it strange that he hadn’t heard the helicopter for a while and drove to a different location to observe the area. During the drive, he saw the helicopter in a nearby gully. He drove back to pick up the ground crew member, responded to the accident site, and initiated an emergency call to 911.

The helicopter was equipped with a bubble window, which was installed on the left door. The pilot was operating the helicopter from the left seat position. A spreader was suspended beneath the helicopter using a 25-ft long line attached to a load hook installed on the belly of the helicopter. The pilot could open the load hook and release the load in flight in the event of an emergency.

A review of the engine logbooks revealed that the spur adapter gearshaft was last accessible when the engine was overhauled in March, 2019, at an engine total time of 6,328.1 hours, which was 1,414.1 hours before the accident.

The helicopter was equipped with a bubble window, which was installed on the left door. The pilot was operating the helicopter from the left seat position. A spreader was suspended beneath the helicopter using a 25-ft long line attached to a load hook installed on the belly of the helicopter. The pilot could open the load hook and release the load in flight in the event of an emergency.

A review of the engine logbooks revealed that the spur adapter gearshaft was last accessible when the engine was overhauled in March, 2019, at an engine total time of 6,328.1 hours, which was 1,414.1 hours before the accident.

Examination of the accident site revealed that the helicopter came to rest in a mostly upright position immediately adjacent to a flat and dry pond area in a gully about 855 ft southwest of the load truck (see figure 1).The fuselage came to rest oriented on a heading of about 060°. The bottom of the fuselage exhibited crushing deformation and the left skid was splayed outward. One main rotor blade was fracture-separated about 3 ft from the rotor attachment point just outboard of the blade doubler and the fracture surfaces exhibited downward deformation. The remainder of the blade was located about 15 ft forward of the helicopter. The second rotor blade remained attached to the rotor mast and exhibited some bending opposite of rotation. The spreader and 25-ft long line were located about 30 ft aft of the helicopter and were not attached to the helicopter, consistent with separation prior to impact. The long-line attachment shackle was unmarred and showed no visible damage. Fuel was observed leaking from the wreckage following the accident.

Figure 1. Helicopter Wreckage

The wreckage was recovered to a secure location and examined. 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. The collective levers displayed continuity to the elbow break on the left collective. All control tubes in the control closet displayed fractures consistent with overload. 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 anomalies were noted with the airframe or flight controls that would have precluded normal operation.

The engine was displaced vertically within the engine compartment, with all engine mount struts exhibiting varying degrees of damage. All but two of the mount struts were found fractured. Cockpit control continuity was not continuous from the collective lever and throttle twist grip through the respective linkages to the power turbine governor and fuel control unit due to impact damage. The pilot throttle was observed in the ground idle position and was immovable. The throttle control linkage, push-pull tubes, and bellcranks in the engine bay were continuous to the fuel control unit (FCU) input control lever. The FCU pointer indicator was positioned 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 the compressor impeller, 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 it was rotated.

The engine was removed from the wreckage and transported to a manufacturer facility for further examination. The engine was disassembled and the spur adapter gearshaft (SAG) O-rings were not present in either the forward or aft O-ring grooves during engine disassembly (see figure 2).

Figure 2. Spur Adapter Gearshaft

The turbine-to-compressor coupling shaft was found fractured into three pieces as shown in figure 3.

Figure 3. Turbine-to-compressor coupling shaft fractured. Photo courtesy Rolls-Royce.

Heavy coking was observed in the forward and aft spline locations of the turbine-to-compressor coupling shaft. Coking was noted 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 the coked material revealed fluorocarbon rubber signatures consistent with O-ring material. A computer tomography (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 (see figure 4).

Figure 4. Restricted oil jet orifices in piccolo tube. Photo courtesy Rolls-Royce.

Allison Engines Commercial Engine Bulletin (CEB) A-72-3108, Rev. 3, September 15, 1985, states:

Leakage through the spur adapter gearshaft & turbine-to-compressor coupling joint reduces oil flow available for turbine shafting lubrication and cooling. When the O-ring between these two shafts seal properly, sufficient oil flows through the concentric gap between the gas producer and power turbine shafts to keep the shafting temperature below the carboning temperature of the oil. However, a damaged or wrong part number O-ring may allow a significant portion of the cooling oil flow to leak back into the gearbox rather than flow between the concentric shafts. The reduced oil flow between the shafts is NOT SUFFICIENT to cool the shafting below oil carboning temperatures. Carbon deposits on the outside diameter of the turbine-to-compressor coupling and the inside diameter of the power turbine inner shaft and turbine-to-compressor coupling can build up until rub occurs. Continued rub could cause coupling or shaft failure.

The engine maintenance manual current when the engine was last overhauled in March, 2019, contained the following within the procedures to install the turbine assembly:

CAUTION – Failure to replace the 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, when performing a turning autorotation the pilot should, “Establish the aircraft on downwind at recommended airspeed at 700 feet AGL [above ground level], parallel to the touchdown area.”

FAA pamphlet P-8740-71, Planning Autorotations, states, “Use a minimum altitude of 700 feet AGL with an entry point on the downwind leg 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 500 ft agl and below 45 knots when below 4,150 lbs.

The State of Montana, Department of Justice, Forensic Science Division performed the pilot’s autopsy. According to the pilot’s autopsy report, his cause of death was multiple blunt force injuries.

The FAA Forensic Sciences Laboratory performed toxicological testing of postmortem specimens from the pilot. Diphenhydramine was detected at 118 ng/mL in subclavian blood and at 675 ng/mL in urine. Acetaminophen was detected in heart blood and urine.

Diphenhydramine is a sedating antihistamine medication widely available over the counter in multiple sleep aids and cold and allergy products. Diphenhydramine can cause cognitive and psychomotor slowing and drowsiness, and often carries a warning about driving and operating machinery. In one driving simulator study, a single dose of diphenhydramine impaired driving ability more than a blood alcohol level of 0.1 g/dL. The FAA states that pilots should not fly within 60 hours of using diphenhydramine, to allow time for it to be cleared from circulation.