Accident Overview
On September 12, 2016, at about 1345 Pacific daylight time, a Bell 206BIII helicopter registered as N6181A sustained substantial damage after landing in remote mountainous terrain on Mount Prophet in the North Cascades National Park, approximately 30 miles northwest of Rockport, Washington. The helicopter was owned by Tony's Trucking, Inc., of Darrington, Washington, and operated by Hi Line Helicopters, Inc., as a public aircraft flight under contract to the National Park Service (NPS). Visual meteorological conditions prevailed, and company flight following procedures were in effect. The flight originated from a helicopter landing zone near Diablo Lake, about 8 miles south of the accident site, at approximately 1330.
The purpose of the flight was to conduct external load operations into the Firn Lake and Skymo Lake areas on Mount Prophet. An NPS employee was on board to assist with external load operations from the ground. After landing at Firn Lake, the pilot reduced the throttle to idle, and the NPS employee exited the helicopter from the left seat. While securing the cockpit door, the helicopter began to move with lateral oscillations. The pilot adjusted flight controls and increased the throttle, which caused the oscillations to cease. The engine was then shut down, and a postflight inspection revealed substantial damage to the transmission system. The pilot reported that the wind at the site was variable at 5 to 10 knots with gusts of plus or minus 5 knots, and that there were no preimpact mechanical failures or malfunctions.
Meteorological Information
The closest official weather observation station was at Bellingham International Airport, Bellingham, Washington, about 55 miles west of the accident site. At 1353, an Aviation Routine Weather Report (METAR) reported wind from 340 degrees at 6 knots, visibility 10 statute miles, clear skies, temperature 75°F, dew point 36°F, and altimeter 30.17 inHg.
Wreckage and Impact Information
Damage photographs supplied by the Department of Interior Office of Aviation Services (DOI OAS) showed substantial damage to the main drive shaft forward coupling, with the forward boot separated from the coupling. The main drive shaft appeared to have contacted the forward firewall, and coupling grease was scattered throughout the aft transmission area. The forward coupling had contacted the top of the isolation mount, and various metal shavings were present. The transmission drag pin spike mount was sheared from the lower transmission housing, and the four drag pin retaining studs were sheared off. The aft transmission deck was damaged, drag pin spike witness ring retaining rivets were sheared from the transmission deck, and the aft drag pin spike witness hole was torn from the transmission deck. The main rotor mast had contact marks with the main rotor static stops. Additionally, the repair station reported that the main rotor pitch links made contact with the cowling and the swashplate inner ring made contact with the sleeve.
Tests and Research
The pylon mount assembly (Bell part number 206-030-539-101) was subjected to dynamic testing consistent with production acceptance testing at Lord Corporation in Erie, Pennsylvania, with FAA inspectors present. After testing, the assembly showed no fault; the dynamic testing results were acceptable given the calendar age of the elastomer, and no elastomer debonding or damage was found on the non-damaged surfaces. The pylon mount assembly had been installed on the accident helicopter on May 17, 2011 (at 6,634.9 hours) due to the previous isolation mount assembly being out of tolerance.
Additional Information
The investigation referenced other Bell 206 accidents (A97W0130 and GAA15LA296) discussing ground mast bumping, pylon whirl, and spike knock conditions. The report noted that the Bell 206 has an underslung, semi-rigid, teetering, two-blade main rotor system, and that mast bumping can occur during ground operations with rotor turning if the static stops contact the mast due to excessive blade flapping or incorrect cyclic positioning. The report also stated that according to Bell, there are no slope limitations for the Bell 206 series, but the U.S. Army OH-58A/C manual limits slope operations to 8 degrees or less.