Rockport, WA, USA
N6181A
BELL 206B
The commercial pilot reported that, after landing the helicopter on a flat area in remote, mountainous terrain, she reduced the throttle to the idle position and the left front seat passenger exited the helicopter. The pilot reported that, while the passenger was securing the door, the helicopter started to oscillate laterally. The pilot adjusted the flight controls and increased the throttle, and the lateral oscillations stopped. The passenger stated that the helicopter "bounced and lurched" while he was securing the door but that the tail rotor did not come in contact with any objects or terrain. The pilot subsequently shut down the engine and conducted a postflight inspection, which revealed substantial damage to the transmission system. The pilot stated there were no preimpact mechanical failures or malfunctions with the airframe or engine that would have precluded normal operation. Damage observed on the pylon mount assembly and main rotor mast was consistent with a mast bumping event, during which the static stops contact the mast due to excessive rotor blade flapping. During ground operations with the rotor turning, the main rotor may be affected by wind gusts and flap to its limits, resulting in a light static stop-to-mast contact. In such an event, mast bumping may manifest itself as a light shudder felt throughout the helicopter. The more extreme the flapping, the more severe the shudder. Mast bumping can also occur during ground operation if the cyclic is incorrectly positioned or is moved sufficiently to tilt the rotor disc to an extreme position.
On September 12, 2016, about 1345 Pacific daylight time, a high skid-equipped Bell 206BIII helicopter, N6181A, sustained substantial damage after landing in remote mountainous terrain on Mount Prophet in the North Cascades National Park, about 30 miles northwest of Rockport, Washington. The commercial pilot and the sole passenger sustained no injury. The helicopter was registered to, Tony's Trucking, Inc. Darrington, Washington, and operated by Hi Line Helicopters, Inc., Darrington, as a public aircraft flight under contract to the National Park Service (NPS). Visual meteorological conditions prevailed at the time of the accident, 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, about 1330. The pilot reported in the National Transportation Safety Board (NTSB) Accident/Incident Reporting Form 6120.1 that the purpose of the flight was to conduct external load operations into the Firn Lake and Skymo Lake areas on Mount Prophet. She reported that a NPS employee was onboard, where his duty was to depart the helicopter after landing, and assist with external load operations from the ground by attaching a long line to the helicopter and then wait to receive the external load. The helicopter arrived at Firn Lake about 1340 and the pilot conducted a high reconnaissance to locate a suitable area for landing and to also identify a suitable spot to place the external load. The pilot reported she selected a landing spot on the south end of the lake in a flat area. After the landing, the pilot brought the throttle to idle and the NPS employee departed the helicopter from the left seat in the cockpit. While the NPS employee was securing the cockpit door closed, the pilot reported the helicopter started to move with lateral oscillations. The pilot adjusted the flight controls, increased the throttle, and the lateral oscillations ceased. The pilot shut down the engine and a postflight inspection was conducted which revealed substantial damage to the transmission system. The pilot reported that the wind condition at the accident site was variable, 5 to 10 knots, and gusting plus or minus 5 knots. The pilot verified that there were no preimpact mechanical failures or malfunctions with the airframe or engine that would have precluded normal operation. During a telephone interview with the NTSB investigator-in-charge (IIC) on October 12, 2016 the pilot stated this was her first time landing at Firn Lake. After conducting the high reconnaissance, she executed an approach to a spot but did not fully set the helicopter down due to suitability concerns with the terrain. The pilot then repositioned the helicopter about 180 degrees to a different spot which was "fairly flat" and landed the helicopter. After the landing and reducing the throttle to idle with the cyclic centered, she discussed with the NPS employee for a few minutes their plan of action for the external loads and he departed the helicopter. The oscillations started as he was securing the cockpit door closed. The pilot stated she then moved the cyclic slightly forward to cease the oscillations, which appeared to make the oscillations worse. The throttle was increased, the oscillations ceased, and the engine was shut down. An air safety investigator (ASI) from the Department of Interior Office of Aviation Services (DOI OAS) reported to a NTSB ASI on September 23, 2016 that the operator was conducting a "call-when-needed" mission for the NPS when the accident occurred. The DOI OAS ASI further reported that after the accident, the helicopter was secured and transported as an external load via helicopter back to the operator's hangar in Darrington. The DOI OAS ASI also submitted on September 29, 2016 a written statement from the NPS employee who reported that the landing area utilized was flat and covered with vegetation and interspersed with basketball-sized boulders. The NPS employee further reported that while he was securing the cockpit door closed, the helicopter "bounced and lurched" and he visually confirmed that the tail rotor system did not strike an object or terrain. On September 21, 2016, an airworthiness inspector from the DOI OAS traveled to the operator's hangar to conduct an inspection of the airframe and engine. In a written report submitted to the DOI OAS, the inspector reported that he was not given full access by the operator to conduct a thorough airframe and engine inspection. The inspector performed a limited inspection and noted substantial damage sustained to the main rotor mast and to the transmission system. METEOROLOGICAL INFORMATION The closest official weather observation station is located at the Bellingham International Airport, Bellingham, Washington, about 55 miles to the west of the accident site. At 1353, an Aviation Routine Weather Report (METAR) was reporting, and stated in part: Wind 340 degrees at 6 knots; visibility 10 statute miles; clouds and sky condition, clear; temperature 75 degrees F; dew point 36 degrees F; altimeter 30.17 inHg. WRECKAGE AND IMPACT INFORMATION The damage photographs supplied by the DOI OAS displayed substantial damage to the main drive shaft forward coupling along 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 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. The 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 with the main rotor static stops as contact marks were present. The repair station assigned to repair the helicopter reported on November 3, 2016 in an inspection report that the main rotor pitch links made contact with the cowling and the swashplate inner ring made contact with the sleeve. TESTS AND RESEARCH Pylon Mount Assembly Testing The pylon mount assembly (Bell part number 206-030-539-101, Lord Corporation part number LB9-1301-1-1-V, serial number LK8458) was subjected to testing consistent with the dynamic requirements of the production acceptance testing produce at Lord Corporation in Erie, Pennsylvania with two aviation safety inspectors present from the Federal Aviation Administration Allegheny Flight Standards District Office (Pittsburgh, Pennsylvania). After the production acceptance testing procedure, a visual examination and disassembly was then performed. Lord Corporation reported that based on the dynamic testing and visual examination of the elastomer and contact area on the top cover, there was no fault found with this component. The dynamic testing results were acceptable considering the calendar age of the elastomer and the dynamic testing did not create additional damage to the part. Lord Corporation further reported that there was no elastomer debonding on the non-damaged top and bottom cover plate surface and there was no elastomer damage on any other molded surface of the bonded assembly. Pylon Mount Assembly History The pylon mount assembly was installed on the accident helicopter on May 17, 2011 (with a helicopter time in service of 6,634.9 hours). According to the aircraft maintenance log supplied by the operator, the pylon mount assembly was installed due to the previous isolation mount assembly being out of tolerance. ADDITIONAL INFORMATION Ground Mast Bumping A97W0130 (Transportation Safety Board of Canada), a Bell 206B accident, discusses ground mast bumping in the Bell 206 series. This report states in part: The Bell 206B helicopter is fitted with an underslung, semi-rigid, teetering, two-blade main rotor system. The teetering design allows the main rotor blades to flap to compensate for asymmetrical lift during flight. One static stop is mounted on either side of the main rotor hub to physically limit the amount of blade flapping. A condition known as mast bumping occurs if the static stops contact the mast, due to excessive blade flapping, during ground operations or in flight. During ground operations with the rotor turning, the main rotor may be affected by wind gusts and flap to its limits resulting in a light static stop to mast contact. In such an event, mast bumping may manifest itself as a light shudder felt throughout the helicopter. The more extreme the flapping, the more severe the shudder. Mast bumping will also occur during ground operation, if the cyclic is incorrectly positioned or is moved sufficiently to tilt the rotor disc to an extreme position. If the static stop to mast contact is severe, pronounced helicopter oscillations may develop and the helicopter can sustain substantial damage. The appropriate corrective action is to immediately reposition the cyclic, toward or near the neutral position so that the rotor disc resumes a flat position. On the ground, at idle RPM, the rotor disc is less stable and more susceptible to larger deviations due to flapping. Pylon Whirl and Spike Knock Conditions GAA15LA296, a Bell 206B accident, identified the pylon whirl and spike knock conditions that the Bell 206 series can be susceptible to. Background information on these two conditions can be found in the factual report for GAA15LA296. Slope Limitations According to Bell, there are no slope limitations for the Bell 206 series. The operator's manual for the U.S. Army OH-58A/C (a single engine, observation type military helicopter produced by Bell, similar to the Bell 206 series), Technical Manual 55-1520-228-10, discusses the OH-58A/C slope landing/takeoff limitations. This manual states in part: Slope operations shall be limited to slopes of 8 degrees or less. Caution is to be exercised for slopes greater than 5 degrees since rigging, loading, terrain, and wind conditions may alter slope landing capability.
The pilot's failure to maintain helicopter control while on the ground, which resulted in mast bumping.
Source: NTSB Aviation Accident Database
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