SAGO, WV, USA
N911RH
Bell 206B
A helicopter was transporting passengers to a remote site when it dropped in a flat attitude and impacted into trees. A witness said he watched as the helicopter approached and circled the local area. As the helicopter flew overhead for the second time, he waved his arms to attract the pilot's attention. As the helicopter continued past him again, he thought that they were taking aerial photographs of the site. He said that the helicopter appeared to be lower and slower on its second pass overhead. Examination of the engine revealed uncommanded engine shut downs. During bench testing, the power turbine governor failed to limit fuel flow, and during disassembly it was revealed that a machine self locking hex nut had backed off the muscles valve stem. Examination of the muscles valve components revealed that the threaded valve stem was below the minimum requirements for a UNJ thread, and the flank angle was too large. The turbine governor, with zero Time Since Overhaul, was installed on the engine in January 1997. The governor was last overhauled at the manufacturer's facility during May-June 1996.
HISTORY OF FLIGHT On October 12, 1997, about 1113 eastern daylight time, a Bell 206B, N911RH, was destroyed as it impacted the terrain in a wooded area near Sago, West Virginia. The certificated commercial pilot and three passengers received fatal injuries. The helicopter was owned and operated by Royale Helicopter Services, Incorporated, Burgettstown, Pennsylvania. Visual meteorological conditions prevailed and no flight plan was filed for the flight conducted under 14 CFR Part 135. According to a Anker Coal Group, Inc. employee, Royale Helicopter Services was hired to fly to coal production sites to obtain photographs for a new brochure. The helicopter departed Burgettstown, Pennsylvania, and flew to Morgantown Municipal Airport, Morgantown, West Virginia, where the President of Anker Coal Group, Inc., his wife, and a photographer boarded the helicopter. The helicopter picked up another company employee at his residence, and then, flew to the first site. Upon landing, everyone, except the pilot, exited the helicopter while the photographer took pictures for approximately 1 hour. One company employee did not re-board the helicopter, but drove a vehicle to the next location. The employee stated that at the new site, he watched as the helicopter approached and circled the local area. As the helicopter flew overhead for the second time, he waved his arms to attract the pilot's attention. As the helicopter continued past him again, he thought that they were taking aerial photographs of the site. The employee stated that the helicopter appeared to be lower and slower on its second pass overhead. The helicopter, approximately a quarter mile away, appeared to be making a left turn when it dropped in a flat attitude into the trees. The employee stated that he saw the tree limbs shaking, but was not certain that it impacted the trees. He quickly drove to the nearest telephone and called the local authorities. The accident occurred during the hours of daylight approximately 38 degrees, 54 minutes north latitude, and 80 degrees, 12 minutes west longitude. PERSONNEL INFORMATION The pilot held a commercial pilot certificate with ratings for airplane single and multiengine land, rotorcraft helicopter, and instrument airplane and helicopter. He also held a flight instructor certificate for rotorcraft-helicopter. The pilot held a mechanic certificate with ratings for airframe and powerplant. His most recent Federal Aviation Administration (FAA) Second Class Medical Certificate was issued on February 10, 1997. The pilot was the President, Chief Pilot, and Maintenance Officer of Royale Helicopter Services. He attended the Bell Helicopter customer training academy offsite location at Helicopter Aviation Services (HAS), Mount Pleasant, Pennsylvania, on October 23, 1996. On his pilot's registration form, he wrote that he possessed 15,000+ hours of flight time, with 9,000+ hours of total time in this model. The check pilot for the course wrote in the remarks section, "Completed 206B Recurrent Training. Good pilot-Safe in all maneuvers." AIRCRAFT INFORMATION The helicopter was a Bell 206B with an Allison 250-C20 engine. The last annual/100 hour inspection was completed on October 9, 1997. On the date of the inspection, the airframe had a total time of 5,970.6 hours, and the engine had a total time of 5,927.9 hours. WRECKAGE AND IMPACT INFORMATION The wreckage was examined at the accident site on October 13, 1997. The examination revealed that all major components of the helicopter were accounted for at the scene, and that the helicopter came to rest on its left side, on an approximate magnetic heading of 280 degrees, at a ground elevation of about 1,760 feet above mean sea level (MSL). The wreckage was located in a wooded area on a hill with a down slope of about 30 degrees. The nose of the helicopter was crushed inward, with all the Plexi-glas broken out. The landing skids were separated from the fuselage, and the fuel tank was ruptured with the surrounding soil soaked with fuel. The tail boom was bent downward, the tail rotor drive shaft was intact, with minimal damage to the tail rotor. One blade of the tail rotor was bent and the other blade was straight and stuck in the ground. The pitch change tube was fractured near the bend in the tail boom, but exhibited continuity to the tail rotor. Minimal damage was found to the leading edge of both the main and tail rotor blades, with score marks found on the underside of one main rotor blade. The engine remained intact exhibiting minimal damage, with the engine compartment found very clean and no signs of fuel or oil leakage. The fuel control throttle position was found near flight idle, and the throttle linkage was jammed at that position. The fuel shut off switch was found in the "on" position, and confirmed by sighting of the valve in the open position. Flight control continuity for the collective, cyclic, and anti-torque was confirmed. Fractures of the collective, cyclic, and anti-torque tubes, similar to overload, were found at the bottom of the control tunnel (broom closet). A small bag, containing a 5/64 inch Allen wrench, a copy of page 3-160 and 3-161 of the 250-C20 Series Operation and Maintenance Manual, a set of duck bill pliers, and a hemostat clamp, was found in the baggage compartment. On October 14, 1997, the wreckage was removed from the wooded area and taken to a hangar. The engine's gas producer and power turbine drive trains exhibited continuity, and rotated freely. The linear actuator was fractured at the forward clevis attachment point. A Chandler Evans (CECO) fuel system was on the engine. The fuel filter was clean, contained fuel, and the by-pass button was not popped. All fuel, air, and oil lines were tight, and no leaks detected. The main rotor drive shaft and the tail rotor segmented drive shaft exhibited minimal indications of rotation at impact. The engine was removed from the wreckage and transported to a test facility. MEDICAL AND PATHOLOGICAL INFORMATION An autopsy was performed on the pilot, on October 14, 1997, by James L. Frost, M.D., Deputy Chief Medical Examiner, State of West Virginia, South Charleston, West Virginia. The toxicological testing report revealed negative for drugs and alcohol. TEST AND RESEARCH The engine was received at Airwork Corporation, Millville, New Jersey and found in acceptable condition for operational testing. On November 4, 1997, the engine was started, accelerated, and stabilized at 100% N2. A dynometer load was applied and at 91% N1, an uncommanded engine shut down occurred. The uncommanded engine shut down repeated itself on subsequent tests at the same power setting/parameters. After telephone discussions with representatives from CECO, the power turbine governor unit and the fuel control were removed from the engine and taken to West Hartford, Connecticut, on November 5, 1997. On November 6, 1997, both the power turbine governor unit and the fuel control were bench tested "as received" at CECO. The power turbine governor was found to "allow the same quantity of fuel flow entering the governor to be discharged by the governor" and the fuel control was slightly above service limits. A systematic plan was organized to disassemble the governor. The disassembly began with the inspections of the mechanical linkage (lever assembly), speed sensor assembly and fixed orifice, and the main diaphragm assemblies. No anomalies were found. Inspection of the muscles valve assembly, which consisted of a diaphragm, diaphragm cup, threaded valve and a self-locking hex nut, revealed that the hex nut, P/N 94289, had backed off the threaded valve stem, with less than one turn of thread engagement remaining. The valve assembly was removed from the governor and taken to a binocular microscope for photos. After photos were taken, the nut fell off the threaded valve stem before the exact "as found" position was marked. The governor was reassembled utilizing a replacement muscles valve assembly and a spring, and tested on the flow bench. The governor responded normally, with slightly rich at higher speed settings, but the acceleration slope of the unit was within limits. After supplemental testing of the governor, other assemblies not previously inspected were checked and found in functional condition. The failed muscles valve assembly and its related spring were transported to the Materials Laboratory Division, National Transportation Safety Board, Washington, D.C. On December 8 and 9, 1997, additional testing was conducted at the test facilities of Allison Engine Company, Indianapolis, Indiana. The governor, with a replacement muscles valve assembly and spring, was placed on the engine, which was specially instrumented to record the parameters. Several test runs of the engine were performed to confirm normal function and performance of the core engine and verify the instrumentation installed. Then, a test of the engine with the muscles valve retaining nut backed off to its "as found" position was conducted. The engine was accelerated to maximum power, at which the engine experienced a droop in N2 rpm, and the measured fuel flow to the engine would not exceed 220 pph. The retaining nut was tightened to its normal location and during the final run, the engine functioned normally. ADDITIONAL INFORMATION Engine records showed that at an engine total time of 5,727.3 hours, the turbine governor, with zero Time Since Overhaul (TSO), was installed on January 22, 1997, by Helicopter Aviation Services (HAS). The time recorded at the accident site was 5,973.2 hours. Review of CECO's records indicated that the governor was received for maintenance four times. The last overhaul on the turbine governor was dated May to June 1996, at CECO's facility in West Hartford, Connecticut. The muscles valve removed from the turbine governor was machined, assembled and installed at that time. The turbine governor "muscles valve" components were examined by the Office of Research and Engineering, Materials Laboratory Division. According to the Materials Laboratory Factual Report, "the threaded valve stem appeared to be undamaged. The outside diameter was 0.110 inches (the specification requires 0.1120 - 0.1069), the root diameter was 0.0768 inches (the specification requires 0.0832 - 0.0798) and the pitch was 0.025 inches. The root radius was 0.002 inches (the specification requires 0.0045 - 0.0038) and the flank angles measured were between 62 and 63.5 degrees ("V" threads are 60 degrees). The nut was placed on the threaded valve stem and was found to rotate very easily, by hand, to the very end of the thread with no sign of resistance." These discrepancies indicate that the nut was either used previously on another threaded valve stem of the correct dimensions, or installed correctly, would not lock onto the threaded valve stem and loosened by the vibration of the helicopter. A 250-C20 Series Engine/Engine Fuel System Trouble Shooting guide was found by the Allison Engine representative. Section III was labeled "Power Turbine Governor Trouble Shooting, CECO Governor Model MC-40." The index section listed various Governor Components of which "Muscle Valve, Malfunctions" were numbered 2 and 3, and found on pages numbered 1 and 2. On page 1, listed under Governor Malfunction No. 2 was: Muscles Valve Remains in the Closed Position. Listed in step "d" were the results of this failure in flight: "(1) Fuel flow goes to minimum, (2) Low N1 RPM, (3) Low T.O.T., (4) Low Torque, (5) N2 under-speed if collective pitch is not decreased, (6) Power turbine governing is lost, (7) Extreme loss of power, (8) Powered flight not possible, (9) Auto-rotation landing is necessary." According to the Operation and Maintenance Manual, the 5/64 inch wrench found on site of the accident was for light off adjustment on the MC-40 fuel control, not the turbine governor. The helicopter wreckage, except the engine and caution warning panel, was released on October 16, 1997, to Mark C. Thompson, a representative of the owner's insurance company. The engine was released to the insurance company on January 5, 1998. The muscles valve components and the caution warning panel were released on March 17, 1998. Other participants in the investigation were: John Cheris FAA FSDO-03 Principal Inspector Scott Scheurich Allison Engine Company Senior Accident Investigator W./D. Hall Royale Helicopters Service Company Pilot/Secretary Russell Comstock Chandler Evans Company Customer Support Manager Ed Pawlina Chandler Evans Company Contract Engineer Rich Linsky Chandler Evans Company Senior Materials Engineer
The improper overhaul of the turbine governor by the manufacturer, which resulted in the loss of power.
Source: NTSB Aviation Accident Database
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