SEA97FA196

While performing external load logging operations, the helicopter's main rotor separated from the helicopter and the helicopter crashed. Metallurgical examination of the main rotor mast revealed a fatigue fracture in the upper spirolox groove in the damper support splines. Based on the mast's historical service record, the fractured mast, a Bell part number 204-011-450-001 mast assembly with a part number '204-040-466-9' mast tube (not a valid part number according to Bell), had 4,006.7 hours at the time of the mast separation. An FAA airworthiness directive (AD), AD 97-14-12, applicable to Bell 204B, 205A, 205A-1, 205B, and 212 civil helicopters but not to surplus military UH-1 helicopters, limited the life of part number 204-011-450-001 masts to 6,000 flight hours or a retirement index number (RIN) of 300,000 (the fractured mast had a RIN of 200,335, computed according to the method specified by that AD.) The most recent main rotor mast AD directly applicable to the accident helicopter, AD 89-17-03, limited mast life to 15,000 hours and contained no requirement to establish or track RIN. The 'thin-walled' Bell part number 204-040-366-9 mast tubes installed in part number 204-011-450-001 masts were ordered removed from U.S. Army UH-1 helicopters by a 1984 U.S. Army emergency safety-of-flight message.

***This report was modified on July 5, 2013.

History of flight

On August 27, 1997, approximately 0915 Pacific daylight time, a US Helicopter (formerly Southern Aero) UH-1B restricted-category former military helicopter, N896W, operated by Horizon Helicopters Inc. of Rancho Murieta, California, collided with terrain after experiencing a main rotor mast fracture while conducting a 14 CFR 133 external-load aerial logging operation in the Mount Baker-Snoqualmie National Forest approximately 4 nautical miles north-northeast of Skykomish, Washington. The helicopter was substantially damaged in the accident, and the airline transport pilot, who was the helicopter's sole occupant, was fatally injured. The local flight had departed from a service landing near the accident scene. Visual meteorological conditions (with showers over the mountains) were being reported at Paine Field, Everett, Washington (approximately 38 nautical miles west of the accident site), and no flight plan had been filed.

Logging ground crew members in the vicinity of the accident stated that the helicopter had been airborne for approximately 45 minutes at the time of the accident. The ground crew members reported that immediately prior to the accident, the pilot attempted to lift a load of two logs but that the load became hung up in trees. The pilot then released the load, and the ground crew subsequently started to reconnect the larger of the two logs to the helicopter's external load line. The witnesses then reported that they heard a loud "pop" and the engine RPM increasing, upon which the helicopter fell to earth. The helicopter's fuselage came to rest inverted on sloping terrain. One of the witnesses reported that he saw the helicopter's main rotor continue to fly for 30 to 45 seconds after the helicopter crashed.

The accident occurred during the hours of daylight at 47 degrees 47.6 minutes North and 121 degrees 18.4 minutes West.

Aircraft information

The helicopter, UH-1B serial number 62-4576, was originally manufactured by the Bell Helicopter Company for the U.S. Army (the Bell Helicopter data plate on the accident helicopter indicated that the aircraft was Bell manufacturer's serial number 636), and received civil type certification under FAA type certificate H3SO, originally held by Southern Aero Corporation of Ozark, Alabama (the type certificate is currently held by US Helicopter Inc. of Ozark, Alabama.) The accident helicopter had been issued a special restricted-category FAA airworthiness certificate, with an original date of issuance of June 28, 1990. The operator reported that at the time of the accident, the airframe total time was 9.176.6 hours.

The accident helicopter had been modified by installation of an AlliedSignal (formerly Lycoming) T53-L-13B turboshaft engine per FAA supplemental type certificate (STC) number SR00073DE, held by Garlick Helicopters Incorporated of Hamilton, Montana. The T53-L13B engine is rated at 1,400 shaft horsepower (SHP) versus the 1,100 SHP of the originally installed T53-L-11D engine. Garlick Helicopters, the STC holder, stated to the NTSB that the STC installation is designed to enhance high-altitude and hot-day performance by virtue of the T53-L-13B engine being capable of producing maximum allowable horsepower to a higher density altitude than the T53-L-11D engine, but that the drivetrain remains limited to 1,100 SHP or 50 PSI torque pressure, and that no increase in drivetrain loads is authorized for a -13 engine installation.

A hand-inscribed data plate on the accident helicopter's main rotor mast, which did not bear an inspector's stamp, identified the mast as a Bell part number "204-011-450-1", serial number H9-2007. The most reliable manufacture records kept by Bell Helicopter Textron on the masts indicate that the actual serial number of the mast was N9-2007, and that this mast was manufactured by Textool, a supplier of the masts to Bell Helicopter Textron, and originally installed on a UH-1D helicopter manufactured for the U.S. Army on February 16, 1967. The "thin-walled" mast tubes (Bell part number 204-040-366-9) installed in Bell part number 204-011-450-001 mast assemblies were ordered removed from U.S. Army helicopters in a Safety of Flight Emergency Message issued by the Army (CDRAVSCOM message UH-1-84-08) on July 18, 1984. This message ordered the immediate grounding of all U.S. Army UH-1 series helicopters equipped with "thin-walled" part number 204-040-366-9 mast tubes until "thick-walled" mast tubes (Bell part number 204-040-366-15) were installed. The message did not state why the action was being ordered, beyond stating that a "command decision" had been made to ground all UH-1s equipped with thin-walled masts until thick-walled masts were installed.

According to the commercial historical service record for this mast supplied by the operator, the mast assembly was equipped with a part number "204-040-466-9" mast tube (according to Bell, this is not a valid Bell part number.) The mast was initially installed on a civil-registered helicopter (N394HP) on November 15, 1988, at 1,285.0 hours since new and zero hours since overhaul. The main rotor mast commercial historical service record indicated that the mast was later installed on a UH-1B helicopter, N87729, at the time that helicopter was involved in an accident at Smith Cove, Alaska, on May 22, 1991 (NTSB accident number ANC91LA059). In that accident, according to the NTSB's brief of accident, "the pilot stated that he was maneuvering just above the tree level when the engine quit." A copy of the operator's accident report (NTSB Form 6120.1/2) indicated that the aircraft landed "very hard" and that the main rotor struck nearby tree stumps in that accident. Post-accident examination of N87729's engine revealed that a turbine wheel had disintegrated in flight, resulting in a total loss of engine power. The main rotor mast historical service record indicated that the mast was removed from N87729 on June 30, 1991, at 1,500.8 hours since new and 215.8 hours since overhaul. The reason for this removal was given on the commercial historical service record as "hard landing." The mast was subsequently overhauled by Garlick Helicopters Incorporated on June 9, 1992, and was subsequently reinstalled on another helicopter, N94NW, on June 11, 1993, prior to being installed on the accident helicopter.

The mast was last overhauled on June 6, 1996, and was installed on the accident helicopter on June 15, 1996, at 8,049.6 aircraft hours. The commercial historical service record indicated that at the time of installation on N896W on June 15, 1996, the mast had 2,879.7 hours since new. At the time of the accident, the main rotor mast had accumulated an additional 1,127.0 hours for a total time since new of 4,006.7 hours.

The FAA-approved maintenance manual for the Southern Aero UH-1B, U.S. Army Technical Manual (TM) 55-1520-219-20, specifies that the mast be replaced in the event of "sudden stopping of [the] main rotor with power on." TM 55-1520-219-20 specifies that after any hard landing (defined as "any incident in which the impact of the aircraft with the ground causes severe pitching of the main rotor, allowing static stops to severely contact the mast..." and which does not involve sudden stoppage), to "Inspect mast for indentation caused by hard contact and static stop for flattened or distorted condition." Hard landing evaluation criteria established by TM 55-1520-219-20 for masts are as follows:

(1) If inspection reveals yielding or deformation in the area which would be contacted by the main rotor static stops or other obvious damage, the mast assembly should be considered unserviceable and non- reparable.

(2) If post inspection does not reveal above discrepancies, the component should be returned to depot for evaluation overhaul.

On June 27, 1997 (two months before the accident involving N896W), t