SKYKOMISH, WA, USA
N896W
Southern Aero UH-1B
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), the FAA issued Priority Letter Airworthiness Directive (AD) 97-14-12, applicable to Bell model 204B, 205A, 205A-1, and 212 civil helicopters but not to UH-1 or other type surplus military helicopters originally manufactured by Bell. This AD superseded an earlier AD, 89-02-07, and mandated the following actions with regard to applicable helicopters equipped with Bell part number 204-011-450-001 and other main rotor masts: creation of component history cards or equivalent records using a Retirement Index Number (RIN) system; use of a specified method given in the AD for tracking increases to the accumulated RIN; and retirement of the mast at a specified RIN. For helicopters equipped with part number 204-011-450-001 masts, the AD directed that the RIN initially be computed by multiplying total flight hours on the mast to date by 50, and mandated removal of the mast on or before attaining 6,000 hours time in service or an accumulated RIN of 300,000, whichever occurred first. Compliance with AD 97-14-12 was required before further flight of Bell 204B, 205A, 205A-1, and 212 helicopters, unless accomplished previously. Using the RIN computation method specified by AD 97-14-12, and the reported time in service on the mast which fractured in the accident, the fractured mast had accumulated a RIN of 200,335 at the time of the accident. AD 97-14-12 was subsequently superseded by AD 98-24-15, which was published as a final rule with request for comments in the Federal Register on November 23, 1998. This AD, which became effective December 8, 1998, contained requirements generally identical to those in AD 97-14-12 but stated that it added Bell model 205B helicopters to the applicability (AD 97-14-12 also stated that it added Bell 205B helicopters to the applicability of the original AD, 89-02-07.) The new AD also referenced Bell Helicopter Textron Alert Service Bulletins (ASBs) published prior to the Bell Helicopter Textron ASBs referenced in AD 97-14-12. The most recent AD directly applicable to Southern Aero UH-1B helicopters equipped with part number 204-011-450-001 main rotor masts was AD 89-17-03, which became effective September 5, 1989. This AD mandated removal of masts from service at 15,000 hours time in service for those masts with less than 14,900 hours time in service on the effective date of the AD. AD 89-17-03 does not contain any requirement to establish or track RIN on the masts, nor to retire the masts at any particular RIN. The part number 204-011-450-001 main rotor masts have external dimensions that allow them to be interchanged with certain other Bell main rotor masts, but differs from those masts in having a thinner mast tube wall thickness. Garlick Helicopters Incorporated, which holds a type certificate for UH-1B former surplus military helicopters as did Southern Aero, reported to the NTSB that in response to the accident involving N896W, it issued Alert Service Bulletin UH1-97-06 on September 26, 1997, directing removal of all part number 204-011-450-001 main rotor mast tubes from service on Garlick UH-1 type helicopters. The helicopter's maximum gross weight was 8,500 pounds, and its external load limit was 3,600 pounds. The maximum allowable external load weight for the accident helicopter further varied according to fuel load, from 2,838 pounds maximum at 900 pounds fuel to 3,538 pounds maximum at 200 pounds fuel. At the estimated fuel weight on board at the time of the accident (approximately 250 pounds), the maximum external load weight is 3,488 pounds. One of the logging ground crew members, who witnessed the accident, estimated the total weight of the two Western hemlock logs the pilot initially tried to lift at 3,100 pounds, with the larger of the two (which the pilot was to lift in the second attempt) weighing approximately 2,400 to 2,500 pounds. WRECKAGE The helicopter wreckage was examined at the accident site by investigators from the NTSB and Bell Helicopter Textron on August 28, 1997, and in a follow-up examination at the facilities of Rotor-Technics, Incorporated, Renton, Washington, on September 9, 1997. The crash site was on sloping terrain in the wooded area where the logging operation had been in progress. The terrain sloped downhill in a generally west-to-east direction in the accident area. Trees in the area were estimated to be approximately 120 to 130 feet tall. Investigators found the helicopter's main rotor assembly approximately 50 yards north-northeast of the main wreckage. The main rotor assembly was present in its entirety; both main rotor blades remained attached to the rotor hub and the top section of the fractured main rotor mast. The main rotor was present from tip to tip, with no readily apparent evidence of a foreign object strike prior to the mast separation. The main helicopter wreckage, consisting generally of the complete helicopter less the main rotor assembly and top of the fractured main rotor mast, lay inverted with its nose on a heading of 152 degrees magnetic. The top of the fuselage and cockpit area was crushed inward. The tail boom of the helicopter was detached from the main fuselage at the rear of the main fuselage, and was also broken in mid-boom, with the broken tail boom sections laying generally in their normal position in relation to the main fuselage. The tail rotor assembly, along with a portion of the 90-degree gearbox, was separated from the vertical fin. The tail rotor assembly and 90-degree gearbox section remained together as an integrated assembly, with the exception of one tail rotor blade being broken in two. The separated tail rotor and 90-degree gearbox components lay approximately adjacent to the helicopter's vertical fin. The helicopter main wreckage was approximately at the south end of two generally north-south oriented, freshly cut felled Western hemlock logs, which the logging ground crew reported were the two logs the pilot initially tried to lift during the accident sequence. These two logs straddled a standing tree, which was observed to have branches freshly stripped off approximately its lower half (i.e., to a height of approximately 60 feet above ground level.) The larger of the two logs was measured to be 95 feet long and tapered from 14 inches diameter at its base to 4 inches diameter at its top. The smaller of the two logs was broken into two sections. One section was measured to be 32 feet long, tapering from 8 inches diameter at its base to 7 inches diameter at the break. The other section was measured to be 20 feet long and tapered from 7 inches diameter at the break to 6 inches diameter at the top. The helicopter's external load line remained attached to the helicopter. From the helicopter, the line went approximately 60 to 70 feet up into trees, then back down to ground level. The external load hook was on the ground adjacent to the logs, with one end of a choker attached. In the cockpit, the helicopter's external load indicator was captured at an indication of approximately 1,100 pounds. Other than the fractured main rotor mast, no evidence of preimpact breaks in continuity of engine-to-main rotor power transmission, engine-to-tail rotor power transmission, or main or tail rotor flight control linkage was noted. No evidence of preimpact airframe structural failure other than the fractured main rotor mast was noted. No evidence of an inflight loss of engine power was noted. There was no evidence of fire anywhere in the wreckage. MEDICAL AND PATHOLOGICAL INFORMATION An autopsy on the pilot was performed by the King County, Washington, Medical Examiner's Office, Seattle, Washington, on August 28, 1997. The cause of death was given as extensive cerebral and visceral lacerations of the head, chest, abdomen, and extremities. Toxicological tests on the pilot were performed by the FAA Civil Aeromedical Institute (CAMI), Oklahoma City, Oklahoma. The CAMI toxicology tests screened for carbon monoxide, cyanide, ethanol, and drugs, and detected the following drugs: 0.063 ug/ml chlorpheniramine in blood; chlorpheniramine in urine; 0.009 ug/ml diphenhydramine in blood; diphenhydramine in urine; pseudophedrine in blood and urine; phenylpropanolamine in blood and urine; and ephedrine in urine. No carboxyhemoglobin, cyanide, or ethanol was detected. TESTS AN
A fatigue fracture in the main rotor mast, which resulted in an in flight separation of the main rotor from the helicopter. Contributing to the accident was insufficient Federal Aviation Administration airworthiness standards and requirements for the aircraft.
Source: NTSB Aviation Accident Database
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