Gila Bend, AZ, USA
N175SF
Tamarack Helicopters Inc UH-1H
While the helicopter was maneuvering at low altitude after an agricultural application, its 90° tail rotor gearbox separated. The commercial pilot performed an autorotation, and the helicopter rolled over on impact. The gearbox had been installed about 300 flight hours before the accident. Postaccident examination revealed that the gearbox separation was the result of fatigue fracturing of its six attaching studs, which, once failed, liberated the gearbox from the input quill sleeve. Fatigue is typically the result of insufficient preload in the fastener often due to insufficient torque during installation or by the loss of preload after installation. Fretting wear on the studs and in the holes indicated that there were small amounts of relative movement between these components and thereby also indicated insufficient preload. However, the interface between the input quill housing and the vertical fin showed no significant fretting or wear. The locations of the main fatigue origins in the studs and the fretting location between the studs and fin holes were both consistent with normal torsional loads in the tail rotor drive system. The studs exhibited a large percentage of fatigue penetration before final stud separation and relatively small overstress zones. These signatures are indicative of a low mean stress and a large number of small tail rotor inputs (loads) consistent with repetitive aerial application flight profiles. The helicopter, which was originally designed and manufactured for the military, was never made commercially available. As such, individual owners and companies were responsible for certification and continued airworthiness of this and other surplus military aircraft. A review of National Transportation Safety Board records indicated two accidents involving similar failures on this helicopter type. Both were surplus military helicopters flying in the restricted category and used in highly repetitive cycle work that would require significant tail rotor inputs.
HISTORY OF FLIGHTOn October 5, 2016, about 0900 mountain standard time, a Tamarack Helicopters Inc. UH-1H, N175SF, rolled over during a forced landing near Gila Bend, Arizona, after the 90° tail rotor gearbox separated. The commercial pilot sustained minor injuries, and the helicopter was substantially damaged. The helicopter was registered to and operated by Farm Ag Enterprises, under the provisions of Title 14 Code of Federal Regulations Part 137 as an aerial application flight. Visual meteorological conditions prevailed, and no flight plan had been filed.The local flight departed a private airstrip about 0700. The pilot reported that he had just completed a series of passes over a cotton field, and was maneuvering the helicopter for a return to the fuel truck. The helicopter was moving at about 20 knots and climbing, when after reaching about 100 ft above ground level he heard a loud bang. The helicopter then began to spin to the right, and would not respond to foot pedal inputs, so he lowered the collective and initiated an autorotation. The helicopter was still spinning as it descended, and as it approached the ground the skids made contact with cotton bushes, and the helicopter rolled onto its left side. During the impact the tailboom partially detached from the aft bulkhead. The tail rotor assembly, which included both tail rotor blades and the outer case of the 90° gearbox, was located about 30 ft from the helicopter (See Figure 1). The input quill and sleeve assembly of the case remained attached to the vertical fin. (See Figure 2) Figure 1 – Tail Gearbox Rotor and Rotor Assembly Figure 2 – Vertical Fin with Input Drive Quill to 90° Gearbox AIRCRAFT INFORMATIONThe helicopter was manufactured in 1969 as a UH-1H, and after military services it was demilitarized, and ownership transferred to the Sheriff's Department of Seminole County, Florida. FAA records indicated that a condition of transfer was that the helicopters sole purpose be restricted to, "Airborne Fire Suppression, Air Search and Rescue, water/Marine Rescue, Disaster Response, SWAT Response, Medivac, Regional Support" In 2007, the helicopter was then sold to a private company which specialized in charter, forestry, and contract flight services. In January 2014 it was issued a special airworthiness certificate in the restricted category after being disassembled and converted through a supplemental type certificate (STC) held by Tamarack Helicopters. The helicopter was purchased by the accident operator on April 4, 2016, 279.1 flight hours after the conversion, and 20.4 hours before the accident. The helicopter was maintained under a continuous airworthiness inspection program, and according to the maintenance logbooks, the most recent inspection event was for a 50-hour inspection, on June 7, 2016, 68.8 flight hours before the accident. The most recent documented maintenance action requiring the disassembly of the 90° gearbox was for its overhaul in August 19, 2013, following which it was installed on the helicopter during the conversion to a Tamarack UH-1H. AIRPORT INFORMATIONThe helicopter was manufactured in 1969 as a UH-1H, and after military services it was demilitarized, and ownership transferred to the Sheriff's Department of Seminole County, Florida. FAA records indicated that a condition of transfer was that the helicopters sole purpose be restricted to, "Airborne Fire Suppression, Air Search and Rescue, water/Marine Rescue, Disaster Response, SWAT Response, Medivac, Regional Support" In 2007, the helicopter was then sold to a private company which specialized in charter, forestry, and contract flight services. In January 2014 it was issued a special airworthiness certificate in the restricted category after being disassembled and converted through a supplemental type certificate (STC) held by Tamarack Helicopters. The helicopter was purchased by the accident operator on April 4, 2016, 279.1 flight hours after the conversion, and 20.4 hours before the accident. The helicopter was maintained under a continuous airworthiness inspection program, and according to the maintenance logbooks, the most recent inspection event was for a 50-hour inspection, on June 7, 2016, 68.8 flight hours before the accident. The most recent documented maintenance action requiring the disassembly of the 90° gearbox was for its overhaul in August 19, 2013, following which it was installed on the helicopter during the conversion to a Tamarack UH-1H. ADDITIONAL INFORMATIONBell Helicopter Inc. was the original designer and manufacturer of the UH-1 series helicopter, which was built for military purposes and never commercially certificated. The FAA registration database lists about 450 UH-1H's currently registered in the US under more than 12 different manufacturers names, including Bell. The NTSB accident database included two other accidents involving UH-1 series helicopters with similar separations of the 90° gearbox. A gearbox separation with similar fatigue damage to the gearbox attachments studs occurred on September 21, 1999 involving a Garlick Helicopters version of the UH-1H, which was being utilized for logging operations. The probable cause was determined to be, "Fatigue failure of the 90-degree gearbox attachment studs, resulting in separation of the tailrotor and 90-degree gearbox from the vertical fin". See NTSB accident number SEA99LA171. On October 7, 2012 the tailrotor gearbox separated from a Garlick Helicopters UH-1H during firefighting operations with a Bambi bucket. Data was limited because the gearbox and four studs were not recovered. The NTSB materials laboratory report indicated an uneven use of sealant at the gearbox mounting flange, and a resultant lack of mounting bolt preload. The probable cause was determined to be, "The separation of the tail rotor gearbox and tail rotor assembly as a result of fatigue to the tail rotor gearbox input quill assembly attachment studs." See NTSB accident number WPR13FA006. A representative from Tamarack Helicopters stated that the 90° gearbox is typically removed and overhauled after 1,200 flight hours, or if grey material indicative of fretting is observed at any of the gearbox mating surfaces. He further stated that fretting damage to one or two attachment studs along with associated ovaling of the pass-through holes on the vertical fin structure was not uncommon (and that maintenance documents exist for quantifying and repairing such damage), but he has never seen damage to all mounting studs before. TESTS AND RESEARCHThe tail rotor 90° gearbox, input quill, and upper portion of the vertical fin and sleeve assembly were examined at the NTSB Office of Research and Engineering, Materials Laboratory Division. A complete report is contained in the public docket. Examination revealed that the six studs which attached the gearbox case to the input quill sleeve assembly on the fin had fractured at the gearbox housing joint line. The threaded portion of the fractured studs remained within the gearbox case. All six stud fracture surfaces displayed topographies and arrest lines indicating bending fatigue cracking. In each case, the fatigue initiated in the thread root radii on the same side of each stud relative and perpendicular to the center of the bolt hole radius. The fatigue cracks propagated diametrically across each stud terminating in small overstress regions, with a small area of secondary reversed bending fatigue cracking at the opposite side of each stud. The fatigue cracks penetrated between 89% and 98% of each stud cross section. (See Figure 3) Figure 3 – Threaded Portions of Fractured Studs Within the Gearbox Case. (Excerpt from the NTSB Materials Laboratory Report) Magnified examinations of the corresponding stud sections from the vertical fin revealed fracture areas with fatigue cracks that had initiated at either the first, second or third full thread from the grip portion of the stud. The thread roots were smooth and typical of rolled thread roots, and yellow paste material consistent with zinc chromate paste was present in all thread roots. The unthreaded grip sections of all studs exhibited fretting wear on the side opposite the main fatigue crack origin areas. In most cases the wear scars corresponded to the length of the stud that passed through the vertical fin structure, and in some instances, was present on the adjacent thread crests. The locations corresponding to stud passage on the fin structure were worn and slightly out-of-round (See Figure 4). Examination of the interior of the fin holes also revealed wear and damage corresponding to the fretting on the studs. Figure 4 - Stud Passage on Fin Structure. (Excerpt from the NTSB Materials Laboratory Report) The interface between the input quill sleeve housing and the vertical fin showed no significant fretting or wear. According to the helicopters illustrated parts manual, the studs were 2.875 inches long, and the AN126420 type. Three studs were tested for hardness and material composition. All met the appropriate specifications required by AN126420.
The separation of the 90° tail rotor gearbox assembly as a result of fatigue to the tail rotor gearbox input quill assembly attachment studs due to the helicopter experiencing highly repetitive agricultural flight profiles for which it was not designed.
Source: NTSB Aviation Accident Database
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