Saturday, May 18, 2019

Bell UH-1H, 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, N175SF: Accident occurred October 05, 2016 in Gila Bend, Maricopa County, Arizona

The National Transportation Safety Board did not travel to the scene of this accident.

Additional Participating Entity:
Federal Aviation Administration / Flight Standards District Office; Scottsdale, Arizona

Aviation Accident Factual Report - National Transportation Safety Board:

Investigation Docket  - National Transportation Safety Board:

Location: Gila Bend, AZ
Accident Number: WPR17LA002
Date & Time: 10/05/2016, 0900 MST
Registration: N175SF
Aircraft: Tamarack Helicopters Inc UH-1H
Aircraft Damage: Substantial
Defining Event: Part(s) separation from AC
Injuries: 1 Minor
Flight Conducted Under: Part 137: Agricultural 

On 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

Pilot Information

Certificate: Commercial
Age: 33, Male
Airplane Rating(s): Single-engine Land
Seat Occupied: Right
Other Aircraft Rating(s): Helicopter
Restraint Used: 4-point
Instrument Rating(s): Helicopter
Second Pilot Present: No
Instructor Rating(s): Helicopter
Toxicology Performed: No
Medical Certification: Class 2 With Waivers/Limitations
Last FAA Medical Exam: 06/01/2016
Occupational Pilot: Yes
Last Flight Review or Equivalent: 02/15/2016
Flight Time:  3778.8 hours (Total, all aircraft), 118.6 hours (Total, this make and model), 3714 hours (Pilot In Command, all aircraft), 64 hours (Last 90 days, all aircraft), 17.9 hours (Last 30 days, all aircraft), 1.5 hours (Last 24 hours, all aircraft)

Aircraft and Owner/Operator Information

Aircraft Make: Tamarack Helicopters Inc
Registration: N175SF
Model/Series: UH-1H
Aircraft Category: Helicopter
Year of Manufacture: 1969
Amateur Built: No
Airworthiness Certificate: Restricted
Serial Number: 69-16713
Landing Gear Type: High Skid;
Seats: 2
Date/Type of Last Inspection: 06/07/2016, Continuous Airworthiness
Certified Max Gross Wt.: 9500 lbs
Time Since Last Inspection: 69 Hours
Engines: 1 Turbo Shaft
Airframe Total Time: 5157.9 Hours as of last inspection
Engine Manufacturer: HONEYWELL
ELT: Not installed
Engine Model/Series: T5313B
Registered Owner: Farm Ag Enterprises
Rated Power: 1400 hp
Operator: Farm Ag Enterprises
Operating Certificate(s) Held: Agricultural Aircraft (137)
Operator Does Business As:
Operator Designator Code: 7FAG 

The 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.

Meteorological Information and Flight Plan

Conditions at Accident Site: Visual Conditions
Condition of Light: Day
Observation Facility, Elevation: KBXK, 1021 ft msl
Distance from Accident Site: 27 Nautical Miles
Observation Time: 1615 UTC
Direction from Accident Site: 5°
Lowest Cloud Condition: Clear
Visibility:  10 Miles
Lowest Ceiling: None
Visibility (RVR):
Wind Speed/Gusts: 10 knots /
Turbulence Type Forecast/Actual: /
Wind Direction: 90°
Turbulence Severity Forecast/Actual:
Altimeter Setting: 29.89 inches Hg
Temperature/Dew Point: 24°C / 1°C
Precipitation and Obscuration: No Obscuration; No Precipitation
Departure Point: Gila Bend, AZ
Type of Flight Plan Filed: None
Destination: Gila Bend, AZ
Type of Clearance: None
Departure Time: 0700 MST
Type of Airspace: Class G

Wreckage and Impact Information

Crew Injuries: 1 Minor
Aircraft Damage: Substantial
Passenger Injuries: N/A
Aircraft Fire: None
Ground Injuries: N/A
Aircraft Explosion: None
Total Injuries: 1 Minor
Latitude, Longitude: 32.975556, -112.728611 (est) 

Tests And Research

The 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.

Additional Information

Bell 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.

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