# NTSB WPR21LA107: Hughes OH-6A, N6639R, Horseshoe Bend, ID, February 5, 2021 - Type: NTSB accident investigation - Classification: Accident · Serious injuries · Class 3 · Limited · Final report - Date: February 5, 2021, 14:00 local (21:00 UTC) - Place: Horseshoe Bend, ID - Coordinates: 44.00019, -116.13546 (estimated) - Nearby events: https://openairdocs.org/map.md?lat=44.0002&lon=-116.1355&mi=10 - Phase of flight: [Maneuvering](https://openairdocs.org/ntsb-phases/450) (OpenAirDocs' grouping) - Defining event: [Maneuvering-low-alt flying: Loss of engine power (partial)](https://openairdocs.org/ntsb-events/342) - Highest injury: Serious injuries - Injuries: 1 serious, 1 minor - Status: Completed - Report: Approved by Office Director - Investigation class: Class 3 - NTSB office: Western Pacific Region - Parties: Rolls Royce, Rolls-Royce Corporation, MD Helicopters, USDA APHIS Wildlife Services, FAA - NTSB case key (mKey): 102608 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=WPR21LA107 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/102608/pdf - NTSB docket: https://data.ntsb.gov/Docket?ProjectID=102608 - Page: https://openairdocs.org/ntsb/WPR21LA107 ## Probable cause A partial loss of engine power due to a leak in the fuel cell outlet valve. ## Findings NTSB's coded findings; those marked are part of the probable cause. - [Fuel system](https://openairdocs.org/ntsb-findings/01022800): Aircraft › Aircraft systems · Failure (probable cause); cases with it: [Hughes OH-6A, 1 case](https://openairdocs.org/ntsb-findings/01022800/cases.md?model=4470905), [All aircraft, 181 cases](https://openairdocs.org/ntsb-findings/01022800/cases.md) - [Terrain: mountainous/hilly terrain](https://openairdocs.org/ntsb-findings/03021010): Environmental issues › Physical environment · Effect on equipment; cases with it: [Hughes OH-6A, 1 case](https://openairdocs.org/ntsb-findings/03021010/cases.md?model=4470905), [All aircraft, 329 cases](https://openairdocs.org/ntsb-findings/03021010/cases.md) - [Misc hardware](https://openairdocs.org/ntsb-findings/01071400): Aircraft › Fluids/misc hardware · Not inspected (probable cause); cases with it: [Hughes OH-6A, 1 case](https://openairdocs.org/ntsb-findings/01071400/cases.md?model=4470905), [All aircraft, 41 cases](https://openairdocs.org/ntsb-findings/01071400/cases.md) ## Sequence of events The events NTSB coded, in order, by phase of flight. The defining event best describes what happened. - [Maneuvering-low-alt flying: Loss of engine power (partial)](https://openairdocs.org/ntsb-events/342) (defining event); cases with it: [Hughes OH-6A, 2 cases](https://openairdocs.org/ntsb-events/342/sequence.md?model=4470905), [All aircraft, 1,858 cases](https://openairdocs.org/ntsb-events/342/sequence.md) - [Emergency descent: Off-field or emergency landing](https://openairdocs.org/ntsb-events/440); cases with it: [Hughes OH-6A, 2 cases](https://openairdocs.org/ntsb-events/440/sequence.md?model=4470905), [All aircraft, 3,365 cases](https://openairdocs.org/ntsb-events/440/sequence.md) ## Analysis While conducting wildlife damage management operations at low altitude over hilly terrain, the pilot of the helicopter noted a loss of main rotor rpm. To regain main rotor rpm, he reduced the engine power demand by descending into a draw; however, the helicopter impacted terrain, rolled over, and came to rest on its left side. Two days before the accident flight, the pilot experienced a similar loss of main rotor rpm; however, he was able to safely land the helicopter. The pilot was unable to replicate the loss of main rotor rpm and continued the flight. The pilot believed that the event was an isolated anomaly and did not notify maintenance personnel. Postaccident examination of the helicopter revealed an air leak located in the fuel cell outlet valve near the firewall. Subsequent examination of the fuel cell outlet valve revealed circumferential scratch marks around the barrel of the pipe. Damage due to cross-threading was observed on the first two threads. When the O-ring was removed, metal chips were located embedded throughout the O-ring. A review of the maintenance records indicated that the fuel system components were inspected during the 100-hour inspection, along with correspondence from the maintenance facility that a vacuum check of the fuel system was conducted during that inspection, with no anomalies noted. No entries were found in the maintenance records of any work done to the fuel outlet valve. The approved maintenance manual for the helicopter contained multiple warnings that air in the fuel system will cause a power reduction or flame out. It could not be determined when, or how, the fuel cell outlet valve developed a leak. ## Factual narrative On February 5, 2021, about 1400 mountain standard time, a Hughes OH-6A helicopter, N6639R, was substantially damaged when it was involved in an accident near Horseshoe Bend, Idaho. The pilot sustained serious injuries and a passenger sustained minor injuries. The helicopter was operated as a public use flight. The purpose of the accident flight was to conduct wildlife damage management operations. While maneuvering at low altitude over hilly terrain, the pilot noted a loss of main rotor rpm. The pilot maneuvered the helicopter into a draw to reduce the engine power demand and regain rotor rpm. The helicopter impacted sloped terrain, rolled over, and came to rest on its left side. The pilot reported that, two days before the accident flight, he experienced a similar loss of rotor rpm while over flat terrain and was able to safely land the helicopter. While on the ground, he utilized the “BEEP” switch (N2 governor control switch) on the collective to increase rotor rpm, but it had no effect. He then changed the position of the throttle from fly to idle and observed the engine instruments for any abnormalities. With no abnormalities identified, he again changed the position of the throttle from idle to fly and regained main rotor rpm. He then utilized the “BEEP” switch and all engine controls returned to normal operations. While remaining on the ground, he repeated the process several times attempting to duplicate the loss of main rotor rpm with no success. As there had not been a prior malfunction and unable to duplicate the loss of rotor rpm, the pilot believed the event as an isolated anomaly and did not notify maintenance personnel. Postaccident examinations of the helicopter revealed substantial damage to the fuselage and tail boom, as well as an air leak at the fuel cell outlet valve that connected the fuel supply line from the fuel shutoff valve and the frangible in-line fuel valve on the self-sealing engine fuel inlet hose located at the firewall. (see Figure 1.) The fuel cell outlet valve along with the engine fuel inlet hose were shipped to the NTSB (NTSB) Materials Laboratory for further examination. Figure 1: view of fuel line leak (photo provided by MD Helicopters) Flight control continuity was confirmed from the cyclic pitch control and collective pitch control systems to the main rotor swashplate. Anti-torque pedal control continuity was established from the pedals up to tail rotor pitch control tube. The engine was removed and shipped to the manufacturer for additional examination and testing. The engine was installed in a test-run cell, where it was started and ran for about an hour with no mechanical anomalies identified that would have caused the reported sudden loss of engine power. Of note were two decelerations and a singular brief engine stall; both events were within the manufacturers experience of a mid-life engine and would not have contributed to the reported power loss. According to the engine manufacturer, the accidenthelicopter had a non-pressure-boosted fuel system that was utilized for normal operation. The engine fuel pump generated a vacuum at its inlet to draw the fuel from the tank to the engine fuel delivery system. A leak in the fuel delivery system prior to the engine can produce air entrainment in the fuel being supplied which could result in the interruption of homogenous fuel flow. Maintenance records provided by the operator indicated that a 100-hour inspection was completed on May 19, 2020. Correspondence with the maintenance facility revealed that a 100-hour inspection was conducted in accordance with the Rolls Royce maintenance and operation manual, 250-C20 Section 72-00-00. No other fuel system troubleshooting or repairs were completed. A vacuum check of the fuel system was conducted during the 100-hour inspection, with no anomalies noted. The Rolls Royce maintenance and operation manual, 250-C20 Section 72-00-00, titled “Engine Fuel and Control System -- Maintenance Practices,” contained multiple warnings that, “Air leaks in the fuel system or the pneumatic sensing system can cause flameouts, power loss or overspeed.” The fuel cell outlet valve was examined by the NTSB Materials Laboratory. A pressure test utilized soapy water as part of its testing protocols; soap bubbles formed at the barrel of the pipe and fuel outlet valve, which indicated the presence of an air leak. Visual examination of the fitting had circumferential scratch marks on the barrel of the pipe, as seen in Figure 2. A visual examination of the fitting threads revealed that the first two threads were damaged due to cross-threading. The O-ring was removed with both outer and inner diameter surfaces flattened/compressed. Metal chips were found embedded in the O-ring; when the O-ring was removed, a metal chip dislodged and revealed an impression in the rubber. Figure 2: View of optical microscope images showing circumferential scratch marks and pile-up material on the outer diameter surface of the pipe barrel (photo provided by NTSB) A review of the helicopter’s maintenance records could not determine the age of the fuel cell outlet valve, or if any maintenance had been performed on the valve. ## Aircraft - Registration: [N6639R](https://openairdocs.org/deregistered/N6639R-2021-08-07), registration cancelled 2021-08-07, S/N 66-07874 (OpenAirDocs) - Reported make and model: HUGHES OH-6A - Registry model: [Hughes OH-6A](https://openairdocs.org/model/hughes-oh-6a) (registry code 4470905) (OpenAirDocs) - Category: Helicopter - Serial number: 66-078784 - Airframe hours: 14627.5 - Engines: Alison 250-C20B 420 hp turbo shaft - Damage: Substantial damage - Flight: Public aircraft - federal, Public use - Operator: Usda Aphis Ws Atoc - Registered owner: Usda Aphis Ws Atoc - Airworthiness certificate: None - Last inspection: Annual, May 16, 2020, 130 hours before - Departed: KMAN, Nampa, ID - Destination: KEUL, Caldwell, ID - Flight plan: Company VFR - Equipment: ADS-B, Handheld GPS ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 52; injuries: serious; flight instructor, commercial; airplane ratings: single-engine sea, single-engine land, multi-engine land; instrument ratings: airplane, helicopter; medical: class 2, without waivers/limitations; last flight review August 12, 2020; seat: right; restraint: 4-point; flight hours: 17,000 total, 1,500 in this make and model, 17,000 as pilot in command, 75 in the last 90 days, 40 in the last 30 days - Other flight crew; age 61; injuries: minor; seat: left; restraint: 4-point ## Weather - Conditions: Visual (VMC), Day - Wind: 320° at 19 knots, gusting 30 - Visibility: 10 statute miles - Lowest ceiling: 6,000 ft overcast - Temperature: 46 °F, dew point 30 °F - Altimeter: 30.06 inHg - Observed at: KBOI, 26 nm away - METAR: KBOI 052053Z 32019G30KT 10SM SCT042 OVC050 08/M01 A3006 RMK AO2 PK WND 32030/2045 SLP185 T00831006 51009 ## FAA reports on this event (1) The FAA's own accident and incident reports, matched to the aircraft by N-number and date (OpenAirDocs). - [Accident 20210205001189A](https://openairdocs.org/incident/20210205001189A), Feb 5, 2021: Hughes OH-6A, Horseshoe Bend, ID, Substantial damage. During a predator mitigation flight and while in a hover the pilot moved to line up a shot on a wolf and at very low airspeed and altitude the engine spooled… ## Other investigations on N6639R (1) From the NTSB's public API. Aircraft are matched to registry aircraft by N-number and event date, checked against the reported serial number, make and model; the registry's history has gaps, so older matches can be wrong. Compare the reported make and model. - [NTSB LAX05TA008](https://openairdocs.org/ntsb/LAX05TA008), Oct 15, 2004: Hughes OH-6A (N6639R), Jacumba, CA, substantial damage. Probable cause: the pilot's failure to maintain adequate clearance from the mountainous terrain, which resulted in the helicopter's tail rotor blades impacting rocks. A factor in the accident was the pilot's… ## Docket (10 files) The files NTSB released with the investigation on September 13, 2022, on NTSB's docket system: https://data.ntsb.gov/Docket?ProjectID=102608. 13.7 MB in all. - Materials laboratory factual report 21-076 (item 1, PDF, 13 pages, 20 photos, 2.2 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=12910729&FileExtension=pdf&FileName=Materials%20Laboratory%20Factual%20Report%2021-076-Rel.pdf - Pilot/operator aircraft accident report, NTSB form 6120.1 (item 2, PDF, 9 pages, 3.6 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=11978197&FileExtension=pdf&FileName=WPR21LA107%20opertor%206120_Redacted-Rel.pdf - Flight crew records of phone conversations (item 3, PDF, 4 pages, 190 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=13027325&FileExtension=pdf&FileName=Flight%20Crew%20ROC-Rel.pdf - Maintenance facility record of e-mail correspondence (item 4, PDF, 1 page, 213 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=14446007&FileExtension=pdf&FileName=Record%20of%20e-mail%20correspondence%20(Maintenance)-Rel.pdf - MD helicopters summary of findings (item 5, PDF, 5 pages, 1.6 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=13027430&FileExtension=pdf&FileName=MD%20Helicopters%20summary%20of%20findings-Rel.pdf - Rolls royce engine run notes (item 6, PDF, 2 pages, 76 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=13027410&FileExtension=pdf&FileName=WPR21LA107%20RR%20Notes%20Engine%20Run%20N6639R_Redacted-Rel.pdf - Evidence control form (item 7, PDF, 3 pages, 48 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=13100351&FileExtension=pdf&FileName=Evidence%20Tracking%20sheets_Redacted-Rel.pdf - Statement of party representatives to NTSB investigation (item 8, PDF, 4 pages, 1.5 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=13027343&FileExtension=pdf&FileName=Certification%20of%20Party%20Representative%20Forms_Redacted-Rel.pdf - Release of aircraft wreckage, NTSB form 6120.15 (item 9, PDF, 1 page, 400 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=13026929&FileExtension=pdf&FileName=Wreckage%20release_Redacted-Rel.pdf - Photo array (item 10, PDF, 4 pages, 3.8 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=13055775&FileExtension=pdf&FileName=Accident%20Photos-Rel.pdf From the NTSB's public API. Aircraft are matched to registry aircraft by N-number and event date, checked against the reported serial number, make and model; the registry's history has gaps, so older matches can be wrong. Compare the reported make and model. --- OpenAirDocs.org is an independent research project of [Contrail Labs](https://contraillabs.com/). It is not affiliated with the FAA or the NTSB. Source documents are U.S. government works in the public domain.