Hughes OH-6A accident near Horseshoe Bend, Idaho, February 5, 2021
On February 5, 2021 at about 9:00 pm local time, a Hughes OH-6A (helicopter), registered N6639R, was substantially damaged in an accident during maneuvering (low-alt flying) near Horseshoe Bend, Idaho. It was a public-use flight (federal) under public-use (government) rules. 1 person was seriously injured and 1 person had minor injuries. The weather was visual conditions (good weather).
Preview. This page is not yet listed for search engines.
The NTSB's probable cause their words, unchanged
A partial loss of engine power due to a leak in the fuel cell outlet valve.
Source: NTSB aviation accident database, copy made October 4, 2026. Docket and reports at the NTSB.
What the record shows
- Date
- February 5, 2021 · about 9:00 pm local time
- Place
- Horseshoe Bend, Idaho · map
- Type
- Accident
- Injuries
- 1 person was seriously injured and 1 person had minor injuries.
- Weather
- visual conditions (good weather)
- Aircraft
- Hughes OH-6A · all OH-6As on the register
- Registration
- N6639R · no longer on the register · serial 66-078784
- Damage
- Substantial damage
- Flight
- Public-use flight (federal) · public-use (government) rules
The NTSB's narrative final · quoted from the NTSB record
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.
The complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.
The factual record from the NTSB's investigation tables, in plain English
What happened, in order
- Loss of engine power (partial) during maneuvering (low-alt flying) defining event
- Off-field or emergency landing during emergency descent
The NTSB's findings
- Aircraft › Aircraft systems › Fuel system › (general) › Failure
- Environmental issues › Physical environment › Terrain › Mountainous/hilly terrain › Effect on equipment
- Aircraft › Fluids/misc hardware › Misc hardware › (general) › Not inspected
Pilot
- Certificate: flight instructor, commercial pilot
- Ratings: multi-engine land; single-engine land; single-engine sea; instructor: airplane single-engine; instructor: helicopter; instrument: airplane; instrument: helicopter; rotorcraft: helicopter
- Flight time: 17,000 hours in all; 1,500 in this make and model; 75 in the last 90 days; 40 in the last 30 days; 17,000 as pilot in command
- Last flight review: August 12, 2020
- Medical certificate: Class 2 (without waivers/limitations)
- Seat: rgt
- Injury: serious injuries
Other crew
- Seat: left
- Injury: minor injuries
The aircraft
- Airframe total time: 14,627.5 hours
- Last inspection: annual inspection, May 16, 2020; 130 hours since
- Maximum gross weight: 2,550 lb
- Seats: 2
- Landing gear: fixed
- Engine: Alison 250-C20B (turboshaft); 7,594 hours total
The flight
- Departed from: KMAN Nampa ID at 4:38 pm
- Destination: KEUL Caldwell ID
Weather at the time
- Light: daylight
- Wind: from 320° at 19 knots, gusting 30
- Visibility: 10 statute miles
- Sky: overcast at 6,000 ft; scat at 6,000 ft
- Temperature: 46°F (8°C), dew point 30°F (-1°C)
- Altimeter: 30.06 inHg
- Observation at 1:53 pm from KBOI, 26 miles away
Weather report (METAR): KBOI 052053Z 32019G30KT 10SM SCT042 OVC050 08/M01 A3006 RMK AO2 PK WND 32030/2045 SLP185 T00831006 51009
Injuries
| Fatal | Serious | Minor | None | |
|---|---|---|---|---|
| Flight crew | 1 | |||
| Passengers | 1 |
Documents from the investigation the NTSB's docket: the evidence folder behind the report
The docket, the folder of records gathered during an investigation (maintenance records, photographs, witness statements, examinations), has not been read from the NTSB for this case yet; the list appears here once it has. Open the docket at the NTSB.
Other NTSB records under N6639R the same tail number, which may have belonged to a different aircraft at the time
About this page
Everything above comes from the NTSB's public accident database: the narrative, probable cause and findings are the NTSB's own words, and the coded tables behind its report (pilot, aircraft, flight, weather, injuries, sequence of events) are written out in plain English. Pilots' ages, home towns and medical details are left out on purpose. The documents and photographs are the NTSB's docket, the folder of records gathered during the investigation, shown as the NTSB released them (the NTSB redacts some personal details first) and listed under the NTSB's own titles. This site's own text never names pilots, passengers or anyone else involved. A preliminary report describes what is known soon after the event and can change; the final report, with the probable cause, usually follows one to two years later and this page is refreshed when it does. Case number WPR21LA107.