Beech H-35 accident near Swan Lake, Montana, August 10, 1999
On August 10, 1999 at about 11:30 pm local time, a Beech H-35, registered N7975D, was substantially damaged in an accident near Swan Lake, Montana. It was a personal flight under general aviation rules (Part 91). 2 people were seriously injured. The weather was visual conditions (good weather).
The NTSB's probable cause their words, unchanged
Detonation/pre-ignition due to use of improper grade and type of fuel (automotive) and the pilot-in-command's failure to use proper fuel specified by manufacturer's directives.
Source: NTSB aviation accident database, copy made October 5, 2026. Docket and reports at the NTSB.
What the record shows
- Date
- August 10, 1999 · about 11:30 pm local time
- Place
- Swan Lake, Montana · map
- Type
- Accident
- Injuries
- 2 people were seriously injured.
- Weather
- visual conditions (good weather)
- Aircraft
- Beech H-35 · all H-35s on the register
- Registration
- N7975D · no longer on the register · serial D5265
- Damage
- Substantial damage
- Flight
- Personal flight · general aviation rules (Part 91)
The NTSB's narrative final · quoted from the NTSB record
The pilot observed a rpm overspeed with oil temperature rise. Throttling back relieved the overspeed situation; however, he observed a rapid loss of remaining power after turning back toward a potential emergency landing airstrip. He stated that the propeller had continued windmilling until impact. Pistons 1, 2, 4, 5, and 6 had few combustion deposits on their combustion faces. Piston 3 exhibited evidence of detonation, including a hole of about 1 inch diameter, and a combustion surface with the appearance of erosion. As originally manufactured, minimum octane requirement for this engine fuel was 91/96 octane. Teledyne Continental Motors amended the recommended fuel to 100 octane after 91/96 octane fuel became obsolete. Placards on the airplane fuel tanks stated that minimum fuel required was 100 octane. No evidence was found of a Supplemental Type Certificate for use of automotive gasoline in the subject airplane or engine. A sample of the automotive fuel removed from the airplane after the accident was tested: Research Octane Number was 94, Motor Octane Number was 83.8, and anti-knock index was 88.9. The pilot stated that he had been using automotive gasoline in the airplane for approximately 700 hours.
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
Pilot
- Certificate: commercial pilot
- Ratings: multi-engine land; single-engine land; instrument: airplane
- Flight time: 2,158 hours in all; 700 in this make and model; 65 in the last 90 days; 15 in the last 30 days
- Medical certificate: Class 2 (valid medical--w/ waivers/lim.)
- Seat: left
The aircraft
- Airframe total time: 5,360 hours
- Last inspection: annual inspection, April 23, 1999; 10 hours since
- Maximum gross weight: 2,900 lb
- Seats: 4
- Landing gear: retractable
- Engine: Continental O-470G (piston); 0 hours total
The flight
- Departed from: Creston MT at 11:00 pm
- Destination: 3U7 Benchmark MT
- Flight plan: none
- Runway 0
Weather at the time
- Light: daylight
- Visibility: 15 statute miles
- Sky: clear
- Temperature: 90°F (32°C), dew point 0°F (-18°C)
Injuries
| Fatal | Serious | Minor | None | |
|---|---|---|---|---|
| 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.
Everything on this page comes from the NTSB's public records. The narrative, probable cause and findings are the NTSB's own words; the coded tables behind the report are written out in plain English, with pilots' ages, home towns and medical details left out. The documents and photographs are the NTSB's docket, shown as the NTSB released them. This site's own text never names anyone involved. A preliminary report can change; the final report usually follows one to two years later, and the page is refreshed when it does.