# Accident 19990810047199A: Beech H35, Swanlake, MT, August 10, 1999

- Type: FAA accident report (Accident/Incident Data System), as reported
- Date: August 10, 1999, 17:30 local time
- Place: Swanlake, MT
- Damage: Substantial damage
- Phase of flight: Fcd/Prec Ldg From Cruise
- Operating rules: General operating rules (Part 91)
- Location: 47.7407° N, 113.7403° W: From the FAA, an estimate near the named town, within about 3 miles
- Nearby events: https://openairdocs.org/map.md?lat=47.7407&lon=-113.7403&mi=10
- Aircraft: N7975D, a cancelled registration (https://openairdocs.org/deregistered/N7975D-2002-04-01) (OpenAirDocs)
- Registry model: Beech H35 (registry code 1151520, https://openairdocs.org/model/beech-h35) (OpenAirDocs)
- Registry engine: Continental O-470 series (registry code 17026, https://openairdocs.org/engine/continental-o-470-series) (OpenAirDocs)
- Reported make and model: BEECH H35
- Reported engine: CONT O470G
- Fatal injuries: 0
- Pilot flight hours: 2,158 total, 700 in this make and model, 65 in the last 90 days
- NTSB investigation: [NTSB SEA99FA138](https://openairdocs.org/ntsb/SEA99FA138) (OpenAirDocs)
- FAA source: FAA Aviation Safety Information Portal (https://asip.faa.gov/)
- Page: https://openairdocs.org/incident/19990810047199A

## Narrative

As the FAA filed it, in capitals; the FAA replaces private details with ^PRIVACY DATA OMITTED^.

(-23) ON AUGUST 10, 1999, A BE-35 OPERATED BY PAUL WELLS CRASHED INTO THE WATER OF SPOTTED BEAR RIVER APPROXIMATELY 5 MILES EAST OF SPOTTED BEAR RANGER STATION DURING A PLEASURE FLIGHT. THE FLIGHT ORIGINATED AT TOM SAND'S AIRFIELD WITH AN INTENDED DESTINATION OF BENCHMARK (3UY) AIRFIELD. NO FLIGHT PLAN WAS FILED AND VFR CONDITIONS PREVAILED. THE AIRCRAFT WAS SUBSTANTIALLY DAMAGED AND AN AMBULANCE HELICOPTER TRANSPORTED PILOT AND SINGLE PASSENGER TO HOSPITAL IN SERIOUS CONDITION. AIR AMBULANCE MEDIC REPORTED THAT PILOT INDICATED AN ENGINE OVERTEMPERATURE CONDITION/PROP VIBRATION AT 7000 FEET AGL AND ATTEMPTED TO LAND A SPOTTED BEAR AIRPORT. (.4) 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 ER                           OSION. 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 NTSB's investigation of this event (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 SEA99FA138](https://openairdocs.org/ntsb/SEA99FA138), Aug 10, 1999: Beech H-35 (N7975D), Swan Lake, MT, substantial damage. Probable cause: 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.

From the FAA Accident/Incident Data System. Reports are matched to registry aircraft by N-number and event date, checked against the reported make and model; the registry's history has gaps, so older matches can be wrong. Compare the reported make and model. For accidents, the NTSB's own investigation is the fuller record.

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