SWAN LAKE, MT, USA
N7975D
Beech H-35
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.
HISTORY OF FLIGHT On August 10, 1999, approximately 1730 mountain daylight time, a Beech H-35, N7975D, sustained substantial damage during a forced landing in the Spotted Bear river, east of Swan Lake, Montana. The commercial pilot and his passenger were seriously injured. Visual meteorological conditions prevailed for the flight, which had departed a private airstrip near Creston, Montana, en route to Benchmark airstrip, adjacent to the Bob Marshall Wilderness in Montana. In a written statement, the pilot stated that they had crossed the mountain range immediately south of Mount Aeneas and were in the south fork of the Flathead River drainage near the south end of the Hungry Horse reservoir. He said he had to circle once to gain sufficient altitude to cross the mountain ride. At the time, he believed the climb was normal, but sluggish due to high density altitude. He stated that he noticed a slightly higher than normal oil temperature during climbout. After entering the South Fork drainage, He held his altitude at about 7000 feet MSL to allow for cooling. He flew south along the South Fork until the intersection of the Spotted Bear River. He then turned east to follow the Spotted Bear River to near the Continental Divide. He stated that he had climbed to approximately 7800 feet, but needed some more altitude before crossing the Continental Divide. He said he was becoming concerned about oil temperature and oil pressure when the engine quit. He said he tried to restart the engine. He said he turned around in an attempt to reach the Spotted Bear airstrip, however it became necessary to make a forced landing in the Spotted Bear River. Witnesses observed the airplane, but did not hear any indication that the engine was developing power prior to impact. During an interview with Safety Board investigators, the pilot noted that he observed what he described as "a governor failure and propeller overspeed with associated rise in oil temperature." Throttling back relieved the overspeed situation, and he had observed a rapid loss of remaining power after turning back. He stated that the propeller had continued windmilling until impact. He stated that he had been using automotive gasoline in the airplane for approximately 700 hours. Additionally, he noted that he had stalled the airplane approximately 40 feet above the surface prior to impact. WRECKAGE AND IMPACT INFORMATION The aircraft was airlifted from the river to a secure site adjacent to the river by helicopter prior to arrival of Safety Board investigators, due to the possibility of environmental contamination from spilled fuel. The river was approximately two feet deep and 50 yards across at the crash site, which was upstream of Beaver Creek campground. The fuselage aft of the rear windows was deformed downward. The engine and cowling were displaced downward forward of the firewall. The fuselage exhibited extensive wrinkling aft of the firewall and above the upper wing surfaces at the wing-to-fuselage juncture. Both wings exhibited leading edge and lower surface scrapes and dents. The seat pan below the pilot's seat exhibited downward deformation and tearing adjacent tot he front of the spar carry-through. The passenger's seat pan exhibited downward deformation and distortion. The propeller blades both exhibited aft curling, leading edge gouges, and chordwise scratching. Preliminary inspection of the engine at the scene revealed a hole in one piston (number 3). All upper sparkplugs were removed and found to be bright and shiny, with number 3 upper sparkplug exhibiting a surface that appeared similar to a metallic granular coating All four fuel tanks in the airplane contained significant quantities of fuel with smell and color similar to that of automotive gasoline. TESTS AND RESEARCH The engine was partially disassembled and inspected after transport to the salvage facility. Pistons 1, 2, 4, 5, and 6 had little, if any, evidence of combustion deposits on their combustion faces. Piston 3 exhibited a hole of about 1 inch diameter, and a combustion surface with the appearance of erosion. Piston 6 exhibited apparent erosion at its six o'clock position, as installed in the engine. The number 4 rod bearing shells were partially extruded from the bearing journal. Magneto timing was 24 degrees BTDC, normal for this series engine. The engine oil pickup screen, propeller governor gasket screen, and oil sump exhibited metallic particulate matter, and the oil screen was found to be collapsed, with a coating of small metallic particles. Compression ratio for the O-470G engines is 8.0:1. As originally manufactured, minimum octane requirements for fuel were 91/96 octane. In the O-470 Operator's Manual X30097, Teledyne Continental Motors amended the recommended fuel to 100 LL 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 fuel removed from the airplane after the accident was tested by an independent laboratory. Research Octane Number was 94, Motor Octane Number was 83.8, and anti-knock index was 88.9. ADDITINAL DATA/INFORMATION The wreckage was released to the pilot's insurance company claims representative on September 22, 1999. The wreckage was at Discount Aircraft Salvage, Deer Park, Washington, at the time of release.
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
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