NYC98LA068

The pilot reported that a few minutes after departure, he smelled 'hot rubber' and started to return to the airport. After the turn, the drive belt between the engine and propeller failed. When the pilot realized that the airplane could not reach the runway, he elected to perform a forced landing in a field. Upon touchdown, the nose wheel dug in the soft terrain and the airplane nosed over. The investigation revealed the builder/pilot had used a 4-inch wide cog belt between the engine and propeller. The cogs were worn off and would not rotate the propeller. The belt manufacturer recommended that cog belts not be used in aviation applications.

On February 22, 1998, about 1600 eastern standard time, a homebuilt Kit Fox Vixen, N955F, was substantially damaged during a forced landing in Weyers Cave, Virginia. The certificated private pilot and passenger were not injured. Visual meteorological conditions prevailed for the local flight which departed from Shenandoah Valley Airport (SHD), Weyers Cave, about 1545. No flight plan had been filed for the flight which was conducted under 14 CFR Part 91.

In a telephone interview, the pilot reported that he had flown over 3 hours on February 22. During the accident flight, he was cruising north of the airport, about 3,800 feet, when he first smelled burning rubber, and headed toward the airport he had just departed from. A few minutes later he experienced a loss of thrust from the propeller, while the engine continued to run normally.

In the NTSB Pilot/Operator Aircraft Accident Report, the pilot stated:

"...Continued to glide to airport at best glide speed. Possibly could have made it but the area just before Runway 05 at SHD is not a good place to try a landing. I chose the field we landed in because I knew I could make this location. The ground was very wet and just after the main gear touched down the front landing gear touched down and buried itself in the soft ground. The nose gear then bent and the plane went nose down and then flipped over up side down slightly to the left side damaging the left wing from the strut to the wing tip...."

Examination of the wreckage revealed that the fuselage was bent, and the right wing was separated outboard of the strut attach point.

The pilot had installed a 4 cylinder water cooled engine, and used a 4 inch wide cog belt to drive the propeller. He reported that he picked the belt used because it was rated for engines up to 300 horse power, and the engine he had installed in the airplane developed 110 horse power. When he examined the belt after the accident, he found that all the cogs had worn off the belt and it would not drive the propeller. The cog belt had accumulated 110 hours at the time of the accident.

A representative of the manufacturer of the cog belt reported that it stated in their catalog that the cog belts were not recommended for aviation applications.

A check with the technical director of the Experimental Aircraft Association revealed they were aware of the limitations of cog belts in aircraft engines. He also reported that the belt used was similar to those used to drive superchargers on racing cars with V-8 engines. The problem with the belts was their lack of ability to absorb the shocks of the power stroke as individual cylinders fired, which became more noticeable as the number of cylinders on the engine decreased.

The pilot reported that he was unaware of any limitations on the use of cog drive belts.