Amateur-built STOL CH701 N701WJ accident near Byron, California

Probable cause

The pilot/builder's improper construction of the cowling, which resulted in inadequate venting of the engine, its overheating, and a partial loss of engine power.

NTSB analysis

The pilot stated that he fabricated and installed a cowling on the airframe prior to the flight. During the initial climb, the engine experienced a partial loss of power and the water temperature climbed above the maximum temperature. The pilot made a forced landing at the departure airport and encountered wind during the approach. The airplane was pushed off the runway and landed hard in the grass; the airplane came to rest inverted.

The engine manufacturer provided instructions to builders to ensure there is proper venting over the engine. The pilot stated that he did not use any plans to build the cowling and did not perform any tests of the venting. The cowling was open on the bottom but did not have aft venting. As a result, the air pressure behind the radiator likely increased to be equivalent to that of the ram air, resulting in the engine overheating and experiencing detonation, leading to the loss of engine power.

Text from the NTSB. Open the NTSB docket for WPR20LA035 for the full record.