N701WJZenith Acft STOL CH 701 · 2019

On November 29, 2019, about 1545 Pacific standard time, an experimental amateur built Zenith CH-701 STOL airplane, N701WJ, was substantially damaged when it was involved in an accident near Byron Airport, Byron, California. The pilot was not injured. The airplane was operated under the provisions of 14 Code of Federal Regulations Part 91 as a personal flight.

The pilot stated that he had recently fabricated and installed a cowling for the airplane. After completing an uneventful high-speed taxi test run, he decided to depart and remain in the pattern. The pilot configured the airplane for takeoff on runway 23 and applied full power. He turned onto the crosswind leg of the traffic pattern and felt an abnormal vibration accompanied with a slight loss in engine rpm. He continued onto the downwind leg and the airplane was about 500 ft above ground level (agl) when he was midfield. The airplane started to lose altitude and he observed the water temperature gauge indicating 250° F; maximum operating temperature was 220° F. The pilot further stated that the airplane continued to descend and at about 100 ft agl he attempted to land on runway 30. While trying to align with the runway, the wind pushed the airplane onto the grass. The airplane touched down hard and nosed over, coming to rest inverted. The airplane sustained substantial damage to the wing struts.

The airplane was equipped with the originally installed AeroMomentum AM13 (serial number AM0314A). the pilot stated that the airframe and engine had accumulated a total time of 115 hours.

A representative from AeroMomentum stated that because of the design of the airplane, specifically with slats on the leading edge, it was capable of being configured at an unusually high angle of attack during departure. With such a high angle of attack, the bottom of the airplane would have a high air pressure at the firewall. With no venting aft, the air pressure behind the radiator would likely build to be equivalent to that of the ram air. This would result in overheating and the engine would subsequently experience detonation.

The AeroMomentum installation manual stated that:

The radiator must be mounted so that there is a minimum of 3 inches of water differential air pressure between the front and back. If you do not have this differential air pressure there will not be adequate airflow for cooling. You can use a water manometer to verify this required pressure and airflow. In general, the inlet should have ram air and the outlet should be in a low-pressure area. The inlet area must be no less than 40% the area of the radiator (more for slower aircraft) and the outlet should be about twice the total inlet area. Keep in mind aerodynamics and how they affect the airflow both in and out of the cowling. Sharp corners at the lower edge of the firewall can have a profoundly reduce the exit airflow. High pressure at the bottom of the aircraft especially during climb can reduce the exit airflow. Obstructions like the tailpipe can reduce the exit area and airflow.

The manual further stated that "high engine temperatures can also cause preignition, detonation and knocking that will reduce power and can cause catastrophic engine failure."

The pilot stated that he built the cowling with no plans and did not perform any tests of air pressures over the engine. The cowling was open on the bottom but did not have aft venting.