N78DWMantooth THORP T-18 · 1980

On August 28, 2021, about 1537 central daylight time, an experimental amateur-built Thorp T-18 airplane, N78DW, sustained substantial damage when it was involved in an accident near San Antonio, Texas. The pilot and passenger sustained minor injuries. The airplane was being operated as a Title 14 Code of Federal Regulations Part 91 personal flight.

According to the pilot, he was planning a short cross-country flight with his passenger. Prior to takeoff, he fueled the airplane with 10 to15 gallons of fuel and added one quart of oil to the engine. He performed an engine run-up that included checking the carburetor heat and departed runway 17. The takeoff and initial climb were normal, but as he turned the airplane onto the downwind leg to depart the traffic pattern, the engine began running rough and lost total power. The pilot attempted to regain engine power by adjusting the engine mixture, but no change in engine performance was noted. Unable to maintain altitude, he performed a forced landing into trees. The airplane came to rest on its nose and sustained substantial damage to the forward fuselage and both wings.

Postaccident examination by a Federal Aviation Administration (FAA) inspector revealed there was at least 2 gallons of fuel remaining in the single header-style fuel tank, which was punctured. Fuel was found in the carburetor bowl, and no contamination of the carburetor inlet screen was observed. The engine was rotated by hand through the propeller which produced compression/suction at each cylinder. All spark plugs were removed and exhibited normal wear signatures. The magnetos were removed for further examination.

Examination of the left magneto revealed the internal components were damaged, and a weak spark was produced on two of the four towers when the magneto was rotated. The two other ignition towers were damaged. The right magneto would not rotate due to damage; the internal coil, capacitor, and points were undamaged, and no problems were noted. Examination of the engine revealed no evidence of preimpact mechanical anomalies that would have precluded normal engine operation.

According to a carburetor ice probability chart, the atmospheric conditions at the time of the accident were conducive to serious icing at glide power. FAA Special Airworthiness Information Bulletin (CE-09-35) – Carburetor Icing Prevention, stated in part:

…pilots should be aware that carburetor icing doesn't just occur in freezing conditions, it can occur at temperatures well above freezing temperatures when there is visible moisture or high humidity. Icing can occur in the carburetor at temperatures above freezing because vaporization of fuel, combined with the expansion of air as it flows through the carburetor, (Venturi Effect) causes sudden cooling, sometimes by a significant amount within a fraction of a second. Carburetor ice can be detected by a drop in rpm in fixed pitch propeller airplanes and a drop in manifold pressure in constant speed propeller airplanes. In both types, usually there will be a roughness in engine operation.