Accident Summary
On August 28, 2021, at approximately 1537 central daylight time, an experimental amateur-built Thorp T-18 airplane, registered N78DW, was substantially damaged when it impacted trees during a forced landing near San Antonio, Texas. The pilot and passenger sustained minor injuries. The flight was conducted under Title 14 Code of Federal Regulations Part 91 as a personal cross-country trip.
According to the pilot, before departure he added 10 to 15 gallons of fuel and one quart of oil to the engine. After performing an engine run-up that included checking the carburetor heat, he departed from runway 17. The takeoff and initial climb were normal. However, during the turn onto the downwind leg to exit the traffic pattern, the engine began running roughly and then lost all power. The pilot attempted to restore power by adjusting the mixture, but the engine did not respond. Unable to maintain altitude, he executed a forced landing into trees. The airplane came to rest on its nose, resulting in substantial damage to the forward fuselage and both wings.
Postaccident Examination
A Federal Aviation Administration (FAA) inspector conducted a postaccident examination. The single header-style fuel tank, which had been punctured, contained at least 2 gallons of fuel. Fuel was found in the carburetor bowl, and the carburetor inlet screen showed no contamination. The engine was rotated by hand via the propeller, producing compression and suction at each cylinder. All spark plugs were removed and exhibited normal wear.
The magnetos were removed for further examination. The left magneto had damaged internal components and produced only a weak spark on two of its four towers; the other two ignition towers were damaged. The right magneto could not rotate due to damage, but its internal coil, capacitor, and points were undamaged and no problems were noted. The engine showed no evidence of preimpact mechanical anomalies that would have prevented normal operation.
Environmental Conditions
In reference 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, notes that carburetor icing can occur at temperatures above freezing when visible moisture or high humidity is present. Icing results from fuel vaporization and air expansion through the carburetor (Venturi effect), causing sudden cooling. A drop in rpm (fixed-pitch propeller) or manifold pressure (constant-speed propeller) often accompanies roughness in engine operation.