What happened
On October 22, 1965, a de Havilland Chipmunk Mk 22, registration HB-TUA, was conducting a training flight at Birrfeld Airfield. The flight, operated by Fliegerschule Birrfeld, was intended for practicing emergency landing procedures.
During the maneuver, the instructor and student were practicing a simulated landing in a nearby field. While flying at approximately 700 feet, the instructor engaged the carburetor heat. As the instructor prepared to reduce power, the student was tasked with selecting a suitable landing site. The instructor identified a field containing a temporary electrical line and instructed the student to perform a go-around to avoid the obstacle.
As the instructor advanced the throttle to initiate the go-around, the engine began to sputter and failed to reach the required RPM. To avoid the electrical line, the instructor attempted to climb slightly, but due to the lack of power, he was forced to execute an immediate turn back toward the airfield. While performing a steep left turn at an altitude of only 5 to 6 meters, the left wing tip of the aircraft struck the ground.
The investigation
The investigation examined the mechanical state of the engine and the cockpit controls. While carburetor icing was ruled out because the engine ran normally after the landing, investigators focused on the fuel mixture control.
Testing revealed that when the mixture control lever was moved forward in conjunction with the throttle, the engine exhibited the same sputtering behavior observed during the accident. The investigation considered the possibility that the student's sleeve may have inadvertently pushed the mixture lever forward while the throttle was being advanced.
Findings
- The primary cause of the accident was a significant loss of engine power during the go-around.
- This power loss was likely caused by the accidental advancement of the mixture control lever.
- The instructor's decision to execute the immediate turn was deemed appropriate given the low altitude and the presence of obstacles, including a hangar and the electrical line.
- The aircraft sustained damage to the left wing, including broken ribs, a dented leading edge, and a bent pitot tube, but there were no injuries to the occupants.