What happened
On February 16, 2019, an Aquila AT01 was conducting a training flight from Bern-Belp to Grenchen. The crew, consisting of a flight instructor and a student pilot, had just completed a touch-and-go landing on runway 24. As the aircraft climbed to approximately 300 ft, the crew experienced light vibrations that rapidly intensified, leading to fears of an imminent engine failure.
The flight instructor took control and declared an emergency via radio. Following instructions from the air traffic controller, the crew initiated a left-hand turn to land on the opposite runway. During this maneuver, the aircraft reached a bank angle of approximately 45 degrees at an indicated airspeed of 80 knots. As the engine power was reduced to idle during the approach, the vibrations ceased. The aircraft landed safely on runway 06, and a subsequent engine run-up showed no immediate defects.
The investigation
SUST examined the engine, a Rotax 912 S, specifically focusing on the dual carburetor system. Investigators found that the float weight in the right carburetor was 6.2 g, while the left carburetor float weighed 7.3 g. According to maintenance standards, the combined weight of the two floats in a single carburetor should not exceed 7.0 g.
While the aircraft was not subject to specific airworthiness directives regarding this issue at the time, the investigation noted that the engine functioned normally after the floats were replaced and the carburetors were cleaned and adjusted.