What happened
On September 17, 2015, a Cessna 152, registration SP-KDL, was conducting a training flight near Łomża, Poland. The crew, consisting of an instructor and a student pilot, was performing a simulated emergency landing procedure at an altitude of approximately 150 meters AGL. Following the simulation, the student pilot attempted to increase power by advancing the throttle. However, the engine RPM failed to increase significantly, peaking only between 1700 and 1800 RPM.
The instructor took control and discovered that the throttle lever moved with much less resistance than normal and had no effect on engine performance. As the aircraft continued to lose altitude, the instructor performed an emergency landing in an unprepared field. Due to the low altitude, the crew had limited options and selected a field with very soft ground. Upon touchdown, the nose gear collapsed, causing the aircraft to capsize. Both the instructor and the student pilot exited the aircraft without injuries, though the aircraft sustained major damage.
The investigation
The PKBWL investigation focused on the mechanical state of the engine and throttle linkage. Physical examination of the wreckage revealed that the bolt connecting the throttle control linkage to the carburetor throttle arm was missing. Investigators conducted a simulation using another Cessuna 152 to study the effects of a disconnected throttle. They found that while a spring on the throttle axis attempted to open the butterfly valve, it was insufficient to maintain level flight, as the engine power stabilized at a level too low for sustained flight.
Technical records indicated that the aircraft was approximately five flight hours away from its next 100-hour inspection, which might have allowed for the detection of the loose connection. The investigation also noted that the failure was likely caused by the bolt falling out due to the complete unscrewing of the castle nut, which could occur if the split pin had failed or been improperly installed.
Findings
- The engine power loss was caused by the disconnection of the throttle control linkage from the carburetor.
- The disconnection occurred because the bolt connecting the linkage to the throttle arm fell out.
- The spring installed on the throttle axis was too weak to open the butterfly valve to a position that would allow for continued level flight.
- The aircraft was operating within its maintenance schedule, but the failure occurred prior to the scheduled 100-hour inspection.