What happened
On August 22, 2015, at approximately 18:32 local time, a Maule MX-7-235, registration HB-KIQ, was performing a glider tow operation at Bex airfield (LSGB). After successfully releasing a glider at roughly 2,600 feet, the aircraft entered a right turn and descended to 2,000 feet to configure for landing on runway 33. During the final approach, the pilot noticed that the engine power failed to respond to inputs, with the engine RPM remaining fixed at 1,000 RPM.
In an attempt to restore power, the pilot activated the electric fuel pump, switched the fuel selector from both tanks to the right tank, and applied carburetor heat. When these actions failed to improve engine performance, the pilot executed an emergency landing in a grassy field located approximately 750 meters southeast of the runway threshold. The landing was completed without damage to the aircraft, and while there were no injuries to the pilot, a passenger was present on board. The engine continued to run at a steady 1,000 RPM throughout the descent and landing, but failed to respond to throttle adjustments even during a post-landing power check.
The investigation
Technical inspections of the aircraft focused on the engine and the throttle mechanism. Investigators discovered that the flexible portion of the throttle control sleeve had become detached from its rigid component. This separation meant that the internal cable within the flexible sleeve was no longer properly guided to control the carburetor.
The investigation examined the mechanics of the throttle, which could be operated either by rotating the knob for fine adjustments or by pressing a central button to allow for rapid linear movement. The separation of the sleeve from the rigid part was attributed to mechanical tension. Specifically, the investigation noted that the gap between the control and the dashboard may have allowed the sleeve to be placed under tension during full-power applications. Additionally, the process of pushing the throttle forward while using the central button and then unscrewing the vernier to the dashboard limit likely caused fatigue-related stress on the sleeve.
Findings
- The primary cause of the power loss was the mechanical detachment of the throttle control sleeve from its rigid mounting.
- This detachment prevented the internal control cable from effectively modulating the carburetor.
- The failure was likely driven by tension and fatigue caused by the physical movement of the throttle assembly against the instrument panel.
- The aircraft's throttle control had not been replaced since the aircraft entered service, and the manufacturer provides no specific time or hourly limit for its replacement.