What happened
On 15 December 2021, a flight instructor and a student pilot were conducting a training flight in a Sling 2, registration ZU-WMM, departing from Wonderboom Aerodrome (FAWB). During a simulated engine failure exercise, the crew discovered that the engine would not respond to throttle inputs and could not reach maximum RPM. Despite attempts by the instructor to troubleshoot the issue by cycling the throttle, the engine manifold pressure and RPM remained at idle levels.
To avoid a more serious situation, the instructor performed an emergency landing in an open field approximately 2.2 nautical miles northeast of the aerodrome. During the landing on a wetland, the spring-loaded main landing gear detached from its underbelly attachment point. The aircraft subsequently struck a perimeter fence, and the left wing collided with a fence pole, resulting in substantial damage to the aircraft. Both occupants escaped the wreckage without injury.
The investigation
Investigators examined the aircraft's Engine Control Unit (ECU) and found that while the engine was operating normally, the throttle position sensor recorded significant irregularities. The data showed that the throttle position was restricted, varying only between 28% and 50% rather than reaching full power.
Physical inspection of the aircraft revealed witness marks on the air-filter housing caused by the throttle lever cable. It was determined that high airflow during flight pushes the air-filter housing backward, which physically obstructs the throttle lever's movement. Furthermore, because the throttle cable attachment bolt does not swivel, the cable tends to bend rather than slide through the sheath, preventing the throttle from reaching the full forward position. This mechanical interference was also observed in two other aircraft within the same training organization's fleet.
Findings
- The primary cause of the loss of engine power was a design susceptibility in the throttle system, where the inner cable buckles instead of feeding through the sheath.
- The air-filter housing movement, driven by airflow, physically limited the throttle lever's travel.
- Although the aircraft had complied with a recent Service Bulletin (SB 19) intended to prevent power control loss, the existing design remained vulnerable to this specific failure mode.
Safety action
Following the accident, the aircraft operator grounded the remainder of its fleet until a permanent solution is identified. There is a broader safety concern regarding the potential for sudden, uncommanded loss of engine power in this aircraft type.