What happened
On 15 December 2021, a Sling 2 aircraft, registration ZU-WMM, departed Wonderboom Aerodrome (FAWB) for a training flight. The flight, involving a flight instructor and a student pilot, was intended to be a routine training session. During a simulated engine failure exercise, the crew discovered that the engine would not respond to throttle inputs and could not achieve the required revolutions per minute (RPM).
Despite attempts by the instructor to troubleshoot the issue by cycling the throttle, the engine manifold pressure remained at idle. To avoid a more serious situation, the instructor took control and performed an emergency landing in an open field approximately 2.2 nautical miles northeast of the aerodrome. The aircraft landed on wet ground, causing the main landing gear to detach from the underbelly. The aircraft subsequently struck a perimeter fence and a fence pole, resulting in the destruction of the aircraft. Both occupants disembarked without injury.
The investigation
SACAA AIID examined the aircraft's Engine Control Unit (ECU) data, which revealed that while the engine was running normally, the throttle position sensor indicated that the throttle was not reaching its full forward position. The investigation focused on the recent installation of Service Bulletin (SB) 19-01102021, which was intended to prevent loss of engine power control.
Technical inspections by the Aircraft Maintenance Organisation (AMO) and the Original Equipment Manufacturer (OEM) identified several critical maintenance errors. It was discovered that the throttle cable retaining bolt had been torque-sealed in a manner that prevented the necessary free rotation required by the service bulletin. Additionally, an excess section of the throttle cable was left protruding, which interfered with the air filter housing. The air filter housing itself was not properly secured, allowing high airflow to push it back and obstruct the throttle lever's movement.
Findings
- The primary cause of the power loss was the air filter housing obstructing the throttle lever cable, preventing the engine from reaching maximum power.
- The maintenance of the throttle system was performed by an unapproved person under the AMO's authorization.
- The installation of the Service Bulletin was flawed; the cable retaining bolt was torque-sealed, which restricted the cable's ability to slide freely.
- Improper cable management, specifically an unclipped excess cable length, contributed to the mechanical interference.
- The air filter housing was not adequately secured against movement caused by airflow.