Event Details
On 15 December 2021 at approximately 1520Z, a Sling 2 aircraft with registration ZU-WMM departed from Runway 06 at Wonderboom Aerodrome (FAWB) in Gauteng province for a training flight under Part 141. On board were a flight instructor and a student pilot. During the flight, after take-off, a simulated engine failure exercise was initiated. The engine could not achieve maximum revolutions per minute (rpm) and did not respond to throttle control inputs. The flight instructor performed fault-finding, including opening and closing the throttle several times, but the engine manifold pressure and rpm remained at idle. The instructor took control and executed an emergency landing on an open field approximately 2.2 nautical miles northeast of FAWB. During landing on a wetland, the spring-loaded main landing gear detached from the underbelly attachment point, causing the nose landing gear to bend. The aircraft then collided with a perimeter fence, and the left wing struck a fence pole. The aircraft sustained substantial damage. Neither occupant was injured; both disembarked without assistance.
Investigation Findings
The aircraft was equipped with an engine control unit (ECU), which was downloaded on 16 December 2021. The ECU data indicated the engine ran normally throughout the flight with no abnormal parameters, except for the throttle position sensor readings. These readings contradicted the pilot and student statements: the throttle sensor varied between 28% and a maximum of 50% until landing, indicating loss of engine power control due to unresponsive throttle movement limited by the air-filter housing.
A manufacturer Service Bulletin (SB) 19, issued on 1 October 2021, addressed potential loss of engine power control related to throttle cables. The accident aircraft had complied with SB 19 on 2 November 2021, 48.9 flight hours before the accident. Despite compliance, further investigation of the throttle system revealed witness marks on the air-filter housing from the throttle lever cable. During testing, the air-filter housing was observed to be pushed back by high airflow during flight, limiting the throttle lever's full forward position. The throttle cable attachment bolt did not swivel; the lock nut caused the cable to bend when the lever was pushed forward, preventing the flexible cable from reaching full position. Two other aircraft in the same training organization fleet showed similar witness marks and interference.
The cause was identified as a throttle system design susceptible to inner cable buckling, preventing it from feeding through the sheath, resulting in loss of engine power control. This problem occurred in both the original and revised SB-19 throttle cable designs. This appears to be the first recorded accident due to this issue. The aircraft's last annual maintenance inspection was on 2 November 2021 at 2429.0 airframe hours; an additional 48.9 hours had been accumulated since. No defects were reported prior to the accident.
Probable Cause
Loss of engine power due to the air-filter housing limiting the movement of the throttle lever cable into full forward position and causing the engine rpm not to reach the desired power. During a forced landing on a wetland, the spring-loaded main landing gear got stuck and detached from the underbelly attachment point; and the aircraft collided with a parameter fence.