Introduction
On 15 December 2021, at approximately 1520Z, a Sling 2 aircraft registered ZU-WMM departed from Wonderboom Aerodrome (FAWB) in Gauteng Province for a training flight under Part 141. On board were a flight instructor and a student pilot. The intention was to return to the same aerodrome.
Event Sequence
After take-off, during a simulated engine failure exercise, the engine could not achieve maximum revolutions per minute (RPM) and did not respond to throttle control inputs. The flight instructor attempted fault finding, including opening and closing the throttle several times, but the engine manifold pressure (MP) and RPM remained at idle and did not increase to maximum power. The instructor then took control and executed an emergency landing on an open field approximately 2.2 nautical miles northeast of FAWB. During the landing on wet terrain, the spring-loaded main landing gear detached from its underbelly attachment point. Subsequently, the nose gear bent, the aircraft collided with the perimeter fence, and the left wing struck a fence pole. The aircraft sustained substantial damage. The flight instructor and student pilot disembarked without assistance and reported no injuries.
Investigation Findings
The aircraft was equipped with an engine control unit (ECU), which was downloaded on 16 December 2021. The ECU data indicated that the engine operated normally throughout the flight with no abnormal parameters, except for the throttle position sensor readings. These readings contradicted the throttle position described by both pilots, varying between 28% and 50% until landing, clearly indicating a loss of engine power control due to an unresponsive throttle.
The accident aircraft had Service Bulletin (SB) 19-01102021 installed on 2 November 2021 at 48.9 hours prior to the accident. This SB was intended to prevent possible loss of engine power control.
Further investigation by the approved maintenance organisation (AMO) revealed the following:
- The air filter housing showed witness marks from the throttle lever cable.
- During recycling of the throttle lever, it was found that during flight, high airflow pushes the air filter housing back, limiting the throttle lever's full forward movement.
- The throttle cable attachment bolt did not swivel because a lock nut was used, causing the cable to bend when the throttle lever was pushed forward, preventing full travel.
- Inspection of two other aircraft in the same training fleet showed similar witness marks and interference.
The original equipment manufacturer (OEM) sent representatives to examine the wreckage on 17–18 December 2021, and later on 4 January and 19 January 2022. Their findings included:
- SB 19 was not properly executed; the cable retaining bolt on the throttle quadrant lever side was torque-sealed in a way that prevented free rotation, contrary to SB requirements.
- The air filter housing was not tied back and limited throttle lever movement.
- A section of throttle cable extended beyond the throttle arm attachment bolt, interfering with the air filter housing.
- The SB was performed by an unapproved person under the AMO's authorization, though dual-inspected by two approved personnel who did not identify the torque-seal issue.
Throttle System Explanation (from OEM)
The throttle mechanism on the engine side consists of a sprung throttle arm attached to a throttle position sensor and a butterfly valve. The arm is sprung towards wide open throttle so that if the cable fails, the engine goes to full power. The throttle lever in the cockpit releases the cable rather than pulling it; the engine-side spring pulls the cable. For proper operation, the cable must slide freely with low friction. SB 19 was issued to address increased cable friction over time by upgrading the cable sheath with a Teflon inner liner. A key aspect is that the cable retaining bolt on the throttle lever must swivel freely to allow natural alignment. The improper torque-sealing of this bolt and the interference from the extended cable section and unsecured air filter housing contributed to the throttle unresponsiveness.
Conclusion
The loss of engine power control was attributed to the improper installation of Service Bulletin 19-01102021. Specifically, the cable retaining bolt was torque-sealed, preventing free rotation; a section of throttle cable was left extending beyond the attachment bolt and interfered with the air filter housing; and the air filter housing was not effectively secured. These issues resulted in restricted throttle movement, leading to the unresponsive throttle during the simulated engine failure exercise.