What happened
On 16 June 2022, a training flight involving a student pilot and an instructor was underway at Klipriver Airfield in Gauteng. The aircraft, a Bush Cat Z194 with registration ZU-EZJ, was performing maneuvers as part of a skills test under visual meteorological conditions. During a precautionary fly-pass over Runway 2/25, the flight instructor attempted to increase power, but the engine failed to respond to throttle inputs.
As the aircraft lost altitude, the instructor attempted to troubleshoot the issue and even had the student pilot attempt to manipulate the throttle from the left side, but the lever remained stuck. Facing obstacles such as power lines, a perimeter fence, and trees at the end of the runway, the instructor opted for a forced landing on a clear patch of land near Runway 20. During the descent, the aircraft struck a barbed wire fence, causing it to stall before it reached the ground. The impact resulted in substantial damage to the aircraft, though both occupants escaped without injury.
The investigation
SACAA AIID investigators examined the aircraft's maintenance history and the mechanical state of the engine controls. The investigation confirmed that the aircraft's recent 100-hour inspection had been completed in May 2022 and all logs were in order. However, a physical examination of the throttle mechanism revealed that the lever was indeed jammed.
Technical analysis identified that a worn-out Nylatron pulley and a Nico press sleeve on the cable had caused the cable to become lodged in the pulley groove. This mechanical interference prevented the throttle from moving beyond a position just above idle RPM.
Findings
- The primary cause of the incident was the jamming of the throttle lever, which prevented the crew from increasing engine power.
- The mechanical failure was specifically due to a Nico press sleeve on the cable damaging the groove of a worn Nylatron pulley, restricting cable movement.
- The instructor followed established emergency procedures for engine power loss, but the physical obstruction of the control prevented a successful restart or power increase.
- Obstacles including trees, lamp posts, and a barbed wire fence limited the available landing options and contributed to the aircraft stalling during the forced landing.