What aircraft was involved and where did it happen?

The accident on 1999-12-28 involved a COSTRUZIONI AERONAUTICHE GIOVANNI AGUSTA S.P.A. AB206B, registration HB-XBA, at Horboden, BE, CH.

What was the probable cause of the accident?

The in-flight failure of the throttle cable strands, which became lodged in the cable insulation, prevented the throttle from advancing and limited engine power.

Throttle Cable Failure Forces Emergency Landing in Sling 2

A training flight in the Eastern Cape ended in a forced landing after a mechanical failure prevented the pilot from increasing engine power.

What happened

On the morning of 26 January 2023, a Sling 2 aircraft, registration ZU-FUS, departed Port Elizabeth Aerodrome for a training flight. The flight, operated under Part 141 regulations, carried an instructor and a student pilot to the general flying area near the Sunday River Mouth to perform spiral dive and recovery exercises.

During the recovery phase of a maneuver, the pilot attempted to increase power, but the engine RPM became stuck at 3,000, failing to reach the required levels. The instructor noted that the throttle lever felt restricted and the engine sound did not indicate any increase in power. As the aircraft began to lose altitude, the instructor took control and attempted to apply full power, but the mechanical restriction persisted. To avoid a more serious accident, the instructor performed a forced landing on a beach at the mouth of the Sunday River. There were 0 fatalities and 0 injuries during the incident, and the aircraft sustained no damage.

The investigation

SACAA AIID investigators examined the aircraft's throttle mechanism following the incident. An inspection by an approved person revealed that several strands of the throttle cable had snapped at the point where the cable attaches to the quadrant. This breakage caused the loose strands to unwind and become lodged within the cable's protective sleeve, which physically prevented the throttle lever from advancing to full power.

Findings

The primary cause of the power limitation was an in-flight failure of the throttle cable due to broken strands obstructing the cable's movement.
The failure was likely triggered by improper maintenance, specifically the over-torquing of the attaching hardware (the AN3 low profile nut) during a previous installation.
The compression loads caused by the washer at the attachment point led to the fatigue and eventual snapping of the cable strands.
This incident marks the second recorded instance of a similar throttle cable defect involving this aircraft type in the AIID database.

Safety action

Following this investigation, the manufacturer released Service Bulletin 0024 to supersede previous instructions. Additionally, a safety recommendation has been made for the manufacturer to update their Service Bulletins to include specific torque values and required tools to prevent ambiguity during cable installation.