What happened
On 16 October 2019, a Cessna 172P, registration ZS-OBD, departed Rand Aerodrome for a scheduled training flight. The flight, conducted under visual flight rules, involved a flight instructor and a student pilot performing straight and level flight exercises. During the return leg of the flight, while cruising at 7,000 feet above mean sea level, the engine suddenly ceased operation.
The instructor assumed control of the aircraft and attempted to restart the engine using the manual FADEC activation, but the engine failed to respond. To avoid a more serious accident, the instructor executed an emergency forced landing in an open field on a farm in Kalbasfontein. Despite the engine failure, the aircraft landed safely, and no injuries were sustained by the two occupants.
The investigation
An investigation by the SACAA AIID focused on the mechanical state of the Thielert Centurion engine and the maintenance history of the aircraft. Investigators examined the engine following the incident and discovered that the engine stoppage was a direct result of a timing chain failure.
Technical analysis revealed that the engine's valve timing had fallen out of synchronization, causing the valves to strike the pistons. The investigation also scrutinized the work performed by the Approved Maintenance Organisation (AMO). It was determined that during a previous cylinder head replacement, the maintenance personnel failed to adhere to the manufacturer's specific instructions. Specifically, the camshaft drive gears were misaligned, and the timing chain had been improperly installed.
Findings
- The primary cause of the engine failure was the failure of the timing chain, likely triggered by excessive tension from the timing chain tensioner.
- The timing chain was incorrectly fitted during recent maintenance.
- The camshaft timing gears were not properly aligned.
- The maintenance organization did not follow the required manufacturer procedures for engine component installation.
- The investigation also noted that the student pilot was operating without a valid license or medical certificate, representing a regulatory contravention by the training organization.