What happened
On 29 October 2008, a Cessna 172 operated by Alfa Flying School was conducting a cross-country training flight from Halim Perdanakusuma Airport, Jakarta, to Penggung Airport, Cirebon. The aircraft, registered PK-SDQ, was occupied by an instructor, a student pilot, and an observer.
While cruising at approximately 5,000 feet near the Purwakarta NDB, the engine suddenly lost power. After the student pilot was unable to restart the engine, the instructor took control of the aircraft and declared an emergency. The crew performed a forced landing on the shoulder of the Jakarta-Cikampek highway. Despite the nature of the incident, there were no injuries to the occupants, and no damage was sustained by the aircraft or surrounding property.
The investigation
Investigators from the NTSC performed a teardown of the Lycoming O-320 engine to identify the source of the power loss. The examination revealed that the camshaft was not rotating because the accessory gear case contained broken components. Specifically, both the crankshaft idle gear and the crankshaft gear had failed.
Metallurgical analysis of the crankshaft gear showed evidence of fatigue failure, with two gear teeth having detached. The analysis also revealed significant wear, pitting, and dents on the gear surface. The presence of visible machining marks and a relatively rough surface finish suggested that the gear had not undergone proper grinding and hardening processes. Furthermore, investigators identified a discrepancy in the part numbers of the installed gears compared to the official parts catalogue.
Findings
- The engine failure was triggered by the failure of the crankshaft gear due to an improper manufacturing process, which caused geometric meshing problems and extraneous loading.
- The failure of the crankshaft gear led to a static overload of the crankshaft idle gear, causing it to break.
- The disintegration of the idle gear stopped the rotation of the magneto and the ignition system, resulting in the engine quitting.
- The aircraft was airworthy and the pilot held a valid license at the time of the incident.