What happened
On 19 December 2011, a Cessna 172P aircraft, registered PK-WTF, was conducting dual flight training from Cakrabhuwana Airport to the Karang Ampel training area. The flight, operated by Wings Flying School, included one flight instructor and one student pilot. During a series of maneuvers, including an engine failure exercise, the student pilot initiated a go-around at 700 feet to return to Cirebon.
As the aircraft climbed toward 1,500 feet, the crew noted engine power dropped to 1,800 rpm and experienced noticeable engine hesitation. The flight instructor took control of the aircraft to manage the situation. As the aircraft descended to approximately 500 feet, the crew declared an emergency. The instructor subsequently executed a forced landing on a muddy coastal area near the Suranenggala checkpoint. The impact caused the aircraft to flip upside down, resulting in substantial damage to the airframe, though both occupants evacuated without injury.
The investigation
The investigation focused on determining the cause of the engine power loss. While the engine itself showed no mechanical abnormalities, inspectors examined the induction system. They discovered a strip of rubber, measuring 12 cm by 0.8 cm, lodged inside the carburetor throttle chamber. This piece of rubber was identified as having originated from the flapper valve within the induction airbox.
Findings
- The engine failure was caused by a piece of rubber detaching from the induction airbox flapper valve.
- The detachment was likely caused by fatigue from the rubber repeatedly bending against the airbox wall when the valve was in the "CLOSE" position.
- The loose rubber entered the carburetor, obstructing the fuel-air mixture and causing improper combustion, which the pilots perceived as engine hesitation.
- The rubber used in the installation was of an improper dimension, being wider than the original component, which contributed to the fatigue and eventual failure.
- The aircraft had undergone a 200-hour inspection approximately 81 hours prior to the accident, which had not identified any issues with the induction system.
Safety action
Following the accident, the operator implemented several changes, including amending the Standard Operating Procedure for forced landing exercises to ensure they are conducted at higher altitudes. The school also reviewed its fuel handling and maintenance programs to improve quality assurance.