What happened
On the evening of October 6, 2017, a privately owned Cessna 172K, registered JA3500, departed Sapporo Airfield for a familiarization flight. The flight, which included a passenger, proceeded over Sapporo City and toward Otaru City before following the coastline toward Ishikari City.
While flying at approximately 1,500 feet, the aircraft's engine suddenly lost power and stopped. The passenger attempted to restart the engine by adjusting the fuel selector lever and the carburetor heat, but these efforts were unsuccessful. As the aircraft's altitude dropped below 1,000 feet, the captain took control and declared an emergency with Sapporo Tower. Recognizing the difficulty of an in-flight restart at low altitude, the captain opted for a forced landing on the sands of Ishikari Beach. The aircraft landed without the use of flaps and came to a halt on the soft sand without any injuries or damage to the airframe.
The investigation
Investigators examined the aircraft's fuel system, the mechanical condition of the fuel selector valve, and the flight crew's monitoring procedures. The investigation focused on why the engine lost power despite the presence of fuel in the left tank.
Technical inspections of the Cessna 172K revealed that the right fuel tank was completely empty at the time of the landing, while approximately 21 liters of fuel remained in the left tank. Traces of water were also discovered in the left tank and the fuel strainers. Furthermore, investigators found excessive play in the hinges of the mechanical linkage connecting the fuel selector lever to the valve body. This wear, likely due to the aircraft's age, meant that the lever could be moved without accurately setting the valve to the intended 'BOTH' position.
Findings
- The engine failure was caused by the exhaustion of fuel in the right tank, which allowed air to enter the fuel system and interrupt the fuel supply to the engine.
- A one-sided reduction in fuel occurred because the fuel selector valve was not properly seated in the 'BOTH' detent, effectively restricting fuel flow from the left tank.
- The crew failed to notice the imbalance in fuel levels because they did not sufficiently monitor the fuel quantity indicators during the flight.
- The mechanical play in the selector valve linkage was a contributing factor, likely resulting from the long-term use of the 49-year-old aircraft.
- The presence of water in the fuel system may have been caused by rainwater entering through deteriorated sealant around the fuel filler caps.