What happened
On July 14, 2023, a Cessna 172R, registration JA10AZ, was conducting flight training at Kohnan Airfield when the engine stopped during the approach phase. The flight, operated by Okayama Air Service Co., Ltd., involved a trainee pilot, an instructor, and a passenger.
While performing touch-and-go training maneuvers, the trainee noticed the engine revolutions dropping from 1,500 rpm to below 1,000 rpm. Upon extending the flaps from 10° to 20°, the engine power failed to maintain thrust, and the propeller began windmilling. The instructor took control of the aircraft and attempted to restart the engine twice without success. Despite the loss of power, the instructor managed a controlled glide, landing the aircraft near the runway aiming point. The aircraft subsequently taxied to a stop on a taxiway. There were no injuries to the three occupants, and no damage was sustained by the aircraft.
The investigation
The JTSB investigation focused on the fuel system and the flight conditions leading to the shutdown. Investigators examined the engine, airframe, and fuel levels. The investigation established that the aircraft had departed with approximately 35 gallons of fuel, but due to the aircraft's vent design and recent uncoordinated flight maneuvers, the fuel levels were unbalanced, with the left tank significantly lower than the right.
Technical analysis of the Cessna 172R fuel system revealed that the engine was functioning normally during ground tests, suggesting the failure was not mechanical. Investigators also reviewed the pilot's maneuvers, noting that the trainee had performed several left-side skids during the training session. Furthermore, the investigation looked into the restart attempts, noting that the instructor's focus on landing procedures may have limited the time spent using the auxiliary fuel pump to clear air from the fuel lines.
Findings
- The engine shutdown was caused by an interruption of fuel flow to the engine.
- Low fuel levels in the left tank, combined with prolonged uncoordinated flight (skidding), likely caused the fuel tank outlets to become uncovered.
- This exposure allowed air to enter the fuel lines, leading to fuel starvation.
- The instructor's priority on landing the aircraft likely prevented a sufficiently long activation of the auxiliary fuel pump to purge the air from the system.
- The aircraft's vent design contributed to an unbalanced fuel load between the left and right tanks during the training maneuvers.