What happened
On October 23, 2010, a Cessna TU206G, registration JA3818, departed Chofu Airfield for an aerial survey mission. The flight was originally scheduled to conclude at Niigata Airport. During the mission, the pilot encountered several deviations from the original plan, including skipping certain survey areas due to equipment data errors and performing additional flight patterns near the Kurobe River.
As the mission progressed, the pilot requested a change of destination to Matsumoto Airport to refuel. However, the Flight Service Center informed the crew that all ramp slots at Matsumoto were occupied. Consequently, the pilot decided to return to Chofu Airfield. During the transit, the engine began to run unsteadily. The pilot attempted to switch the fuel selector between the left and right tanks to mitigate the issue, but the instability persisted.
After declaring a state of 14:10 local time, the pilot initially attempted to divert to Tachikawa Airfield but ultimately returned to a state of emergency to land at Yokota Airfield. The aircraft landed safely at 14:13, but the engine ceased operation less than one minute after entering the taxiway. There were no injuries to the two occupants, and the aircraft sustained no damage.
The investigation
The Japan Transport Safety Board (JTSB) examined the aircraft's fuel system and the accuracy of its instrumentation. Investigators confirmed that both wing tanks were empty upon arrival at Yokota. Subsequent testing revealed that the fuel quantity indicators were providing inaccurate readings. Specifically, the investigation found that the indicators showed significantly more fuel than was actually present in the tanks. For example, when the tanks were filled to show a specific level, the actual usable fuel was much lower than the gauge suggested. The investigation also reviewed the pilot's fuel consumption calculations and the communication between the crew and the Flight Service Center regarding the refueling request.
Findings
- The primary cause of the incident was fuel starvation resulting from inaccurate fuel gauge readings that obscured the true remaining fuel.
- The pilot's fuel consumption calculations were based on an estimated rate that was lower than the actual consumption rate experienced during the flight.
- The pilot did not sufficiently integrate a proper reserve fuel margin into the flight plan.
- The pilot's communication regarding the necessity of refueling was not sufficiently urgent or clear when the initial request to Matsumoto was denied.