What happened
On January 24, 2021, an Embraer ERJ195-200, registration SP-LNH, experienced a fuel leak while climbing to its cruising altitude. The incident occurred in the airspace of the Socialist Republic of Vietnam, near Kon Tum. During the climb, the flight crew identified a leak from one of the wing tanks via instrument indications, which was subsequently confirmed by a cabin crew member through visual observation. Due to the nature of the leak, the crew declared a "MAY DAY" and aborted the flight to the intended destination. The aircraft returned to its departure airport, where it performed a safe landing assisted by airport emergency services.
The investigation
The investigation was conducted by LOT Aircraft Maintenance Services Sp. z o.o. The technical inspection and analysis focused on the mechanical failure within the fuel system. The investigation established that the leak originated from the right-hand wing tank. The analysis also included a review of manufacturer data regarding airflow dynamics at high speeds and the performance of the pressure maintenance system.
Findings
- The primary cause of the fuel leak was a malfunction of the right-hand pressure relief valve.
- At high speeds, the airflow characteristics into the NACA inlet can alter the efficiency of the positive pressure maintenance system.
- This change causes a sudden drop in pressure within the vent tank, forcing high airflow through the float vent valve, which can lead to the valve becoming stuck.
- As the aircraft climbs and external atmospheric pressure decreases, a pressure differential develops between the wing tank and the atmosphere. If the fuel level is above the vent pipe end, fuel is pushed from the wing tank into the overflow tank and escapes into the atmosphere through the NACA inlet.
- The leak ceases automatically once the main vent pipe is uncovered.