What happened
On February 9, 2020, a Boeing 737-800, registration SP-ENV, was repositioned from Katowice Airport to Kraków-Balice Airport (EPKK) in preparation for a scheduled flight to Marseille (LFML). Following the arrival and parking of the aircraft, the pilot performed a pre-flight inspection (PDC). During this check, the pilot observed significant moisture and fuel droplets dripping from the lower surface of the left wing.
The operator's technical coordination and operations departments were notified immediately. Upon inspection by maintenance personnel, it was determined that fuel was leaking from beneath the service cover of the surge tank. To address the issue, technicians transferred fuel from the wing tanks to the center tank. After removing the lower inspection cover of the surge tank, they identified a damaged and deteriorated seal. The faulty component was replaced with a compliant part, and the aircraft was cleared for service. No further leaks were reported following the aircraft's subsequent flight to Marseille.
The investigation
The PKBWL investigation examined the sequence of events leading to the leak and the maintenance history of the aircraft. The investigation established that the aircraft's wing tanks had been filled to maximum capacity at Katowice Airport, as there had been no refueling planned after the arrival at Kraków. The investigation also reviewed the maintenance intervals for the access panel, noting that the visual inspection interval for checking the condition of this specific seal is 10 years.
Findings
- The primary cause of the incident was a damaged and deteriorated seal on the surge tank service cover.
- The leak likely occurred during flight or while taxiing due to centrifugal forces during turns, which caused excess fuel to overflow from the wing tanks into the surge tank.
- It is possible that the leak had existed previously, but because the surge tank was empty during earlier flights, the leakage was not detectable.
- The investigation could not determine exactly when the seal failure occurred or how many operations had been performed with the faulty seal.