Incident Details
On 20 February 2020, a Boeing 737-800, registration SP-ENV, was positioned from Katowice Airport (EPKT) to Kraków–Balice Airport (EPKK) for a planned flight to Marseille (LFML). After landing and parking at the apron, the captain performing a pre-flight inspection noticed that the lower surface of the left wing was heavily moist and dripping fuel. The crew reported the issue to the operator's Technical Coordination Center (MCC) and Operations Department (OPS).
Findings
Technical personnel arrived and determined that fuel was leaking from beneath the surge tank access panel. Fuel was transferred from the wing tank to the center tank, and the lower access panel of the surge tank was removed. Inspection revealed a deteriorated seal, which was then replaced with a compliant part. After updating documentation, the maintenance staff cleared the aircraft for further operation. No fuel leaks were observed after landing at LFML or during subsequent flights.
The investigation noted that the wing tanks had been filled to capacity at Katowice in order to avoid refueling at Kraków for the onward flight. It is likely that during the flight or ground maneuvering, centrifugal force during turns caused excess fuel to overflow from the wing tanks into the surge tank, leading to leakage through the damaged seal. The surge tank seal may have been previously faulty, but without fuel in the surge tank on earlier flights, the leak would not have manifested. Therefore, the time of seal failure and the number of flights with the defective component could not be determined.
Aircraft Information
The Boeing 737-800 (SP-ENV) was registered in the Polish civil aircraft registry on 4 November 2013 and entered service with Enter Air Sp. z o.o. The visual inspection interval for the access panel (including seal condition) was 10 years per Boeing Task Card 57-830-0101. The last such inspection had been performed as indicated in the maintenance records.
Probable Cause
The incident was caused by a damaged (deteriorated) seal of the surge tank access cover.