What happened
On December 7, 1997, a SAS DC-9-81, registration LN-RMM, was prepared for a scheduled flight from Helsinki to London. During the refueling process at Helsinki-Vantaa airport, a maintenance mechanic attempted to manually open the left wing tank fill valve because it had failed to open electrically, likely due to freezing. In the process, the mechanic mistakenly opened the center tank valve instead.
After realizing the error, the crew and the mechanic coordinated a fuel transfer to balance the tanks. The captain opened the crossfeed valve and activated the center tank pumps to move fuel from the center tank to the left wing. Once the gauges indicated the tanks were equal, the captain closed the crossfeed valve. However, because the mechanic had not yet closed the wing's defueling and fill valves, fuel continued to migrate from the right wing to the left wing through the open valves.
Upon takeoff, the aircraft experienced a severe leftward roll of approximately 25 degrees. The pilot had to apply roughly 70 degrees of right aileron input to maintain level flight. The investigation later revealed a massive fuel imbalance: the left wing contained over 4,200 kg of fuel, while the right wing contained only 1,100 kg, creating a difference of 3,100 kg—far exceeding the maximum allowable takeoff/landing imbalance of 681 kg.
The investigation
The investigation examined the fuel system's integrity, including inspections of the piping, valves, and tank seals. Pressure tests were conducted in a hangar to check for structural leaks or pipe failures. Additionally, fuel transfer tests were performed using a similar aircraft, an MD-83 (registration OH-LMG), to simulate the conditions and determine the possible rate of fuel migration between tanks.
Findings
- The primary cause of the incident was the unintended fuel transfer from the right wing to the left wing following the closure of the crossfeed valve.
- The mechanic's insufficient knowledge of the complete fuel system allowed the migration to continue unnoticed.
- The pilots' lack of familiarity with the specific fuel system configurations contributed to the oversight.
- The left wing fill valve failure was attributed to freezing, though it functioned normally once warmed in the hangar.
- The layout of the refueling panel switches did not match the physical order of the valves on the wings, contributing to the initial error.
- The mechanic failed to record the valve malfunction in the technical logbook.