B737 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 188
Human-Machine Interface; Confusion
Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Fuel Issue
Flight Crew Overcame Equipment Problem
Procedure; Human Factors; Aircraft; Company Policy
While at cruise flight; I noticed the autopilot kept increasing aileron trim. I mentioned it to my Captain and he immediately realized we had a 4;000 pound fuel imbalance. All four main tank fuel pump switches were in the 'On' position with the crossfeed valve in the open position. He promptly began balancing fuel. By the time we landed the fuel was balanced. We believe my Captain may have opened the crossfeed valve when he turned off the center tank pumps.I think there were many factors that contributed to this occurrence. Neither one of us flies the classic frequently. Therefore; there is a lack of familiarity with the aircraft. The fuel gauges are closer to the Captain and were not in my scan. The Captain experienced interference from the newer model fuel procedure and out of habit moved the crossfeed valve after turning off the center tank pumps. Also; if the system is functioning correctly; I believe the fuel system should have remained reasonably balanced. From the crew perspective; making sure to occasionally scan the fuel gauges and both crew members verifying each other's switch movements would have helped prevent this situation. From a procedure perspective; having like aircraft with different procedures opens the door to these mistakes. Also; flying another aircraft infrequently makes it difficult to include different gauges in your scan.
B737 First Officer attributed Captain's improper positioning of the fuel pump and crossfeed switches to differences in B737 models flown by the company and to the fact that the Captain and the First Officer usually flew the newer models.