Narrative:

At flaps 5; we noticed that the autopilot was holding some right aileron to stay wings level. At flaps 30; the first officer disengaged the autopilot (he later told me that this was due to the autopilot not maintaining localizer centerline) and he had to use substantial (about 25 degrees of yoke) right aileron to stay wings level. We checked fuel balance; flap gauge; rudder trim and HUD slip indicator; all normal. We elected to land flaps 30; and the jumpseater quickly checked the landing performance numbers. I asked the first officer if he was ok with the landing; and he confirmed that he was. Landing was normal; and we later learned that there was a failed bearing in the left outboard flap that prevented the aft segment from extending. In retrospect; we should have done the memory items for uncommanded roll/spindle failure; gone around; and landed (somewhere else) flaps 1. I'm not sure why it didn't occur to us that this QRH procedure fit our situation. It was a slow onset with the autopilot on; so we didn't get a bang or a sudden change in roll force as is described with a spindle failure in training. The aircraft never really rolled; and it didn't require anything besides aileron to maintain wings level. It's not unusual to have some roll trim change with flaps down so it's just a matter of how much is enough to trigger use of this QRH procedure. Looking back at it; I'd say we crossed the threshold; but it just never occurred to us at the time. Having the runway three miles straight ahead with the aircraft controllable is a pretty strong obstacle to thinking about other actions besides landing. We should have thought about the possibility that we could have had a single spindle failure and that the second one could subsequently have failed.

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Original NASA ASRS Text

Title: A B737 had a flap asymmetry not indicated on the flap gauge because the flap spindles both extended normally except the left outboard aft flap segment had a bearing fail which prevented full extension.

Narrative: At flaps 5; we noticed that the autopilot was holding some right aileron to stay wings level. At flaps 30; the First Officer disengaged the autopilot (he later told me that this was due to the autopilot not maintaining LOC centerline) and he had to use substantial (about 25 degrees of yoke) right aileron to stay wings level. We checked fuel balance; flap gauge; rudder trim and HUD slip indicator; all normal. We elected to land flaps 30; and the jumpseater quickly checked the landing performance numbers. I asked the First Officer if he was OK with the landing; and he confirmed that he was. Landing was normal; and we later learned that there was a failed bearing in the left outboard flap that prevented the aft segment from extending. In retrospect; we should have done the memory items for uncommanded roll/spindle failure; gone around; and landed (somewhere else) flaps 1. I'm not sure why it didn't occur to us that this QRH procedure fit our situation. It was a slow onset with the autopilot on; so we didn't get a bang or a sudden change in roll force as is described with a spindle failure in training. The aircraft never really rolled; and it didn't require anything besides aileron to maintain wings level. It's not unusual to have some roll trim change with flaps down so it's just a matter of how much is enough to trigger use of this QRH procedure. Looking back at it; I'd say we crossed the threshold; but it just never occurred to us at the time. Having the runway three miles straight ahead with the aircraft controllable is a pretty strong obstacle to thinking about other actions besides landing. We should have thought about the possibility that we could have had a single spindle failure and that the second one could subsequently have failed.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.