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.
Synopsis
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.
Second reporter narrative
The aircraft started rolling to the left upon flap deployment. The autopilot was on so there was only the control column that gave any indication of the roll. Once flap 30 was selected; the autopilot could not hold the ILS and ended up turning left. Upon disconnecting the autopilot to manually return the aircraft to the ILS; I could feel the roll and should have executed the uncommanded roll memory item. Since the disconnect was close to the ground and the airplane remained easily controllable; I didn't have time to execute the maneuver as I was trying to figure out what had just happened and land the airplane in the next minute. Take into account that an uncommanded roll doesn't have to have the aircraft roll to a quick 90 degree angle like a spindle failure to follow the memory item.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.