Date: 201209
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Night
Aircraft: B757-200
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class B ZZZ
ATC / advisory: Tower ZZZ
Aircraft component: Trailing Edge Flap
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Distraction; Training / Qualification; Time Pressure; Situational Awareness; Human-Machine Interface
Second reporter function: First Officer; Pilot Not Flying
Anomaly: Aircraft Equipment Problem Critical
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Took Evasive Action; Flight Crew Overcame Equipment Problem; Flight Crew Landed in Emergency Condition; General Declared Emergency; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: I was the Captain and pilot flying. The flight was uneventful until the approach. We were assigned the VOR 13L approach. The weather was a ceiling of approximately 2;800 feet; good visibility underneath the clouds and winds down the runway at just under 10 knots. We were given several speed reductions during vectors to the approach and during the initial phases of the approach. We crossed the IAF at 160 knots (assigned by ATC) and Flaps 20 degrees. We flew the approach using non-ILS procedures using VNAV and LNAV. Passing the FAF I called for gear down and began slowing to final approach speed. At about the same time I was visual on the field; runway and lead-in lights [were seen]; so I disengaged the autopilot and as soon as the gear indicated down and locked I called for Flaps 25 and shortly thereafter Flaps 30. We were at approximately 1;300-1;400 feet MSL. I noticed that I needed a good bit of yoke to the right to keep the wings level after disengaging the autopilot. The First Officer said; 'Hold on a minute' when I called for Flaps 30. I asked him what the issue was and he pointed out that the left trailing edges flaps indicated 25 degrees and the right indicated just over 20 degrees. I directed him to move the flap handle to 25 degrees; as the aircraft handling was acceptable in the previous configuration. Since the aircraft was stable and controllable; I elected to continue the approach to landing rather than go-around. We obtained the Flaps 25 Vref from the FMC and set that in our MCP. I declared an emergency with Tower and had them send the emergency vehicles as a precaution. As this was happening the right trailing edge flaps moved to the 25 position and the rolling forces went away. We continued to an uneventful landing and terminated the emergency with Tower as we cleared the runway. Since the situation occurred close in and then resolved itself; I did not coordinate with the flight attendants. I spoke with Dispatch after the flight.


Second narrative: I looked at the airspeed indicator and then cross-checked the flap indicator and noticed left TE flaps indicated 25 but right TE flaps indicated just a bit beyond 20 (21-22 maybe). I pointed the difference out to the Captain and instinctively brought the flap lever up to the 20 flap detent. I then pulled up the approach page on the FMC and reset the Vref for a flap 20 landing and told the Captain the new approach speed. I believe the Captain said to return the flap lever to 25; so I did.


Synopsis: A B757-200's right trailing edge flap remained less than Flaps 25 with 30 selected. The Captain felt the disagreement and after the First Officer confirmed the flap gauge disparity; Flaps 25 was reselected; an emergency declared and a normal landing completed.