Narrative:

Descending through 3;500 ft for 3;000 ft; 10 NM north of zzzzz (weather was overcast with good visibility and a few scattered clouds between 2;000 ft and 4;000 ft); I began to configure the aircraft. As the flaps were extended from the 15 flap position to the 20 flap position we received a trailing edge flap asymmetry light with the associated EICAS message. The flap indicator showed the left flap extended to 20 flaps and the right flap extended to 15 flaps. The captain requested a turn towards the west; so we could figure out the problem. ATC cleared us on a westerly heading and asked us to climb to 4;000 ft. I climbed to 4;000 ft on a westerly heading. After analyzing the situation the captain declared an emergency and we accomplished the 757 trailing edge flap asymmetry checklist in accordance with the QRH. During the checklist we requested a northerly vector to avoid some scattered clouds and to remain VMC. The controller did not hear our request; so we were instructed to return to a westerly heading to avoid traffic. I returned to a westerly heading. The captain requested a turn towards final once we were prepared for the approach. ATC delayed our turn inbound for other landing traffic. The approach was uneventful. After clearing the runway we had the aircraft examined for hot brakes by the fire chief. Everything checked out fine so we taxied to the gate.

Related reports:

Google
 

Original NASA ASRS Text

Title: B757-200 flight crew experiences a trailing edge flap asymmetry during approach with the flaps moving from 15 to the 20 degree position. QRH procedures are complied with and an emergency is declared before an abnormal flap landing ensues.

Narrative: Descending through 3;500 FT for 3;000 FT; 10 NM north of ZZZZZ (weather was overcast with good visibility and a few scattered clouds between 2;000 FT and 4;000 FT); I began to configure the aircraft. As the flaps were extended from the 15 flap position to the 20 flap position we received a trailing edge flap asymmetry light with the associated EICAS message. The flap indicator showed the left flap extended to 20 flaps and the right flap extended to 15 flaps. The Captain requested a turn towards the west; so we could figure out the problem. ATC cleared us on a westerly heading and asked us to climb to 4;000 FT. I climbed to 4;000 FT on a westerly heading. After analyzing the situation the Captain declared an emergency and we accomplished the 757 trailing edge flap asymmetry checklist in accordance with the QRH. During the checklist we requested a northerly vector to avoid some scattered clouds and to remain VMC. The Controller did not hear our request; so we were instructed to return to a westerly heading to avoid traffic. I returned to a westerly heading. The Captain requested a turn towards final once we were prepared for the approach. ATC delayed our turn inbound for other landing traffic. The approach was uneventful. After clearing the runway we had the aircraft examined for hot brakes by the fire chief. Everything checked out fine so we taxied to the gate.

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.