Date: 201307
Location: ZZZ.Airport, US
Aircraft: B757-200
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Flight phase: Descent
Airspace: Class B ZZZ
ATC / advisory: TRACON ZZZ
Aircraft component: Flap Control (Trailing & Leading Edge)
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Human factors: Human-Machine Interface
Second reporter function: First Officer; Pilot Flying
Anomaly: Aircraft Equipment Problem Critical
Detector: Automation Aircraft Other Automation
When detected: In-flight
Result: General Declared Emergency
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: 'Uncommanded secondary flight control surface movement' [while] descending through approximately 7;000 FT slight right bank; 250 KIAS; autopilot on; heading select; flight level change. Calm; smooth; uneventful flight conditions. The Leading Edge Slat Asymmetry EICAS message and associated warning came on along with a slight movement in the flap indicator to approximately flaps less than 1 (about 1/3 of the movement from up to 1). No configuration change had been initiated; and the flap handle was confirmed in the up position. Post flight walk-around reveled the inboard leading edge devices had moved out of the up and retract position but not to the fully extended TO position (during this entire sequence nothing should have commanded these to move at all). It appears the uncommanded inboard leading edge device movement was the impetus for the Slat Asymmetry EICAS message and system lockout. Descent was stopped at 6;000 FT and holding entered for about 10 minutes to deal with the problem. Normal configuration was locked out and aircraft was ultimately configured with alternate system per the QRH guidance. An emergency was declared CFR requested and a runway change requested due to the QRH configuration requirement to land with no leading edge devices and flaps 20.This configuration required an approach speed 30 KTS faster than normal. [Runway] XL was active runway; with winds 040/8; we requested and used Runway YL due to the longer length. Uneventful approach and landing.


Second narrative: Descending through approximately 7;000 in a slight right bank; 250 KIAS; autopilot on; heading select; flight level change. Calm; smooth; uneventful flight conditions. The Leading Edge Slat Asymmetry EICAS message and associated warning came on along with a slight movement in the flap indicator to approximately flaps less than 1 (about 1/3 of the movement from up to 1). No configuration change had been initiated; and the flap handle was confirmed in the up position. Post flight walk-around revealed the inboard leading edge devices had moved out of the up and retract position; but not to the fully extended takeoff position (during this entire sequence nothing should have commanded these to move at all). It appears the uncommanded inboard leading edge device movement was the impetus for the Slat Asymmetry EICAS message and system lockout. Descent was stopped at 6;000 FT and holding entered for about 10 minutes to deal with the problem. Normal configuration was locked out and aircraft was ultimately configured with alternate system per the QRH guidance. An Emergency was declared CFR requested and a runway change requested due to the QRH configuration requirement to land with no leading edge devices and flaps 20. This configuration required an approach speed 30 knots faster than normal. [Runway] XL was active runway; with winds 040/8; we requested and used Runway YL due to the longer length. Uneventful approach and landing.


Synopsis: A B757-200 flight crew received a leading edge slat assymetry EICAS warning during descent. They leveled; went into holding; performed the associated QRH procedures; declared an emergency requesting a longer runway due to procedural requirements; and landed uneventfully.