Date: 201303
Local time of day: 0601-1200
Location: ZZZ.TRACON, US
Flight conditions: VMC
Light: Daylight
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight phase: Initial Approach
Airspace: Class C ZZZ
ATC / advisory: TRACON ZZZ
Aircraft component: Leading Edge Flap
Component problem: Malfunctioning
Reporter function: Captain; Pilot Flying
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Aircraft Equipment Problem Critical
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Aircraft Equipment Problem Dissipated; Flight Crew Diverted; Flight Crew Executed Go Around / Missed Approach; Flight Crew Landed in Emergency Condition; Flight Crew Overcame Equipment Problem; Flight Crew Requested ATC Assistance / Clarification; General Declared Emergency
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: During approach I was the pilot flying and requested flaps 5. The Leading Edge (LE) flaps moved to the extended position with a yellow 'flaps transit' light. Trailing Edge (TE) flaps moved to a position of slightly less than one with approximately a one-needle width split and stopped moving. We abandoned the approach and requested delaying vectors while we worked the problem. There was no roll or yawing of the aircraft. We had a pilot on the jumpseat so I asked him to calculate the data for our current configuration (TE flaps 1; LE flaps not full extended) and asked the First Officer (pilot not flying) to get ready with the QRH checklist for asymmetrical flaps. The revised data showed a runway stopping margin of less than 50 FT and; since we only had 6;000 LBS [of] fuel; we declared an emergency and requested to divert. We turned that direction while we started the QRH. Just as we were beginning to run the checklist; I saw the needle on the flap gauge suddenly jump to the matched (symmetrical) reading. There was no roll or yawing of the aircraft. When we momentarily moved the alternate flaps switch to the down position; the LE flaps quickly moved to the full extended position (all green lights) and the TE flaps moved slightly and symmetrically to position. We recalculated landing performance; and this configuration gave us a runway stopping margin of approximately 500 FT. We were now 20 miles from our original destination so we requested and received clearance back to that airport. This was my first opportunity to talk to the flight attendants; and I told them that we had a mechanical problem and would be landing in 10 minutes; there was no need to prepare the cabin. It was at this time that I re-evaluated the situation and decided that the proper checklist for us to be running was the Flaps Trailing Edge: Symmetrical. We switched to that checklist and per the QRH and closely monitored flap movement for asymmetry. The revised performance data showed a stopping margin greater than 1;300 FT at flaps 15. We made an uneventful flaps 15 landing in Chicago; and shut down at the gate with 4;000 to 4;100 LBS of fuel on board.I would like to add that having a pilot on the jump seat was a tremendous asset because he was able to keep us constantly updated on performance numbers for different airports and aircraft configurations while we concentrated on flying and running checklists. I am submitting this report because there are questions about my decision to change checklists during the event. Maintenance reset the flaps and ran them multiple times after my arrival with no malfunctions. I left to resume my trip before they determined the cause.


Second narrative: [Narrative 2 contained no additional information.]


Synopsis: B737-700 experienced a LE Flap asymmetry condition during approach; abandoned the approach and ran the appropriate checklists. After running the procedures; subsequent flaps 15 approach was uneventful.