B737-800
Flight Crew Air Transport Pilot (ATP)
Human-Machine Interface; Troubleshooting
Aircraft Equipment Problem Critical; Inflight Event / Encounter Fuel Issue
Flight Crew Diverted; General Maintenance Action; General Declared Emergency
Aircraft; Airport
While switching to Tower frequency on final the pilot flying called extend flaps from 25 to 30; but the indicator showed that one flap was still at 15 degrees. I immediately knew that we could not land at [the destination airport] in this configuration. The First Officer leveled the airplane at 2;000 and I told the Tower that we had a flight control malfunction; would continue straight out; level at 2;000 FT; and get back with them shortly. I directed the First Officer to fly and communicate with ATC while I accomplished the QRH procedure. We determined a diversion to a nearby airport to take advantage of the longer runway was the safest course of action; so we declared an emergency. I entered a diversion report in ACARS; communicated with the flight attendants and then told the passengers what was going on. When we raised the landing gear; however; the landing gear warning horn sounded continuously at cruise power; so we elected to fly gear down. We landed smoothly and normally some 15 minutes later. Blocked in with just 4;200 LBS of fuel on board; so time was of the essence on this divert. The First Officer did a great job flying the airplane and handling ATC while I was working the QRH. ATC handled us very well; were patient and understanding when we were busy accomplishing tasks and didn't load us up with too many questions or unnecessary vectors. I could suggest that the QRH procedure for Trailing Edge Flap Asymmetry be tidied up just a little bit in the decision about using Vref 15 or Vref ICE on step 3. I believe I inadvertently used Vref ICE when it wasn't necessary. It also should include a note to leave the flaps in the final setting after landing; instead of selecting flaps up. This might aid Maintenance in working the problem. I had decided to do this anyway; and the AMT [Aircraft Maintenance Technician] was thankful for that. Due to the facts that it was day six in a row for me (28 hours flying plus 4 hours deadheading); and day five for the First Officer; and due to the rush of excitement in the divert; we both declined to fly another airplane to [the original destination] and ended the trip [at the diversion airport].
A B737-800 flight crew inbound to a short runway suffered asymmetrical trailing edge flap extension at less than landing flap settings.  They elected to divert to a major nearby airport with longer runways.