Flight Crew Air Transport Pilot (ATP)
Flight Crew Type 1236
Distraction; Communication Breakdown; Workload
Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Unstabilized Approach; Inflight Event / Encounter Weather / Turbulence
Flight Crew Executed Go Around / Missed Approach
Human Factors
The weather in San Diego was very gusty and continuous light to moderate turbulence down at our vectoring altitude of 2;000 ft.  I was the flying pilot and had the autopilot on as we were cleared for the approach to runway 9. A glideslope dot before intercept I called for gear down; flaps 15; landing checklist and it was completed. At the intercept the airplane began to porpoise as the autopilot struggled with the choppy air to the point where the flight directors were kicked off. I turned off the autopilot; stabilized the aircraft while maintaining the raw data approach. When I looked out the window; the runway and vasi were already insight so we decided to continue on the approach. I asked for the captain to reestablish the flight directors which he was able to do in short order.  We continued to configure to flaps 25; followed by flaps 30 and the landing checklist was completed. When we started passing over the small hill off the end of the runway we received a 'Too Low Flaps' aural warning.  I asked the captain if there was something wrong with the flaps to which he checked the flap handle and confirmed we were ok and should continue. Over the threshold we received a 'Terrain' warning at which point I looked down and saw that the flaps were actually set to 25 and not 30.  I immediately executed a go-around; we climbed up to 3;000 ft; followed ATC's instructions and returned to an uneventful landing on runway 9. In the debrief with the captain; I learned that he had accidentally moved the flap handle from 25 to 40 on the inital flap 30 call of mine; realized his mistake and tried to return the handle to 30 but overshot and ended up back in 25.  I unfortunately missed the error due to heightened focus on hand flying a very choppy approach.
Aircraft at 180 kt; flap 15; cleared for ILS 9 SAN; intercepted glideslope; started down. Hit turbulence sufficient to disengage autopilot. As Pilot Monitoring was attempting to reestablish flight director and auto flight system while configuring aircraft for landing (approximately 1600 ft AGL). Configured the aircraft (I thought) in accordance with of commands. Read landing checklist and checked annunciation. I thought I had seen correct indications but apparently had not. We got a landing warning for flaps. I checked the flap detention and it was in flaps 25 and moved to flaps 30 but actually moved to 40 and flaps blew back. Got another warning for flaps and Pilot Flying (FP) initiated missed approach. Subsequently got a too low terrain warning but we were already in the missed approach.
B737 crew reported executing a missed approach due to 'Terrain' and 'Too Low-Flaps' aural warnings received on short final. Crew determined that the cause was that the wing flaps were in the Flaps 25 position instead of the required Flaps 30 position.