B737-700
Flight Crew Last 90 Days 140; Flight Crew Type 2312
Confusion
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Inflight Event / Encounter CFTT / CFIT; Inflight Event / Encounter Unstabilized Approach
Air Traffic Control Issued Advisory / Alert; Flight Crew FLC Overrode Automation; Flight Crew Regained Aircraft Control
Aircraft
We were doing the RNAV W to XXR from ZZZZZ. Aircraft was doing fine as we configured. Approaching 1;300 feet AGL; just as the flaps were set to 30; the aircraft pitched over and started a dive. I said; 'what's it doing?' a few times quickly then clicked off the autopilot and recovered the aircraft below 1;000 feet. Tower gave us a low altitude alert. I had the ILS in as backup; and I climbed back to a normal glidepath and landed uneventfully. The Captain and I discussed the situation and we agreed that the aircraft (for some reason) went to 'Speed' and was chasing the airspeed. We all have the aircraft go into speed during arrivals with no indication; but this was a first for me on approach. Had this been night/IMC; it could have resulted in something very bad.
B737-700 First Officer reported that while on final approach the aircraft pitched over and started a dive. After disengaging the autopilot; the flight crew recovered and made an uneventful landing.