Large Transport; Low Wing; 2 Turbojet Eng
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 95; Flight Crew Total 20643; Flight Crew Type 6383
Situational Awareness; Fatigue
Aircraft Equipment Problem Less Severe; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT; Inflight Event / Encounter Unstabilized Approach
Flight Crew Took Evasive Action; Flight Crew Overcame Equipment Problem; Flight Crew Regained Aircraft Control; Flight Crew FLC Overrode Automation
Aircraft; Airspace Structure
We were set up for the RNAV 25 approach with V ref (30) +5 speed [and] FAF altitude correctly inserted into the FMC at the FAF. LNAV and VNAV were the active lateral and vertical modes with autopilot and auto throttles engaged. First Officer was the Pilot Flying. We were in VMC conditions at 2500 feet; (estimated cloud bases at 3000 feet during the descent) and had the runway in sight directly ahead. Prior to the FAF; the Pilot Flying dialed the Decision Altitude into the Altitude window and upon reaching the FAF the aircraft pitched down normally to follow the path.During configuration change to flaps 30 and speed reduction to the final approach speed; the aircraft; for some unknown reason; suddenly pitched down reaching approximately 1500 FPM rate of descent. The Pilot Flying disconnected the autopilot and took over manually to arrest the descent rate; however the GPWS announced 'Obstacle' during the recovery. The Pilot Flying leveled the aircraft to re-intercept the VNAV path and the remainder of the approach was stable and uneventful.I can't remember making the 1000 feet call; but distinctly recall making the 500 feet call and the aircraft stabilized. The remainder of the flight was uneventful. Contributing factors: early report time; extreme cold weather; tail swap in departure airport; pressure to block out on time for the leg; irregular sleep and nutrition during the layover; circadian flip/flops and cumulative fatigue from previous days on the pairing. In addition; the event occurred on my 10th consecutive day of work.
[Report narrative contained no additional information.]
An Air Carrier flight crew reported that after the glideslope was captured some unknown reason the aircraft suddenly pitched down.