B737 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 140; Flight Crew Total 14900; Flight Crew Type 3000
Fatigue; Situational Awareness
Aircraft Equipment Problem Less Severe; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Unstabilized Approach
Flight Crew Became Reoriented; Flight Crew Overcame Equipment Problem
Human Factors; Aircraft
On glide slope ILS Runway 19C IAD; configured; everything normal. Weather 600 OVC RVR 6000+. Autopilot turned off about 1000 AGL; auto throttle still on. Approach was completely normal and stable until at 700 MSL (about 400 feet AGL) all at once the Captain's side (mine) flight director disappeared; FO's still visible; FO called Rwy in sight; I disconnected auto throttles; and the GPWS called glide slope; airspeed showed 8 kts slow; I added plenty of power to correct; we made an adequate and safe landing. But what happened when the FD disappeared? Instantly we were low and slow yet I remember changing nothing. One second everything was normal and stable and the next instant we were low and slow. I don't know what happened to cause the FD to disappear and the GS to go haywire (interference in the critical area?). There was no wind shear to explain away the airspeed loss. Pilot error? I don't know what I did since it was stable the whole way down and required little input. Yes I was ridiculously tired today but neither the FO nor I noticed any radical changes made by me. It was recovered safely and otherwise no big deal except the GPWS GS warning.
B737 Captain reported a low altitude unstable condition occurred rapidly and without obvious cause on an approach to IAD.