Medium Large Transport; Low Wing; 2 Turbojet Eng
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 142; Flight Crew Type 12000
Aircraft Equipment Problem Less Severe; Inflight Event / Encounter CFTT / CFIT
Flight Crew Overcame Equipment Problem; Flight Crew Regained Aircraft Control; Flight Crew FLC Overrode Automation
Aircraft; Airport; Environment - Non Weather Related
The First Officer was flying a visual approach to 31R at DAL. He had the autopilot engaged with approach mode selected. The Localizer and glideslope were both captured and the aircraft was approaching target speed and final configuration. As we were approaching one of the buildings east of downtown; the glideslope began to fluctuate. The aircraft began to pitch up and down to regain the glideslope. As it pitched down; we received a Caution Obstacle Alert. The First Officer immediately disconnected the autopilot and corrected the flight path. We never received a Pull Up Alert. We realized the glideslope was getting some interference and the First Officer flew the aircraft visually without incident. The building was in sight the entire time; but I believe when the airplane began to pitch for the erratic glideslope it sensed a fast closure to the obstacle. After landing; I realized we should have gone around for the caution because it was dark. Since I didn't hear Pull Up; we had the obstacle in sight and the caution went away immediately we decided to land. However; after reviewing our policy I realized at night with a caution; it requires a go-around and that's exactly what we should have done. The glideslope anomalies for both 31L and 31R are nothing new; and I should have reminded the First Officer about them so he could have planned accordingly.
Air carrier flight crew experienced unstable glideslope on short final.