Flight Crew Air Transport Pilot (ATP)
Situational Awareness; Time Pressure; Human-Machine Interface; Confusion; Distraction
ATC Issue All Types; Inflight Event / Encounter Unstabilized Approach
Flight Crew Took Evasive Action; Flight Crew FLC Overrode Automation
ATC Equipment / Nav Facility / Buildings
During ILS/DME Runway 10 approach; aircraft [was] on autopilot tracking slightly below glidepath until approximately 1;800 AGL when nose pitched up slightly and power came up to correct to glidepath then glideslope dropped below horizon and autopilot chased it down resulting in an unacceptable sink rate. Disconnected autopilot and corrected to visual approach. I believe the ILS at MTPP should be tested for accuracy and proper functioning. I fly this [approach] a lot and observe the glide slope and localizer to be erratic constantly. The problem is; it is insidious and if you don't fly this [approach] a lot it can take someone by surprise. It acts like it is working normal then the glideslope slowly drifts up or down slowly. Suggest using only RNAV approaches.
The MTPP Runway 10 ILS glideslope rose slowly then quickly dropped; causing a B737-800 autopilot to chase it at an unacceptable rate down. The autopilot was disconnected and a visual approach completed.