B737-800
Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Flight Crew Total 7800
Situational Awareness; Confusion
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT
Flight Crew Took Evasive Action
Airport; Procedure; Human Factors
Cleared for the visual approach to 9R at PHL airport. The weather was night VMC with unrestricted visibility and light winds. I was the Flying Pilot with the autopilot engaged. We were at 2;000 ft. AGL approaching the 9R LOC and GS; I selected approach mode; called for the gear down and flaps to 15. The Captain (Pilot Monitoring) verified the speed (below 200) and selected the flaps to 15. The airplane turned to final; captured the LOC and GS. The airplane started a descent on G/S. Without warning the GS indication deflected instantly to full down. With the autopilot engaged; the airplane abruptly pitched nose down as it attempted to track the errant GS indication. I quickly disconnected the autopilot and arrested the sink rate; leveling at 1;100 ft. AGL. The aural warning of 'sink rate' activated. Once leveled at 1;100 ft. with the autopilot disconnected; the Before Landing Checklist was completed. I flew manually level to intercept the glide path to a landing (followed the VASI). Meanwhile the Approach Controller told us he had received a low altitude alert and instructed us to contact the Tower. Once the Captain contacted the Tower; we were cleared to land on Runway 9R. At 1;000 ft. AGL the airplane was fully configured; on speed and before landing checklist completed. I continued the visual approach to a normal landing. Both the Captain and I could not figure out why this event occurred. What makes the glide slope to all of the sudden dive to the bottom of the display?? Either there was a system malfunction; or a truck or airplane drove or taxied across the ILS critical area. (Our assumption).
B-737 First Officer reported experiencing glide-slope signal errors resulting in a low altitude alert.