B767 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Unstabilized Approach
Flight Crew Regained Aircraft Control; Flight Crew Overcame Equipment Problem
Aircraft
The crew built bugged and briefed for the visual approach backed up by the ILS. The weather was VFR. It was also briefed to expect ATC to possible keep the flight above the glideslope (GS) intercept at the FAF. We were vectored and told to expect the ILS for runway XY. We were then given a vector and told to maintain 2500 ft MSL until passing [the FAF] and cleared for the visual approach. We turned to intercept the Localizer and descended to 2500 ft. The MCP was Alt hold and the GS only was armed.I as PNF did not catch the approach was selected after altitude capture at 2500 ft and prior to [the FAF]. The autopilot captured the glide slope and began descending on the glide slope to cross [the FAF] as published at 2500 ft MSL. This caused the aircraft to descend below our clearance altitude of 2500 ft at [the FAF]. As soon as the aircraft began descending the auto pilot was disconnected and the crew manually pitched up to regain the assigned altitude. We descended to about 2300 ft before the altitude began to be regained. Prior to reaching 2500 ft the aircraft passed [the FAF]. The crew then flew a visual approach which was stabilized and within parameters of the AOM (Aircraft Operating Manual) prior to 1000 ft. The crew completed the approach and landing uneventfully.
B767 Captain reported that after the autopilot captured the glideslope; it descended below the assigned altitude.