BAe 125 Series 800
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Situational Awareness
Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Aircraft; Human Factors
Potomac Approach give us instructions as we crossed Cavalier fix to proceed directly to Mosby fix (IF) for ILS Runway 1R; and maintain 4;000 ft. Seven miles from the fix we were instructed to [descend to] 3;000 ft. to cross Mosby and cleared for the ILS approach.As we crossed MOSBY at 3;000 ft. I armed approach and allowed autopilot [to] continue approach. As [we] were getting closer to the next fix [Waxin at 1;900 ft. with] GS captured I noticed autopilot [was] not holding 1;900 ft. and continued to descend. I disconnected the autopilot and hand flew the aircraft to the proper attitude on the glidepath. At the same time Tower notified us they are getting low alert altitude warning. We acknowledged the correction and indicated we were on the glidepath and proceeded with our approach to complete the landing.The lesson I learned from this [is] always be prepared to take the airplane from automation and fly the airplane.
Hawker 800XP Captain reported that an autopilot malfunction during approach resulted in an altitude deviation.