Date: 202010
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: VMC
Aircraft: B757-200
Operator: Air Carrier
Operating under: Part 121
Mission: Cargo / Freight / Delivery
Flight phase: Initial Approach
Airspace: Class B ZZZ
ATC / advisory: Tower ZZZ
Aircraft component: Flight Director
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Human factors: Fatigue; Situational Awareness; Distraction
Second reporter function: First Officer; Pilot Not Flying
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Detector: Automation Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Flight Crew Overcame Equipment Problem
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: Cleared for visual approach and cleared to land on RWYXXL. [Commercial flight] canceled their take off clearance so [ZZZ] Tower told us to continue the visual approach with a canceled landing clearance. We were 7.5 miles out direct to the Final Approach Fix in LNAV (Lateral Navigation) and the armed glideslope had captured at about 3;000 MSL providing a constant descent. We were Flaps 5 at 180 kts. To create spacing I called for gear down and flaps 20 and rolled the speed back above 20 maneuvering speed. The plane began to slow and pitch down with flap extension. I called for flaps 25 when the gear showed 3 green and the EICAS (Engine Indication And Crew Alerting System) showed 'Autopilot' indicating it was still working but in a degraded mode. I then saw GS FMA (Glideslope Flight Mode Annunciator) now had a line thru it. At this point I disconnected the autopilot and leveled off visually. The altitude was about 1400 MSL; 1000 AGL. I told the Pilot Monitoring I was going to maintain 1000 AGL until established. Right after I leveled off the [ZZZ] Tower announced low altitude alert. During the debrief the Pilot Monitoring said he saw the glideslope go full scale below us and then shoot up to full scale above us near the autopilot disconnect. I continued to maneuver visually to the runway centerline at 1;000 ft. Once established we cycled the Flight Directors and the glideslope centered back to where we were on the approach. Continued visually and landed. Wrote up the discrepancy in the logbook.  Fatigue didn't cause it; but it may have slowed my processing of everything that was happening kind of at the same time.  Don't allow the glideslope to capture early. Choose another vertical mode until on or very close to the LOC (Localizer).


Second narrative: We were cleared for the visual to [Runway] XXL and subsequently cleared to land approximately 10SW of the airport. We were level at 3000 when approach was armed and glideslope captured. Aircraft began descending on glideslope. ATC canceled our landing clearance and cleared a [Commercial flight] for takeoff on [Runway] XXL. In order to slow and create space the Pilot Flying called for gear and flaps 20. After we received an AUTOPILOT EICAS and the GS FMA (Glide Slope Flight Mode Annunciator) had an amber line through it. The Captain disconnected the autopilot and autothrottles and leveled the aircraft. We were approximately 1400 MSL (1000AFE). About this time [ZZZ] Tower called a low altitude alert. The glideslope indicated full scale deflection below us and shortly after indicated full scale deflection above us. The Captain continued visually onto centerline; the Flight directors were turned off and turned back on and approach rearmed. LOC and GS FMAs activated and appeared to act normal. Autopilot/Glideslope malfunction. ATC canceling our landing clearance and clearing an aircraft for takeoff on our planned runway caused a distraction about the same time as the malfunction.


Synopsis: B757-200 flight crew reported Flight Director anomaly on approach.