Narrative:

Vectored to join the rwy xx ILS inside zzzzz; called the field and cleared for the visual. Disconnected to autopilot to descend and join the glide slope (which was below us). Continued to hand fly since the aircraft wouldn't lock the glide slope outside the localizer. As localizer came in; the flight directors remained on the horizon and unusable. I asked the first officer (first officer) to turn off and turn on both flight directors to regain their use. At that point (just outside ZZZZZ1) the flight directors were giving us good glide slope and localizer information. We put the gear down and commenced to slow to approach speed inside ZZZZZ1 (still hand flying) and I disconnected the autothrottles somewhere around 1000 ft. AGL. Approximately 600 ft. (Inside 3 miles) we received an aural 'glide slope; glide slope' warning. At that time I noticed that while my flight director in the HUD and on the pfd was centered; the glide slope information was deflected all the way to the top of my display. A quick scan of the first officer's pfd showed centered flight director as well. Checking the papis for rwy xx showed 4 red. I leveled off and added power to fly to the glide slope again; readjusted power; and landed uneventfully. No warnings or cautions were displayed.when we reconnected the flight directors inside zzzzz; descending on the glide slope; they came back directly where I wanted them showing me on glide slope and on course. I did not check localizer and glideslope annunciations on the pfd. Is it possible they did not capture the glideslope? I would doubt that since I got a glideslope aural warning. I have no idea why the flight directors both sides had us descending below glide slope inside 3 miles on the approach. Wrote [a maintenance] entry; called [maintenance control] and conferenced in the avionics tech for a verbal debrief on the gripe.I guess I could have caught the error had I been cross checking the glide slope on the right hand side of the HUD display; but I sit back far enough that I have to make an effort to see it; and it's not in my habit pattern on a visual. I'm normally working lineup; glide slope; airspeed; and any crosswind correction looking outside and through the HUD. It was my error for not scanning the PAPI to detect the gradual settle - which I'll put down to fatigue - but the flight director was centered! Long day. Error captured and uneventful landing - but very troubling to see the flight directors do that.

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Original NASA ASRS Text

Title: B767 flight crew reported Flight Director malfunction on approach.

Narrative: Vectored to join the Rwy XX ILS inside ZZZZZ; called the field and cleared for the visual. Disconnected to autopilot to descend and join the glide slope (which was below us). Continued to hand fly since the Aircraft wouldn't lock the glide slope outside the localizer. As localizer came in; the flight directors remained on the horizon and unusable. I asked the FO (First Officer) to turn off and turn on both flight directors to regain their use. At that point (just outside ZZZZZ1) the flight directors were giving us good glide slope and localizer information. We put the gear down and commenced to slow to approach speed inside ZZZZZ1 (still hand flying) and I disconnected the autothrottles somewhere around 1000 ft. AGL. Approximately 600 ft. (inside 3 miles) we received an aural 'Glide Slope; Glide Slope' warning. At that time I noticed that while my flight director in the HUD and on the PFD was centered; the glide slope information was deflected all the way to the top of my display. A quick scan of the FO's PFD showed centered flight director as well. Checking the PAPIs for Rwy XX showed 4 red. I leveled off and added power to fly to the glide slope again; readjusted power; and landed uneventfully. No warnings or cautions were displayed.When we reconnected the Flight Directors inside ZZZZZ; descending on the glide slope; they came back directly where I wanted them showing me on glide slope and on course. I did not check LOC and glideslope annunciations on the PFD. Is it possible they did not capture the glideslope? I would doubt that since I got a Glideslope aural warning. I have no idea why the flight directors both sides had us descending below glide slope inside 3 miles on the approach. Wrote [a maintenance] entry; called [maintenance control] and conferenced in the Avionics tech for a verbal debrief on the gripe.I guess I could have caught the error had I been cross checking the glide slope on the right hand side of the HUD display; but I sit back far enough that I have to make an effort to see it; and it's not in my habit pattern on a visual. I'm normally working lineup; glide slope; airspeed; and any crosswind correction looking outside and through the HUD. It was my error for not scanning the PAPI to detect the gradual settle - which I'll put down to fatigue - but the flight director was centered! Long day. Error captured and uneventful landing - but very troubling to see the flight directors do that.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.