Suspected Flight Director run away after T/O ( B757classic/legacy)
B757 legacy/classic - Need your input / opinion on this rare anomaly/ fault . What transpired was as follow:
1- Normal rotation , normal F/D lag .. Aircraft rotated to 20 degree pitch up ( aircraft light )
2- At 1000 AAL we selected Climb Thrust + VNAV + CMD
3 - Aircraft then went into ALT CAP.. I selected Climb Thrust + Vref 80 + Speed
4- Checked speed above Vref 30+40 and Called for flaps 1
5- Speed dropped and continued reducing: rechecked speed selection on MCP speed window - Vref+80
- No Flags on ADI - No Eicas message .
6-Then noticed that the F/D command bars had increased to an abnormal high pitch up ( CMD ) center auto pilot already engaged :hence the speed drop.
7- AUTO PILOT disengaged and nose pushed down + trim ( we ignored the F/D)
8- Speed increased , clean up and ,after the F/D went back to a more sensible pitch, we re-engaged CMD auto pilot.
Flight then continued as normal .
NOTE - we did NOT recycle the F/D in the air .
Of course I'm just guessing that it's a FD runaway as it could have being something else. Would appreciate your input as to why this happened ?