Suspected Flight Director run away after T/O ( B757classic/legacy)

[camps]

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 ?

[Dufo]

We had a very similar occurence earlier this year on 737-400.
I am searching for crew report.. anyway we couldn't find any anomalies with the systems (QAR/FDM) and no actions/recommendations followed.

[camps]

Thanks . Our aircraft was a particularly old B757 ( early eighties model )
Hopefully we can both get some feedback from any readers out there

[RAT 5]

You say FD Runaway as if the FD controls the A/P. It does not. It reflects what the A/P is going to do to correct an error between the a/c performance now and what the MCP or FMC is commanding. It is perfectly possible to fly A/P ON & FD OFF. So if the a/c pitched up under A/P CMD which did not comply with the MCP selections I'd suggest the problem lies within the A/P and not the FD.
This is my understanding and I welcome to be corrected and learn more.

[EMIT]

The Autopilot was in command and it performed the maneuver that it announced through the FMA.

The active mode was ALT CAP, which is a precalculated path from climb conditions to level flight conditions.

The capture was initiated far below the level off altitude, because of the very high climb rate. During the execution of the capture, thrust was reduced from T/O thrust to CLB thrust, this resulted in too little thrust for the remainder of the capture maneuver.

The situation resembles very much this case (Canada TSB)

Aviation Investigation Report
Aircraft Pitch-Up/Stall Warning on Departure
Air Canada
Boeing 767–233 C–GAUE
Toronto/Lester B. Pearson International Airport
Toronto, Ontario
26 January 2004
Report Number A04O0020

[camps]

RAT 5 - yes you are correct ; totally agree with you.

Just another point I forgot to mention;there is a possibility that during the cockpit preparation setup we forgot to recycle the F/D after we had set the V2 speed in the MCP ; do you think that might have being the cause ? Do you think it might have affected the FMC / AP ?
The manual says that the F/D must be recycled on the ground after setting the V2 speed but the manual doesn't elaborate why ?

[camps]

Thank you EMIT

[flyingchanges]

EMIT has it 100%. I have seen this on several occasions especially with high vertical speed.

[FullWings]

This happens a fair bit. Any time you trigger an altitude capture with an instantaneous rate-of-climb which is not sustainable, if the aeroplane follows the path it will trade speed for height. An A330 was lost at Toulouse because of pretty much that...