Flight director "anomalies"

[teropa]

Hi,A couple of things have started to "bug" me a little lately about the operation of PIC's flight director.Firstly, have you ever noticed that when you engage autopilot (ie. press CMD), the F/D moves to a different position than what it's indicating at the time you're manually flying the plane ?? Once the autopilot is on, it seems that the F/Ds are immediately centered, and the A/P starts maneuvering the plane to satisfy the AFDS. This is weird IMO, because one would expect the F/D behaviour to be exactly the same whether or not the A/P is engaged.This behaviour leads to another weird thing. I don't see a nose down command after passing acceleration altitude (VNAV engaged) at all. I think there should be significant nose-down command from the AFDS after AA, but there is not. If I was to fly the director all the way to flight levels, I would certainly not be able to retract the flaps as the should.Also, on takeoff. I have believed that the F/D should command V2+15 after liftoff, but it does not. It commands a much shallower climb than what would be required to keep even V2+25 which is the upper speed "limit" for initial climb until AA. Any faster than V2+25 and the climb performance is getting rapidly worse.Any comments ? I have thought about this long, and I think this might be a bug in the AFDS, or am I missing something ??regardsTerops. one more thing, yesterday with VNAV on (not on CMD) on initial climb, the F/Ds commanded max angle speed, not 250, after the AA, which was pretty weird... I checked the FMC, and the speed it commanded was exactly the max angle, even though I was on ECON CLB and the orange bug was on 250kts. Again, weird F/D behaviour...

[teropa]

Anyone ? I can't believe I'm alone with this ??Tero

[Guest RyHoffman113]

The F/D keeps me at V2+15 after rotation, commands nose down at accel height though I've noticed that on occasion when using the A/P on initial climbout that the plane forgets to drop the nose until after 2000AGL and I haven't been able to figure out why...sounds like same problem you're having with F/D not commanding nose down pitch at accel height.Ryan

[Guest]

Hi Tero,Ummm maybe you have already know this but for the F/d to give you a correct pitch command you have to set your V2 speed on the MCP panel.It is to this speed that the FCCs then add the 15-25 for the commanded pitch on t/o.Hope this helps.Cheers Sals

[KevinAu]

F/D centering upon autopilot activation is fairly standard behavior or these devices. If the F/D did not center, the aircraft may be subjected to abrupt pitch or roll movements when the autopilot activates and immediately corrects towards the error. By centering the F/D command initially, the autopilot can then correct the variances at it's own pace.

[teropa]

Sals,Yup... I know that :). The thing is, with the FL CH problems I've been encountering and all, I'm thinking maybe it's time for yet another reinstall of PIC, to clean up the table :).thanks for the tip though!Tero

[teropa]

Hi Kevin,Oh, I didn't know it was like that. Makes sense. I knew it had to be a "feature" rather than a bug. Thanks!Tero