767; why LNAV/VNAV doesnt require A/P?

[G-STAW]

Evening pprune,

ive just been scanning through the 767 cbt and it has become aparent that on takeoff, LNAV and VNAV doesnt require the A/P to be engaged for flight controls to move to maintain the FMC flight profile.

but the 767 AOM states that the autopilot moves the flight controls so why does the flight controls move when the A/P is disconnected?

is it that LNAV and VNAV are in some way seperate to the other modes on the MCP?

thanks

G-STAW

[luvly jubbly]

What you talkin' bout, Willis??

[luvly jubbly]

OK, I'll be more helpful..................

Forget the CBT. As you're doing a 767 course, I assume you have already passed the 757 groundschool.... The 767 MCP behaves exactly the same as the 757.

VNAV LNAV VS etc are modes which control the FDs (when selected). The AP simply follows the FD, therefore must be engaged to affect the controls....

LJ

[pdrusch1]

In addition, the flight controls can move as a result of manual control manipulation (hand flying) OR the A/P. The A/C can be hand flown "raw data" that is without a F/D, but at least one F/D must be operational to engage an A/P.

Hope this helps.

[A Comfy Chair]

G-STAW,

Either you've got the wrong end of the stick from the CBT, or the CBT is telling you porkies.

The Flight Controls will only move with a) pilot input or b) autopilot engaged.

The Flight Director will command LNAV and VNAV, but without the A/P engaged, the aircraft will go nowhere unless you point it there.

[Wizofoz]

pdrusch,

Are you sure about that? Long time since I flew the 767, but my last types have been 737NG and 777 which I thought operated in the same way. I think you can engage the A/P without the F/Ds. It engages in basic attitude modes, and then goes into whichever mode is selected.

I'm not sure that "The A/P follows the F/D" is exactly correct. Rather, they both get inputs from the AFDS system.

As to the original poster, you've mis-interpreted something. The only things that can move the flight controls are the Pilot and the Auto Pilot. As Luvely said, LNAV and VNAV are AFDS modes that drive either the F/D or the A/P. They do not autonomously fly the aircraft.

Oh and luvley, not every airline has 757s AND 767s. If got 3500 hrs on the 767, and I've never SAT in a 757!

[Mshamba]

Just to clarify: On a 737 the A/P does NOT need any FD to operate, of course the AP can fly the plane without any FD's activated - what is regular practice by the way. But the two modes LNAV and VNAV do require the FD activated before AP activation is possible. AP is engagable only into basic modes like LVL CHG or HDG SEL if FD's are OFF. You just cannot select LNAV and VNAV on the MCP with FD's off.

For example: If i get a direct on a raw data departure later on, i fly manually "the magenta line", first FD on, then LNAV, then VNAV, if everything's fine and no corrections by the AP to be expected, i engage AP and grab a cup of coffee.

No idea how this works on a 757/767 though, but can hardly imagine there are differencies, at least with the cup of coffee.

[G-STAW]

hello guys,

thanks for the constructive replies much appriciated.

i misread a diagram within the AOM,

so the pilot(whether LNAV/VNAV as been selected)is flying manually (following the FD)until A/P is engage, basically using the F/D to aid the pilot in positioning the a/c to keep on the VNAV profile, correct?

it also states the FCC provide "source information" for the autopilot and the flight director, what is the "source information"?


thanks guys,

G-STAW

[stiffwing]

Mshamba
On my 767 it is most certainly NOT regular practice to fly with an AP engaged with no FD guidance

G-STAW.. the source information refers to the fact that each AP has its source from independent FCC's, and so a small variation between the computed data from each FCC is normal (and desired.. redundancy).. that is, AP "C" gets its info independently from FCC "C"

[Starbear]

G-STAW

The replies given will I hope have cleared up the confusion but just to try and tie it together:

there are 3 FCCs (Flight Control Computers) which will independently compute from many varied inputs the required outputs to either the AP or FD and as others have said Left FCC=Left A/P or F/D etc.

Only one FCC is required for FD operation and both pilots can select that FCC simultaneously for FD display on their respective ADIs. Normally 2 FCCs are required to engage an AP (for internal cross check purposes but later aeroplanes seem to have a mod allowing single FCC for AP engagement) but no FDs are required for AP engagement.

The FCC outputs to the FDs, postion the FD command bars as targets for the pilot to follow for the selected AFDS mode and this can be any mode not just LNAV/VNAV.

The FCC outputs to the AP position the flight controls to follow the selected AFDS mode. If the FDs happen to be on they will reflect (hopefully) the commands from a different FCC (see later) that a pilot would require if manually flying. If FDs are selected off the AP will still fly the a/c as above.

With some authorities it is not permitted to have the FD and AP fed simultaneously from the same FCC and so the FD command bars bias out of view (no flags) when that FCC is utilised for AP purposes. It is for this reason that some company SOPs will specify, for one example, that the Centre AP for normal use with each pilot's FCC switch selected to Left or Right respectively for FD display and also why there are no FDs displayed when all 3 APs are engaged on multi-AP approaches. Some authorities seem to permit this or don't stipulate one way or the other so yo may see some a/c with the FD bars displayed even when using the same FCC (just a pin change I believe).

I hope this helps rather than confuse.

[G-STAW]

ahh,

that explains everything actually,

so basically the respective FCC tells its respective F/D command bar to move in relation to the selected AFDS mode on the MCP, thus telling the A/P to move the flight control to head towards the F/D command bars.

and, even if the A/P hasnt been engaged the F/D will provide guidance to the pilot for manual flight


correct?

cheers guys,

G-STAW

[Starbear]

G-STAW

You have it.

[G-STAW]

fantastic, thanks for the help!


G-STAW