F/D recycle in NPA - 737CL

[mmrassi]

Hi guys,

Can anyone explain me why we have to recycle the FD when we are doing a NPA. As the FCTM says we have to set the miss approach altitude on MCP at 300ft to MDA, and after disengaging the AP we recycle the FD. I know its about the GA but it makes me more confused when we don't have the FD after recycling? I need a good a explanation for this part of system.

Thanks in advance

[Denti]

Just fly it as IAN, no recycling there...

OK, that didn't help, did it?

[Shiny side down]

The flight director is only giving guidance based on the MCP/nav modes-
LNAV, VNAV, VORLOC, G/S(APP), etc

The NPA is only bringing you to a position from which you can complete a visual manoeuvre to land.

If, for example you were following a VOR radial that is very much offset from the runway course, the lateral FD command would be contradicting the actual need to fly towards the runway. So it is now providing bad information. Get rid of the FD.

If you were descending in VS for example, to achieve MDA, once visual with the runway, your vertical profile needs to be consistent with the PAPI or whatever visual cue is needed to touch down in the right place. So the pitch FD that you were using is no longer relevant and may be providing contradictory information. Get rid of the FD.

When you hit the toga buttons, now the FD pops up again to achieve commanded pitch for the V2+ speed, and limited bank to 400' in the initial go around manoeuvre.

I think that covers most of it, with the usual caveats.

[Wizofoz]

The reason for cycling, though, is what happens in a go-around when another mode is selected and the F/D switches are off- the F/Ds dissappear and thrust which was giving 2000fpm now changes (to THR from memoy??)

By cycling you ensure F/Ds are available and the AFDS goes into whatever mode you select.

[172_driver]

This was discussed in another thread a few days ago, so I have it quite fresh.

In case of G/A pressing TOGA will give you "pop-up" FDs to guide you through the go-around and clean up procedure, even if the FD switches OFF. Though, the FDs will disappear at ALT HOLD. Having FDs re-cycled will ensure the FDs remain on even after ALT HOLD.

[cosmo kramer]

mmrassi, to clarify, what confuses you... The need to turn them off at all? Or why they should be turned back on (recycled)?

The recycle (turning back on) has been explained perfectly by 172_driver.

Why to turn them off to begin with, has partly been explained by Shiny side down. If using basic MCP modes, the guidance is no longer appropriate after reaching MDA (or when becoming visual) and should therefore be removed.

However, if you are flying non-precision approaches using LNAV/VNAV for flying the profile (with raw data checks in the background of course), you actually have sensible guidance untill the flare. In principle you could leave them on.

My guess is that that the requirement to turn them off (and then back on), is either to keep a uniform procedure whatever using LNAV/VNAV or basic modes. That and/or to relieve Boeing of responsibility from whatever you are doing below MDA.

Quite obviously the maneuver from MDA to landing should be accomplished visually. However, if going into a pitch black airport in the countryside with no moon and no visual cues otherwise, and using LNAV/VNAV, with a visually confirmed good guidance, I personally would postpone the recycle or leave it on altogether. I am a big fan of raw data and hand flying in general, but have a sound respect for darkness.

[Shiny side down]

I would have to say that the procedure to recycle them is common across the board to maintain consistency in the procedures.
It's there in the NG manual, in selected cases.

Assuming that approaches are bringing you to a final position on the centreline, then great. The aircraft in question is the classic, however. Will this be conducting LNAV/VNAV or RNAV approaches?

[mmrassi]

Thanks all guys!
Very helpful answers.

One question raised regarding the answer of our friend 172_driver and that is about this statement: "Having FDs re-cycled will ensure the FDs remain on even after ALT HOLD."

Is the FD supposed to turn off after ALT HOLD?


Thanks alot

[Fullblast]

Yes, it is. If you leave the fd switches off and press toga the fd bars will appear anyway, then off again at alt capture, if you leave'em on the fd stays on even at level off.

FB

[RAT 5]

FD's off: they will pop up in TOGA. When you go out of TOGA in either roll or pitch that mode change will cause the respective FD bar to disappear. Thus if you select a roll mode HDG or LNAV the roll bar will disappear: if you select a pitch mode, or if it is activated automatically, e.g. ALT ACQ at MAA, then the pitch bar will disappear. Therefore each bar is independent. Recycling and displaying the green FD (with FD's out of sight) will ensure that after TOGA pop up they will remain in view and giving correct guidance when you move out of TOGA and into other modes.