The purpose of recycling FDs on a 737NG on final approach

[Papa_Golf]

Can somebody tell me why could this be needed after disconnecting the automatics after a NPA?

I've heard lot of different answers...

[keep_her_lit]

If you didn't and had a go around you'd get flight directors back but only until alt acq, whereas if you've recycled them then they'll remain on throughout the g/a and level off.

That's how it's been explained to me at some stage anyway

[rossjmc]

Recycling FD's

My take on it is that as it's a NPA there's no useful guidance to be gained from the FD's below minimums and we should be visual anyway with the runway. Depending on your company's procedures if the MCP is set to the Minimums prior to descending from the platform then we would get ALT ACQ and ALT HOLD and the guidance would then be indicating for you to level off which isn't very desirable.

By recycling them we are arming them again in the event we have to carry out a G/A.

What the poster above me is referring to is if you just switched them off. The guidance would be available all the way up until we either engage the A/P, select a different pitch mode or indeed we get ALT ACQ and then they will disappear completely.

[Dream Land]

Due to inferior AFCS!

[Denti]

There is actually no need to recycle them at all. NPAs are flown in IAN and since there is no difference to flying an ILS the FD behaves exactly the same, it will automatically switch to LNAV on go around and behave the same as from an ILS go around in the pitch axis.

We don't have the recycling in our SOPs anymore, only thing we do is rearm them if flying a FD off approach so we will have LNAV guidance during go around.

[rossjmc]

Recycling FD's

We don't fly IAN approaches and don't have the option to in any case.

That would depend on whether or not an operator has that option on their aircraft.

[Natstrackalpha]

Recycle FDs

Strangely, they do this on the A320 during a touch and go during base training.

Selects 50% FD,Flaps 2 and HDG V/S, check trim in the green band, Sel flaps 2, "Go!" TOGA, rotate at 140 or VAPP.

For a low go around would they not recycle the FD then on the A320?

[eagleflier]

Selects 50% FD

Like really? Is it me or the bus just seems complicated.

[cav-not-ok]

i think thats 50% N1

[de facto]

By recycling them we are arming them again in the event we have to carry out a G/A.

Absolute and utter

[rossjmc]

As i've stated above.

"The guidance would be available all the way up until we either engage the A/P, select a different pitch mode or indeed we get ALT ACQ and then they will disappear completely."

selective reading de facto??

[Denti]

Indeed, even without IAN recycling the FD is simply not needed. Apart from rearming when doing a FD off approach (and even that is not really necessary -> pop up) there is no reason to recycle the flight director on the NG.

That depends on your ordered options. On our planes, even the very old ones (we were a launching customer on the 800), the FD won't disappear, they will automatically engage in LNAV at 400' RA and continue in TOGA pitch mode until level off or another mode is selected. But even at level off they will not disappear, just showing the ALT AQU path based on current speed and thereafter ALT HOLD.

[rossjmc]

That makes sense alright. If i'm honest i had wondered why we needed to recycle them at all really. Previously our procedure was to set the MDA rounded up to the nearest 100' before descending from the platform and it would obviously capture in ALT ACQ and ALT HOLD as we approached minimums.

So then as we disconnected we would recycle FD's and set the MAA. I had thought it was just a procedure that had hung on since the days of that old way of doing it.

[fulminn]

also you need to recycle the FDs otherwise if you press toga the roll bar will give indication to maintain present track, not runway heading..and also because when you get alt acq the pitch bar disappear!