A320 HDG to NAV “wiggle”
 

When modifying the to waypoint, we have to select heading prior to inserting the modification. When pushing to reengage NAV often times there is a slight but noticeable bank to return to the green line. Since heading was selected for only a few seconds, why the “waggle” to get back to the green line? (1994-2000 model 319/320’s)

Since the TO point is changed I suppose there will be some deviation off track unless the same way point is reinserted. So the aircraft will reposition on track with some bank when back to NAV.

This is your definitive answer*, Cap Mongo and as a proof, next time note the wind direction. If the return to NAV commands a turn into the wind, you have your answer with confirmation.

*With respect to the aircraft switching from HDG to NAV. The "TO" waypoint could very well be the same airspace fix or navaid as before.

Why select HDG?

GF: The FMS will not allow changing the active "to" waypoint with LNAV engaged. Therein lies the answer as well, because if you change that, you will modify the active leg (actually create a new one) and flight path adjustment could be due.

The OP answered himself unknowingly (my underline):

Thanks, FD, didn’t fly the ‘Bus, not an issue on some other types. Enter the DIRECT TO, agree on it, then press EXECUTE.

So in the, less than 2 seconds I: pull for heading, insert, and push for NAV we will have gone that far off course? Is not the new waypoint created spacially dynamic between the A/C PPOS and the follow on waypoint?

I will take a look at that tomorrow.

isn't the new waypoint normally one of the waypoints of your flight plan?

so assuming the waypoints on your flight plan are not all on a great circle, which they aren't or it would already be a "direct", changing the point you navigate towards will change your heading.

let's say it in another way: why change where you want to go when you want to go straight?

There are times when I want to add a waypoint in between my PPOS and TO waypoint. As an example, I sometimes add a 30nm wayoint from the airport for a 250/10000 reminder. When you insert it, the MCDU make you pull into HDG and then insert it. Going back into managed, on some aircraft, puts them into a bank and you have to execute a DIR to arrest the bank.

Altcrznav
Can you explain the need for this? There is already a speed limit point on the ND which shows you where you will reach 10000 and transition to 300kts.

When cleared direct to an airport, there is no waypoint that will give me VNAV guidance to maintain a profile that puts me at 30nm/10000'/250kts. I add that into the MCDU manually so that I have a TOD point to reference.

3x Table. :E

Pardon me, but I am not satisfied with the responses.

I am proceeding direct to waypoint ABC which is 300 NM in front of me. I decide to create a new waypoint which will be ABC -100NM. I open the FIX INFO page. I type in ABC and 100 in radius. I select 4L which drops the radius into FPLN. That radius fix remains dynamic until I select heading.(does it not?) I select heading, insert, then select NAV. I can do that in one second, yet I still get a wiggle.

Yeah I do that as well. The technology is there to assist us in the manner we determine necessary. The aircraft is merely a tool, we are the brains.

Sure you could just remember 3:1, then again we could still have a FE, and we could still fly one leg a day, or ....

CM: I hear you.

Modifying the TO waypoint does mean
- not only doing a DCT-to some other place (2 keystrokes with (L)NAV still engaged)
- but also adding a intermediate new waypoint along the active leg.

For the latter, which you brought up, some of the modifications require pulling HDG, otherwise FMS refuses accept the change with NAV mode engaged.

Use cases? 20 NM before IDESI, 10 MIN before VESAR from the top of my head.

I would understand the "wiggle" to get back to the center line, if the green line moved away from actual PPOS during the reprogramming, but that is somewhat unlikely. At the same time, I do not recall such behaviour (to the extent I would be displeased enough to rembeber).

What FMS verison are we talking about?
Is the wiggle always towards the same side? (*)
Are you using onside FMS and onside AP (the normal way: FMS2 + AP2 and v.v.) (**)

* = if it is just a mathematical inprecision, the behaviour should be reasonably random, i.e. 1/3 left, 1/3 right and 1/3 nothing

** = presumably programming via FMS 2 with AP1 engaged could create a tiny discrepancy that would need to be compensated for?

Why not have the MCDU backing up your 3:1?

Does it do it if there's no crosswind component?

My guess is that since you're disengaging nav (however briefly) when you re-engage it, it might not be using any of its previous knowledge of crosswind correction, and therefore having to re-figure it out from scratch and starting out turning the nose directly to the fix, in the process.

There are only a few seconds you say between HDG pull and return to NAV mode?

Perhaps the waggle you see could be an auto flight system peculiarity to your aircraft type or company configuration. My earlier answer was based on a return from HDG mode to NAV after at least a few minutes.

That said, it still might be informative to see if the winds and any XTE relate to the direction of the course correction once returning to NAV mode.

Speaking of peculiarities, I believe someone told me when there is an AC bus failure on the Airbus, one of the ailerons or spoilers will raise a bit and this is "normal". I wonder if this behavior relates to the auto flight and is in some way connected to your waggle.

Since ABC in in the Flt Pln why not insert a PD, ABC/-100? No need to go to fix.

vilas: I think it is EXACTLY what is is doing. Either PD waypoint or FIX radial/radius and ABEAM. Thus an intermediate WPT gets inserted onto the active leg, which reportedly - as desribed - makes the FD/AP uneasy.