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Looking for answers on real/sim behavior on A320 Managed/Managed non-ILS (2D or 3D) approaches.

Airbus says to only press the APPR arm button when on the final leg with the Final App Fix as the "TO" waypoint on the Nav Display. So, what happens if I press the APPR arm button when:

  1. the "TO" waypoint is not the FAF?
  2. when I am established on the intercept to the final approach course, e.g.: When the "intercept" line is solid?
  3. When I am in HDG over NAV (blue) when on a heading to intercept the approach course?

Answers can be technical. I know a lot about aviation, just not much about A320, (and maybe a little out of date).

Much appreciated in advance.

EDIT: FCOM FCOM APPR pb when FDP is TO waypoint

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I have never heard that the APPR pushbutton should only be pressed when on the leg to the FAF. Pushing the button earlier will simply arm the respective modes and they will engage once the engagement criteria are met. In fact, the A320 FCTM recommends to arm the approach modes earlier, but to remove points from the flight plan which will not be overflown when on radar vectors:

FCTM images

When ATC gives radar vector and clears for final approach course interception, the crew will [for] managed approach

  • Select HDG according to ATC
  • Select APPR p/b on FCU
  • Check on FMA the final approach mode engagement

(Airbus A320 FCTM - Normal Operations - Non Precision Approach)

You can find all the details about arming and engagement conditions for the APP NAV and FINAL modes in the A320 FCOM:

The NON ILS approach includes the following managed modes:
APP NAV mode for lateral guidance
FINAL mode for vertical guidance

[...]

Arming Conditions

The pilot arms the APP NAV and FINAL modes by pressing the APPR pushbutton on the FCU, if all of the following conditions are met:

  • The aircraft is above 400 feet AGL.
  • The flight plan is valid (lateral and vertical profile).
  • The active flight plan has selected a NON ILS approach.
  • GA mode is not engaged.

The FMA displays "FINAL" and "APP NAV" in blue. If NAV mode was already engaged, APP NAV engages immediately.

[...]

Engagement Conditions

APP NAV and NAV modes engage under the same conditions:
If NAV mode was engaged, AP NAV engages immediately. If HDG/TRK is engaged, APP NAV engages when the intercept conditions are met (aircraft track line must intercept the flight plan active leg).

FINAL mode engages, if:

  • APPR phase is active, and the deceleration point has sequenced, and
  • APP NAV is engaged, and
  • Crosstrack error is less than 1.5 NM, and
  • FINAL is armed, and
    • The aircraft intercepts a descending leg of the vertical flight path, or
    • In V/S (FPA) or OPEN DES, the aircraft intercepts a level-off segment of the vertical flight profile, with an FCU-selected altitude different from this level-off segment.

APP NAV Engagement

(Airbus A320 FCOM - Autoflight - Flight Guidance - Non Precision Appproach Mode)


Note: I am assuming you were talking about managed FINAL APP mode here. However, some Airbus A320s are equipped with FMS Landing System (FLS), which works similar to a precision approach like ILS:

The FLS (FMS Landing System) [...] is an Airbus option offering a solution to fly 99% of approaches that are not ILS/MLS, with a barometric vertical profile. It offers lateral and vertical guidance for a straight-in instrument approach, referenced from the aircraft position, along a trajectory retrieved from the FMS navigation database.

Airbus FCU modes for approach

(Airbus: Safely Flying Non-Precision Instrument Approaches)

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  • $\begingroup$ I added FCOM citation. Airbus terminology difference between FDP and FAF..? So, what you are saying is that as long as lateral Nav is established, then APPR pb is selected even if, say there are 3 waypoints before the FAF (If A, B, C were all in a line and you intercept at between A and B) and cleared on heading to intercept. So, is HDG over NAV blue considered laterally managed? $\endgroup$
    – WhoShock
    Oct 5, 2023 at 19:27
  • $\begingroup$ @WhoShock FDP is probably just the FAF in these cases. I think what the FCOM meant is that you have to clear waypoints before the intercept point from the flightplan, if you are radar vectored (see first quote in answer). But that intercept with the flight plan could hypothetically happen well before the FDP, that is just not typical for radar vectors. HDG over NAV blue is not yet laterally managed, but it will be once NAV (or APP NAV) becomes active. $\endgroup$
    – Bianfable
    Oct 6, 2023 at 5:58

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