When I enable Approach Mode in Airbus 320, I start gaining altitude

Hi, so I set up a low alt ILS flight form KMIA 8L to KFLL 16L. As I am heading toward final, I have the glidescope diamond pretty centered before I hit the approach button. I press approach when I am intercepting the runway line and the bottom diamond starts marching over. This usually has worked in other flights, but in this one as soon as I press Approach, the plane starts gaining altitude instead of continuing to follow the glideslope down. If I let it go I just fly over the airport and then the AP starts going around or something. Why is this not working? Its done the same thing about 3 times in a row.

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First which Airbus, stock or FBW? You don’t activate approach when it is centered already, that’s too late. You have to capture the localizer first, some 12-15 miles away (3000ft AGL) from the airport at an intercept angle of 90 deg or less. You activate approach prior to intercepting the LOC. If you are referring to the appr mode in the FMC than that should have been engaged much earlier by a waypoint or manual activation.

I saw that too. Still it is not quite clear why that happens. I thought may be some not visible Star or other routing? Assumption is that in the described situation it should still be possible to activate the approach mode… why would that be too late?

When locked in (LOC and GS) the AP is responding to the signals that is receiving. It’s actually straightforward. The error in the simulator could be due to the specific airport or specific plane. Or it could be some other transient behavior that we don’t understand because we don’t know the software design.

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We ask that people use Community Support when asking for help from others. I have moved your topic there.

You may also want to double check the MCDU, both your descent calculations and the RAD/NAV page to make sure the localizer frequency for your runway is imput correctly.