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Ok, i am on approach, AP on, ILS frequency is entered, NAV1 select, NAV1 picked up beep sound, Plane align with runway, ALT HOLD at 3000. The moment I press APR the plane falls from the sky.

Hi

How is this MF Related ???

Why do you press APR so late?
Sounds strange to me.... But i can also not see if you fly with VNAV-LNAV i not know if 3000 Alt is correct for that Airport.....

So. You must Confirm you know 100% how to fly a ISL Aproach and how to handle your Aircraft there.
I recommend to watch a Youtube Video how ILS Aproaches work ( for exactly the Aircraft/Addon and Sim you use).

Well, in my experience, you should fly into the glide slope with armed APPR from below. As soon as the glide slope is entered, the system automatically should engage APPR mode.

It sounds weird to me that engaging approach mode in an autopilot system while locked on the ILS should somehow inhibit an airplane from performing its aerodynamic tasks. This in mind I would kindly ask what behaviour the plane is exactly showing.

It sounds like you are in the autopilot NAV mode while aligning with the ILS localizer and the altitude is being held by the autopilot ALT mode. So far so good.

Now you need to wait for the glideslope indicators, the two yellow triangles to appear on the HSI BEFORE pressing the APP button on the autopilot. In your case autopilot is holding the altitude at 3000 but when you press the APP button, the autopilot will now control altitude by the glideslope signal. If that signal is not available, no glideslope signal, then the plane pitches down because the autopilot can't "lock-on" to a signal that's not available. Then effect on pitch is like disconnecting the ALT mode on the autopilot.

When you press the APP button you are in effect telling the autopilot to cancel the ALT hold and use the GS(glideslope) signal for altitude control. Make sure the GS signal is available before pressing the APP button.

After read this again....

There is a situation you would need this!

If you get a clearance for LOC Only.
Done on some Airports or in rush hour situations.

For example your cleard "LH745 Turn Left heading 270 ...Cleared for Localizer Runway 24"
Means you can join the LOC but your not allowed to capture the GS for now. You remain in your current or cleared for Altitude.
Done for example to avoid conflict and sepration issues if a other aircraft is still to close on a parallel runway.
So you will get the ILS or Glideslope Clearance a little delayed.

Whatever.
Topic is pretty old. I expect the user Johann meanwhile did the landing !