Oh man there is a lot of miss information in this as far as how APPR now interacts with the autopilot and setting it up. The tutorial posted is a good reference, but the system has been updated since then.
Let me walk you through this step by step. Let’s begin by talking about the ILS approach.
The ILS approach is made up by two individual radio signals sent out from the center of the runway know as the Localizer and the Glide slope. Each of these has a very distanced purpose. The localizer is used for lateral navigation which keeps you aligned with the center of the runway. The Glide slope is used for vertical navigation to keep you at a constant angle of descent on the approach.
Now let’s talk about how to set up APPR.
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You will need to have the autopilot engaged you can either have NAV selected so long as your flight plan intercepts the localizer of the runway you wish to use. Or you can use HDG.
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You need to have your altitude set to be below the glide slope after you intercept the localizer. Most ILS approaches are 3° so at 10NM DME from the runway the glide slope will be at 3,000 feet above the runway. If you intercept at 5NM it’s going to be 1,500 feet.
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You need to select the ILS that you will be using so you get rid of auto tune this can severely screw you up. Which can typically be done within 25 NM of the airport so long as your facing the relative direction of the runway.
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Once all this has been established you can then arm APPR.
Scenario KSAN runway 09 ILS approach approaching from the east.
Flying between the middle and outer airspace rings
AP on, HDG 270, altitude 2,000, VS what ever gets you there, speed below 200 KTS.
10 NM west of KSAN HDG 180 or 360 depending on if you were north or south of the field. Runway 09 ILS selected, APPR on, Altitude 2,000 feet.
45-30 seconds from the center of the red cone turn HDG 120 or 030 again depending on direction.
At this point we are well below the glide slope, we are on a 30° intersecting course of the localizer, our APPR is armed and ready. Once the localizer needle begins to move this is where APPR begins to take over lateral guidance. Vertical guidance is still with the AP as we haven’t reached the glide slope. Once the glide slope centers then APPR has been fully activated.
If you come in above the glide slope APPR will never fully activate. Hope the step by step helps let me know if you have any questions.