How do I know Auto Pilot will intercept an ILS in a 737-800?

I'm practicing ILS approaches by hand flying a 737 from airport A to airport B which has an ILS runway. What I'm trying to understand is how well positioned do I need to be for the Auto Pilot to take over and intercept the ILS.

 

I know that I need to receive the ILS Nav 1 signal and be

 

I know that I need to be approaching the ILS by hand flying but are there any guidelines as to when the autopilot will be "happy" to take over where my hand flying leaves off?

 

If the autopilot can't execute the ILS intercept/approach is there any visual indication in the cockpit that it can't do what I'm asking?

 

So far, I've found that if I'm approx. 20 km out and heading in approx. the right direction, APP seems to do fine. At other times it rejects what I'm asking but I don't know it has until by approach is totally off.

 

I know GPS makes it simple but for the purpose of this exercise I don't want to use GPS.

 

TIA

Basically, if you are receiving the localizer signal on the receiver and L-R the needle is not at either extreme, the AP will track the localizer (assuming the GPS/NAV switch is on NAV). If you are outside the FAF (Final Approach Fix) and still below the glideslope and tracking the localizer, then when you press the APPR button on the AP it will also intercept the glideslope as it reaches centered on the GS needle.

Good advise above. I think about it this way:

Max range for ILS in FSX is around 21.5 miles so outside that range, you will NOT have any chance of a GS track.

 

Between 3000-4000 ft above ground altitude, 20 miles out, the GS needle will snap to the top of the display. If the NAV/GPS button is in NAV mode and APP activated, you are guaranteed to track the GS when it crosses the center between 9-13 miles from the threshold.

-Pv-

It's important to note that a number of airports in the flight sim may have localizer only with no glide slope indication.

^^^

Yes, and when this happens the GS pointer will not snap to the top ~ 21 miles.

-Pv-