I've been simming for a long time, but I have a an ILS question. What distance from the landing runway do pilots in real life capture the glideslope? Should they target capture at the FAF, or capture it just as soon as possible (10-15 miles out)? I know ATC will vector them to where they can capture the localizer, but is it pilot's discretion as to what distance they actually capture, or are there specific rules that must be followed?

For your typical 3 degrees glide slope angle

A simple figure would be 10 miles at 3000 feet and 5 at 1500.In real world the pilots would refer to charts such as Jeppesen , for exact data.

 

You should be able to pick up an ILS signal strong enough to use about 18NM away within 10 degrees of the runway centerline and 10 NM when you are within 35 degrees of the runway centre line.

 

If you know trigonometry

Tangent (glideslope)=height/distance.

For a poor mans glide slope, each mile is worth 300 feet in altitude. Multipliy miles from runway threshold by 300 (Feet) for the correct altiitude the airplane should be at on a 3 degree glide slope.

Curt, good question!

 

Actual approaches differ from runway to runway, and the approach procedure in each case will provide profile guidance in terms of what height (and thus what distance) to intercept the glideslope at.

 

If you aren't so worried about adherence to real-world procedures, try this: My personal rule of thumb for this is actually associated with the minimum height for establishing a stable approach. I use 1,000ft AGL for an instrument approach and 500ft AGL for a visual. I want to be in the landing configuration, at the right speed, on profile by the time I descend through that height, otherwise I go around.

 

So, assuming that the weather conditions would prevent a visual approach (i.e. cloud base down towards minimums), I'd try to be established on the ILS (or VOR if that's what's available) about 10 miles out. That gives you time to get the aircraft configured and stable well before 1,000ft.

 

If it's a nice day with a cloud base above 2,000ft or so, and I expect to go visual, I will intercept as close as 5 miles and aim to be stable passing through 500ft.

 

Each to their own of course, but intercepting much further out than 10 miles would probably be excessive, unless the approach procedure explicitly calls for it.

Thanks for the feedback. After thinking about it, I guess ATC commands what altitude to descend to, and capturing glideslope at that altitude is a must...whether it's 5, 10 or 15 miles out.

Curt, I totally got off subject above, but for real world ILS capture point operations, you shouldn't arm or capture the glide slope until ATC has cleared you for the ILS approach. If they tell you to join the localizer, you should only push VOR LOC and capture the localizer.

You can follow and stay on the GS and follow it down, or you can just descend below it to the attitude ATC cleared you to and capture there after you've been cleared for the ILS approach. Once ATC clears you for the ILS approach, then you should push APP. You don't want to arm APP and capture the glide slope before being given clearance because if they don't give you clearance, or tell you to maintain a new altitude, your not going to be able to to that because the airplane AFDS is basically stuck in APP mode. To get out of APP mode, and put the AFDS back in other flight modes, it’s just a big pain in the you know what!

after you've been cleared for the ILS approach

 

Thanks John, that does clear it up for me.