ILS approaches and stepdowns

[Bringupthebird]

You may find yourself below the Class B airspace on some approaches if you merely intercept the GS as far out as you want, a consideration to turbine -powered aircraft pilots.

 

[nosehair]

Examples, people, examples. Real-live approach plates that show step-down minimums higher than the glideslope.

 

[A Squared]

I don't have the plate handy, but I heard this is true at KLAS also, whereby the G/S goes below the floor. Just a rumor.

Congratulations, you get the prize!

I've been interested in the subject for a while, and every time it comes up, I ask for real life examples where the GS will take you below a stepdown. You're the first to point to one.

The ILS 25R at Las Vegas has a stepdown above the GS. (unless I've screwed up my calculations)

http://www.naco.faa.gov/d-tpp/0703/00662IL25R.PDF

According to my calculations if you fly the GS, you'll cross LARRE intersection at 6146 MSL, and the minimum altitude there is 6500 ... so the GS would have you 354 ' below the Minimum altitude. Bear in mind that the obstacle clearence protection on the intermediate segment is 500 ft, so you could be getting pretty close to some rock out there.

So, how does that fit with the TERPS?

The TERPS says that for an intermediate segment

"the MAXIMUM (thier emphasis) permissible gradient is 300 feet per (nautical) mile..." sec 242(d)

To cross LARRE at 6500 MSL, then get down to the GSIA of 3800 MSL before RELIN (approximate location of FAF) would require a descent of 370 ft/NM or more.

Hmm, sounds like more than the maximum permissible gradient. The answer lies in a note at the end of the section that I'd overlooked before which says

"NOTE:when the descent gradient exceeds 300 ft mile, the procedure specialist should assure a segment is provided to prepare the aircraft speed and configuration for entry into the final segment This segment should be a minimum length of 5 miles and its descent gradient should not exceed 300 feet per mile

Apparently the PRINO-LARRE segment would fufil this requirement.

So, there's a maximum permissible gradient, but you're permitted to exceed the maximum.

 

[time builder]

I think you meant 25L http://204.108.4.16/d-tpp/0703/00662I25L.PDF

 

[Say Again Over]

If you are correct about that, I think NOS should get an award of the year for the most misleading graphic display, the bold line indicating the G/S is clearly depicted above all step downs.

 

[A Squared]

I think you meant 25L http://204.108.4.16/d-tpp/0703/00662I25L.PDF

Yes, I did. Thanks, good catch. I had both the plates open and I guess I lost track of which one I was looking at.

 

[atldc9]

ON the 11-0 page for the PRM ILS's into ATL, there is a note that reads, 'following the glideslope will ensure compliance with all crossing restrictions.'

Never noticed it before.

FWIW

 

[Seadogrun]

Congratulations, you get the prize!

I've been interested in the subject for a while, and every time it comes up, I ask for real life examples where the GS will take you below a stepdown. You're the first to point to one.

The ILS 25R at Las Vegas has a stepdown above the GS. (unless I've screwed up my calculations)

http://www.naco.faa.gov/d-tpp/0703/00662IL25R.PDF

According to my calculations if you fly the GS, you'll cross LARRE intersection at 6146 MSL, and the minimum altitude there is 6500 ... so the GS would have you 354 ' below the Minimum altitude. Bear in mind that the obstacle clearence protection on the intermediate segment is 500 ft, so you could be getting pretty close to some rock out there.

So, how does that fit with the TERPS?

The TERPS says that for an intermediate segment

To cross LARRE at 6500 MSL, then get down to the GSIA of 3800 MSL before RELIN (approximate location of FAF) would require a descent of 370 ft/NM or more.

Hmm, sounds like more than the maximum permissible gradient. The answer lies in a note at the end of the section that I'd overlooked before which says

Apparently the PRINO-LARRE segment would fufil this requirement.

So, there's a maximum permissible gradient, but you're permitted to exceed the maximum.

Just wondering how you calculated that you would reach Larre at 6146?

My calculations show reaching Larre on the GS at 6528, which is above the 6500' minimum.

 

[nosehair]

Just wondering how you calculated that you would reach Larre at 6146?

My calculations show reaching Larre on the GS at 6528, which is above the 6500' minimum.

I'm waiting on that one, too.

 

[Seadogrun]

Oops. I think I see my mistake. Now I get something simular to you, but to get an accurate answer you need to know where the glideslope attena is. Generally it is between 750 - 1250 from the runway.

My mistake was I started with the 14.2 dme at larre and converted it to feet which is 86279 and did the trig to figure the height. The problem is is that the 14.2 dme is not off the LOC, but is off LAS VOR. So wrong answer for me.