TERPS question?

[peerlesscowboy]

My understanding is that minimum altitudes published for each progressive approach segment guarantee a certain (and progressively shrinking) minimum of obstruction clearance.
Question; does that guaranteed minimum obstruction clearance begin right at the fix defining the beginning of the approach segment? Or is there an assumed descent gradient factored in? IOW for example, if I were to descend straight down immediately after crossing the FAF on a non-precision approach would I be guaranteed obstruction clearance?

 

[RussR]

Yes, and no.

Obstacle clearance usually begins before the fix, at the earliest the fix can be received (called the fix displacement area), which depends on what type of fix it is. This is because, as you've undoubtedly seen, sometimes it's a little hard to determine when exactly you're at the fix (say, for instance, a crossing VOR radial, wavering in and out like they sometimes do).

However, in the specific example of the FAF, there is what's called a "7:1" area that starts at the beginning of the FDT and extends towards the MAP at a 7:1 ratio for 1 nm (so, for every 7 feet you go forward, it drops 1 foot). This allows the designer to eliminate from consideration for the MDA any obstacles that fall under this surface right near the FAF. However, you're starting off with the obstacle clearance you had in the intermediate segment anyway.

The presumption, of course, is that you WON'T (and can't) go straight down upon reaching the FAF. And that's a pretty steep descent anyway.

 

[dtuuri]

I have a link in the 4th column, 4th row to TERPS here: http://www.avclicks.com/lessons.html

You can find the pertinent part in Volume I, page 36-2, paragraph 289. The 7:1 gradient only applies to FAF and step-down fixes.

dtuuri

 

[peerlesscowboy]

Thank's fella's, so then there is a descent gradient factored in altho' it's pretty steep.

 

[poadeleted20]

My understanding is that minimum altitudes published for each progressive approach segment guarantee a certain (and progressively shrinking) minimum of obstruction clearance.

Correct.

Question; does that guaranteed minimum obstruction clearance begin right at the fix defining the beginning of the approach segment?

No.

Or is there an assumed descent gradient factored in?

Yes. You'd have to look in in the TERPS or ask Wally Roberts to find out what it is in each situation, but IIRC, it's astonishingly steep.

IOW for example, if I were to descend straight down immediately after crossing the FAF on a non-precision approach would I be guaranteed obstruction clearance?

No.

 

[John Collins]

The 7 to 1 slope only applies to the first 1 NM past the fix, after that point, one can point the nose down as much as they feel comfortable with, because the obstacle clearance is the same from there on. Slope of 1 to 7 for the first mile and then drop off the cliff to the minimum altitude.