TERPS for PAPI

[Stepclimb]

I tried to find this on the web, but could not. Maybe some one with a knowledge of TERPS can chime in.
An ILS glide slope gives a certain number of feet of terrain clearance at the outer marker (or intercept point) and that terrain clearance tapers down to something on the order of 166 feet of clearance at DA (DH).
Any one know if that same clearance applies to VASI/PAPI's when they are installed and what TERPS criteria applies.
Please provide web links if you find anything.
-Regards
Stepclimb

 

[A Squared]

The clearence below a VASI is a theoretical surface which angles up from the near VASI projector, at an angle 1 degree less that the VASI angle. it projects up at this angle until it intersects the obstacle clearence height of the fianl approach. It's a lot easier to draw than it is to describe. Also, the way it is depicted it relates more to a non-precision approach than an ILS. The TERPS I have don't mention a PAPI, but I don't have all the latest revisions. I don't have a web reference for this, other than the TERPS file on the FAA website:

http://av-info.faa.gov/terps/directives%20page.htm

It's a huge file, 28 megs. the info can be found in Chapter 2, Section 251 (b)

 

[A Squared]

OK, here's the rest of the story:

A PAPI has a slightly different Obstacle clearence criteria than a VASI. It is one degree below the angle which gives you a one light low indication. IE three red, one white. On a 3 degree PAPI, you'll get that at a glide angle 2 degrees, 50 minutes above. The obstacle clearence is at 1 degree, 50 minutes, projected from a point 300' in front of the PAPI. Here's a pdf file which explains it better and has diagrams.


http://www.faa.gov/arp/publications/acs/5340-30/5340-30part2.pdf

You can deposit a finders fee equal to one 6 pack of premium beer in my Pay Pal account

 

[Stepclimb]

Contradictions

A Squared,

Thanks for the links.
While digging around those pubs, all I could find in the TERPS (under visual segment) was mention of 10000 feet of obstacle clearance plane starting 200 feet from the threshold. This 2sm (roughly) seems to cover the approach lighting system, but not the 4NM mentioned in AIM 2-1-2. Also no specific mention of VASI/PAPI in TERPS-only visual glide slope guidance.

The advisory circular 150 indicated in 2(d) that PAPI's Obstacle clearance surface (OCS) starts 4 SM from the point of origin. (usually 1000 feet down the runway.
The AIM 2-1-2(a) states that VASI's provide obstacle clearance 4 NM from the runway threshold.
Perhaps the PAPI clearance plane was designed after the VASI and the author didn't bother to check the specifics for the former.
-Stepclimb

 

[88_MALIBU]

An ILS glide slope gives a certain number of feet of terrain clearance at the outer marker (or intercept point) and that terrain clearance tapers down to something on the order of 166 feet of clearance at DA (DH).

What is the terrain coverage at the GS interception point? Trivial, but I am curious...

 

[100LL... Again!]

Around 1000' or so, but I believe it can vary based on a few factors.

 

[A Squared]

What is the terrain coverage at the GS interception point? Trivial, but I am curious...

The obstacle clearence for the intermediate segment of any approach is 500', so as you reach the gs, you know you have 500'. now GS intercept is where you transition from the intermediate segment ot the final segment, and a different set of rules apply for the final segment of an ILS, Like Stepclimb said, it tapers as you go. FOr distances from the "ground point of intercept (essentially where the glide slope hits the runway) greater than 10,975 feet (where do they come up with these numbers) the required Obstacle Distance = distance from GPI X 0.01866 + 75' or, for a typical ILS where you intercept the GS 5 miles out, about 640'

 

[Stepclimb]

More on required obstacle clearance (ROC)

A Squared has it right.
I was a little off on my original post about the ILS ROC at DH. I come up with 660 feet at 5 miles and 111 feet of obstacle clearance at a standard DH of 200 feet.
In regards to ILS ROC's, I remember reading something about the FAA adopting ICAO standards for new ILS's designed or revised after 1996. The ICAO standards have higher vertical obstacle clearance in the final segment but have a much much smaller lateral primary and secondary area. I think the ICAO ILS areas are the same as U.S. MLS standards (about 2/3 less lateral clearance at 5nm from the runway).



Things get interesting the more one reads into TERPS. I was re-reading about the visual segment that I mentioned in my second post. It starts 200 feet from the threshold and extends out 10000 feet. It turns out that if an obstacle penetrates a 34:1 slope but is just below a 20:1 slope, the only thing that changes is the minimum visiblility for the approach: it must be no less than 3/4 mile. Now for a little math.......

20:1 is 302 ft/nm. A 3 degree glide slope is 318 ft/nm. On an ILS with mins of 200ft (DH) and 3/4 sm, an aircraft could conceivably pass just a few feet above an obstacle while centered on the glide slope once past DH. Yikes!

It gets worse. If the 20:1 slope is penetrated by an obstacle, the approach designer bumps the visibility minimums up to 1sm or greater. In this situation, the DH usually is 250 ft or higher. If one were to encounter an ILS with mins of 250 ft (DH) and 1sm, you could hit an obstacle below DH even if you remained on the glide slope!

Be careful out there!

-Stepclimb