Circling Approach Minimums - Descent Rate Constraint

[Mark Hargrove]

Circling approach minimums apply when the runway heading is more than 30 degrees offset from the final approach course and/or the descent rate would be more than 400 ft / NM. What are the end points of the distance measurement for that constraint? From the FAF to the threshold? From the MAP to the threshold? Something else?

-M.

[Keith Smith]

FAF to the threshold, using the TCH (threshold crossing height) as the governing figure on the threshold end.

[Peter Grey]

Simple Answer: The beginning of the usable landing portion of the runway.

Complicated Answer:

based on the distance from the plotted
position of the FAF/PFAF or stepdown fix to the
plotted position of the final end point (FEP). The FEP
is a point on the FAC equal to the distance from the
FAF/PFAF to runway threshold (RWT) coordinates (or
displaced threshold coordinates when applicable) or
from FAF/PFAF to the edge of first usable landing
surface for circling only aligned procedures.

Reference is volume 1 Chapter 2 Paragraph 252 of FAA order 8260.3B (TERPS)

[Mark Hargrove]

OK, the definition is clear (even Peter's, after looking up all 93 acronyms). What I'm a bit puzzled about is the point of this constraint with this definition. Let's use, oh, I don't know, the Catalina Airport VOR/DME or GPS-B approach as a reference for a minute, but let's reorient the runway so that it's aligned with the the approach course to remove the more-than-30-degrees from the approach course constraint. So now we're approaching Catalina Rwy 17 from RIGLI. RIGLI is 3.2 miles from the runway, and if we're at the MEA of 2300 feet there, we have 700' down to the runway, a descent gradient of 700'/3.2NM = 218 ft/NM. This is easily within the <400 ft/NM constraint, so with our re-oriented runway this approach seems like it would qualify as a valid 'straight in' under TERPS.

If we continue on towards the MAP, though, even slipping down to the MDA of 2220 feet, and finally see the airport just before reaching the MAP (which is 1 NM from the runway threshold), we have a descent gradient of 600'/NM to land. At a 90 kt approach speed we're going to cover that last mile in about 40 seconds, which would require a descent rate of about 1000 FPM to reach the runway (if I did my math right). That seems like it would be a little reckless, so I'm started thinking there is something I'm missing about this whole process.

I pulled up the VOR Rwy 19L approach at KSFO and took a look. The MDA for that approach is 460 feet with a threshold elevation of 10 feet, so call it 450 feet from MDA to runway. The MAP is 0.2 miles from the threshold(!). If I see the runway environment just before the MAP in a 737 at 140 kts, I'm going to cross the threshold in about five more seconds. Even acknowledging that my touchdown zone is well beyond the runway threshold, that seems like a lot of altitude to lose really quickly.

So it seems to me like the MAP in two very different cases still has the same problem -- it's too close the runway to permit a safe descent if the runway environment doesn't become visible until just before the MAP.

What have I got wrong here?

-M.

[gavink42]

The M41 VOR/DME or GPS 18 is the same way. The MAP is *at* the runway edge and the straight-in minimum is 1080'. But the airport elevation is 553'. So, if you see the runway just before the MAP, there's no way to properly descend the remaining 527' and land on this 3000' runway. There's the circling option, but you're already below minimums for that (1120'), so missed approach would be the proper response.

My personal feeling about this approach is to "drive and dive" to 1080' to see if I break out. If not, I climb back up to 1120', so I'm not below circling minimums if I do break out very close to the field.

[Keith Smith]

There is absolutely no guarantee of being able to execute a straight in descent from the MAP at the MDA on non-precision approaches.

[gavink42]

There is absolutely no guarantee of being able to execute a straight in descent from the MAP at the MDA on non-precision approaches.

That's the reason I prefer the "dive and drive" method!

[Keith Smith]

some people compute a point by which they need to have the field in sight on a straight in approach to be able to make the field.

I am not a fan, but the technique exists. I prefer to take the situation in its merits. winds will affect your effective max descent gradient.

[twharrell]

What about the VOR/GPS-A approach into Santa Monica? It's circling only, despite being aligned with Rwy 21, but note where the MAP is located! If the ceiling is at or just above minimums, you'll be well beyond the landable runway environment. I wonder what the thinking was behind that approach design.

Todd

[Keith Smith]

it's typical for airports with co-located navaids to have the navaid be the missed approach point. Again, there is no expectation that spotting the field at the MAP means you'll be able to descend safely for the field at that point. It's simply an indication of where the missed approach segment begins.