Non-precision approaches for which constant descents are not approved?

I noticed an interesting note -- circled in red pencil below -- on the KEEO RNAV (GPS) RWY 03 approach the other day:

Why would the FAA apparently prohibit constant-descent final approaches to this airport, given the operational benefits they provide to aircraft of many types? (While props can get away with a lot more on approach than a jet can -- diving and driving is still more workload than maintaining a constant descent.)

• Without hunting for more details, I suspect that it's highly mountainous terrain, and the ridge(7000') between WAGOK and the runway might be too close for decent obstacle clearance. Aug 1, 2015 at 4:17
• Your MDA is well above the ridge east of WAGOK, and WAKOG protects you from the ridgeline right under the FAF. Inside WAGOK you're good down to the MDA as fast as you can get there, so lack of a published descent angle shouldn't be driven by terrain at/after WAGOK, as far as I can tell.
– Ralph J
Aug 1, 2015 at 7:39
• @slookabill: You do realise that that ridge isn't much higher than the runway itself (6415'), right? Mar 23, 2019 at 20:42

1 Answer

Probably because it's so shallow: you have 8+ NM to descend about 1100'. On a 3 degree angle, you'd typically decsend 1100' in a little over 3 miles. I'll leave it to good folks with calculators to work out exactly what angle this would be, but it's under 1.5 degrees, so probably below whatever limit TERPS gives for depicting an angle there.

One solution would be to remain at the FAF altitude until reaching a calculated "fly-off" point, at which time you would descend at a 3 degree angle until reaching published MDA + 50', at which point you continue or go around. But that probably takes specific FMC software & perhaps a tailored approach chart, so with only this one your option pretty much looks to be "dive" (not that far) "and drive" (quite a bit).

Though that gets you to an MDA that is 1700'+ above the TDZE...

ACTUALLY, ON FURTHER CONSIDERATION: It isn't too shallow, it's a steep path from FAF to TDZE, 3000' in 8 NM (would be 9NM at 3 degrees). And the angle to do that probably doesn't quite clear the WAGOK step-down, meaning your vertical path becomes even steeper at that point than it starts out.

So rather than publish gradient***s*** steeper than (whatever the TERPS limit is), and increasing after the step-down fix, they just say NA & let you "make it work" on your own.

I notice that there is no VDP published here, either...