Is this a good puzzle for students?

[Turbo-Arrow-Driver]

This is not the first time these approaches at KAPC have been discussed here.

Recently, I was planning some practice approaches and and considered the RNAV 36L at Napa County (KAPC). Actually there's three of them, X/Y/Z. It took quite some time to go through these plates to figure out what all the differences are. This might be a good training puzzle for students. Have any CFII's used them before?

Here's just one question you could dream up:

Say you want to fly one of these approaches LNAV with an older non-WAAS GPS. Comparing RNAV X and Z, the MDA for Z is 20 feet higher (600 vs 580) but Z requires a better climb gradient on missed (480 vs 410). Why?

I think I know the answer. Or maybe I've got it wrong. What do others think?

 

[Rgbeard]

I think.......

They're worried about the tall tower at the departure end (481' AGL). Going lower means needing to climb harder.

 

[luvflyin]

This is not the first time these approaches at KAPC have been discussed here.

Recently, I was planning some practice approaches and and considered the RNAV 36L at Napa County (KAPC). Actually there's three of them, X/Y/Z. It took quite some time to go through these plates to figure out what all the differences are. This might be a good training puzzle for students. Have any CFII's used them before?

Here's just one question you could dream up:

Say you want to fly one of these approaches LNAV with an older non-WAAS GPS. Comparing RNAV X and Z, the MDA for Z is 20 feet higher (600 vs 580) but Z requires a better climb gradient on missed (480 vs 410). Why?

I think I know the answer. Or maybe I've got it wrong. What do others think?

Maybe because of the Missed Approach Point. The X is ZAPGO, 0.3 from the Runway. The Z doesn't actually specify a named MAP. RW36L is the only thing there. So with the Z you start it later which is closer to the obstructions to the North. Other than the Z having a lower visibility minimum, I would see no reason to have bothered putting LNAV minimums on the Z

 

[Turbo-Arrow-Driver]

Maybe because of the Missed Approach Point. The X is ZAPGO, 0.3 from the Runway.

That was my guess...you get to go 0.3nm further before MAP with the Z approach, so would have to climb that much steeper to miss obstacles on the missed. The extra 20 feet higher seems a lot less important than the extra 0.3nm.

 

[luvflyin]

That was my guess...you get to go 0.3nm further before MAP with the Z approach, so would have to climb that much steeper to miss obstacles on the missed. The extra 20 feet higher seems a lot less important than the extra 0.3nm.

If what we are guessing is right, the closer MAP created the need for the extra 20 feet but I wouldn’t say it’s any less important.

 

[Turbo-Arrow-Driver]

The different climb gradients give you about 1000 feet of clearance on the 481 foot obstacle (I guessed it is about 12,000 feet from the RW36L MAP on the Z procedure. I didn't think that much obstacle clearance was required on the MAP segment.

I also see that the X approach requires 410 ft/nm to 2000 feet, but the Z approach is 480 ft/nm to 2200 feet? Why the different altitudes? There's also a small hill peak at 1690 MSL about 4.8nm from the RW36L MAP. Perhaps the difference in altitudes (2000 vs 2200 feet) has to do with clearance from the slope of the hill.

Regardless of the reasons, I think this is a good learning example because of all these seemingly minor differences.

 

[MauleSkinner]

I’d say the “puzzle” is best presented as “Which of these approaches can you or can’t you fly, and why?”