Remote altimeter setting, use of baro vnav

[poadeleted20]

That's not my understanding at all Ron, for a non precision approach. Once you go past the FAF, you can descent immediately to the mda, no worrying about a 40:1 plane clear of obstacles.

Your understanding is incorrect. There is a maximum gradient from the FAF down to the MDA -- it's not a cliff, so there can be obstructions which the MDA doesn't clear by 250 feet (250 being the minimum clearance for a GPS approach) near the FAF. But that is an additional restriction, not the only restriction. From the point where the 40:1 plane intersects the 250-foot-below-MDA plane (250 being the minimum clearance for a GPS approach), the 40:1 plane overrides during that last portion of the approach, thus giving you additional clearance between the MDA and the highest obstacle close to the runway.

Since there seems to be a lot of confusion, I'm going to call Garmin and see what they say.

 

[azure]

Hmm. Perhaps you guys (Stan, Ron), you guys are talking past each other? I know of quite a few approaches with stepdowns PAST the FAF, where you can't descend to MDA until you pass the stepdown, but I think they may all be VOR based. Perhaps on standalone LNAV approaches there aren't stepdowns past/inside the FAF?

I'm not sure the point is germane to what Ron and Stan are talking about, but I flew this one yesterday.

 

[John Collins]

Your understanding is incorrect. There is a maximum gradient from the FAF down to the MDA -- it's not a cliff, so there can be obstructions which the MDA doesn't clear by 250 feet (250 being the minimum clearance for a GPS approach) near the FAF. But that is an additional restriction, not the only restriction. From the point where the 40:1 plane intersects the 250-foot-below-MDA plane (250 being the minimum clearance for a GPS approach), the 40:1 plane overrides during that last portion of the approach, thus giving you additional clearance between the MDA and the highest obstacle close to the runway.

Since there seems to be a lot of confusion, I'm going to call Garmin and see what they say.

Ron,

I don't think you are correct regarding any additional 40-1 restriction during the last portion of any approach, much less a LNAV approach, I have gone thru the TERPS involved and it is not there. The only use of the 40 to 1 slope is for a departure procedure or missed approach procedure as the OCS (Obstacle Clearance Surface).

In the case of a LNAV approach, the 250 ft ROC (Required Obstacle Clearance) applies all the way to the MAP which in most cases is at the threshold.

You may have been thinking of the OCS used for a procedure with a glideslope such as an ILS or LPV, which is calculated to be 104/GS angle, or for a 3 degree slope, it is 34 to 1. The OCS starts at a point 200 ft or more from the threshold depending on the TCH (Threshold Crossing Height).

For the LNAV/VNAV glidepath there is another OCS slope calculation and offset that varies based on several factors. It is offset from the threshold by much greater distance than for a LPV or ILS to compensate for temperature variations in Baro-VNAV equipment.

For the space between the DA and the threshold there is a GQS (Glideslope Qualification Surface) that applies to obstacles on approaches with a glidepath and defines how high obstacles can be in the visual segment, but this doesn't apply to a LNAV approach.

 

[John Collins]

I'm not sure the point is germane to what Ron and Stan are talking about, but I flew this one yesterday.

The RNAV (GPS) RWY 27 approach at KAMN is a good example. It would be a LNAV+V. My rough calculation of the altitude of the Glidepath (GP) at the stepdown DUREY (minimum 1460 ft MSL) would be 1467 feet. A more precise calculation using the TERPS manual is 1463 feet. In either case the GP is at or above the step-down fix. The GP angle is 3.09 degrees, about 328 ft/NM and the Threshold Crossing Height (TCH) is 53 ft.

The fact that a VDP is depicted means that the visual segment from that point is clear of obstacles on a 20 to 1 slope. Furthermore the fact that there is a gray feather on the chart from the VDP point to the touchdown point indicates that the visual segment is clear on a 34 to 1 slope, the same clearance required for an ILS.

 

[gismo]

Furthermore the fact that there is a gray feather on the chart from the VDP point to the touchdown point indicates that the visual segment is clear on a 34 to 1 slope, the same clearance required for an ILS.

So that's what the grey "feather" means here. Thanks for that tidbit.