Decision Height

[MauleSkinner]

That's just it - I won't ever really know. If I'm at the MDA and execute a miss because I don't see anything, my momentum *will* carry me below it, and I'll never have seen the PAPI/VASI.

From a practical standpoint, my automation should keep me from going below the proper glidepath - which is why we validate and brief that the coded descent path in the FMS is correct. Obviously we train for this stuff, and have all the tools to execute these approaches safely. But like Sluggo63, I'm just curious about the TERPS protections behind it all.

Check out this article, Which includes this paragraph:

You can get a better idea about the obstacle environment below the MDA when a VGSI is installed at the runway you’re approaching. VGSI have hard surfaces at an angle of roughly 1 degree below the published VGSI angle, so as long as you stay on the visual glidepath, you will remain clear of obstacles. Granted, it’s not an exact substitute for the visual segment because the VGSI surface is limited to 4 miles from runway intercept, and is a different shape than the visual area, but at least it tells you something.

I'll let Wally post direct quotes or paraphrasing from TERPS, but as I indicated in my previous post, being at or above the VASI/PAPI, which you will if you follow your OpSpec procedures, guarantees obstruction clearance in the initial dip through MDA on the missed.

 

[kayoh190]

We also add 50' to MDA non precisions... This is common practice I believe.

We used to do it too, that's why it was so interesting that it changed. Especially since we're not very sophisticated in the manner in which we do LOC and LDA approaches - we drive to .4nm before the FAF, then push the nose over to a Flight Path Angle that matches what's on the chart. Sometimes we can't exactly match it because FPA on the guidance panel is set to tenths of a degree, while what's listed on a chart can be in hundredths. So if the descent angle was 3.06 on the chart, we'd set a -3.1 FPA, knowing that we'd always be above the published MDA. Not sure that I'm happy with doing that now.

I can't think of the last time I've flown a green needle non-precision approach to minimums (outside of training, of course), but it's interesting to learn the nuts of bolts of this stuff behind the OpSpec that allows it to happen.

 

[kayoh190]

I'll let Wally post direct quotes or paraphrasing from TERPS, but as I indicated in my previous post, being at or above the VASI/PAPI, which you will if you follow your OpSpec procedures, guarantees obstruction clearance in the initial dip through MDA on the missed.

Ah - that makes sense. This is the kind of stuff I've been looking for. When you ask these kinds of questions in the school house, you usually just get a blank stare and a "because our procedures say we can!"

 

[MauleSkinner]

Ah - that makes sense. This is the kind of stuff I've been looking for. When you ask these kinds of questions in the school house, you usually just get a blank stare and a "because our procedures say we can!"

One key indicator that training is going to be worthless to me is when the instructor says, "I'll teach you everything you need to know about..."