Circling minima lower than LNAV/VNAV DA

[mfisher]

RNAV 35 KHSD
Circling minima 1780, category A & B
LNAV MDA 1780
LNAV/VNAV DA 1881

Why?

 

[MauleSkinner]

Probably something to do with an unlighted object in just the wrong place, since straight-in 35 or circling to 35 is not authorized at night.

 

[mfisher]

Still not getting it. Doesn't explain why the precision approach is 1881-2.5 but both LNAV and circling is 1780-1.

 

[MauleSkinner]

VNAV isn't a precision approach, but it does have different obstacle criteria than an approach with an MDA.

 

[luvflyin]

AIM 5-4-5 g. Figure 5-4-12 tells ya all about it.

 

[RussR]

Circling MDA is based on the higher of:
- the altitude determined by obstacles within the circling areas, or
- the highest non-vertically guided MDA.

So, the reason the circling MDA in this case is the same as the LNAV MDA and not the LNAV/VNAV DA, is simply that the rules don't call for it to be based on the LNAV/VNAV line of minima.

That's the criteria answer. As for why the rule is written like that, you'd have to ask the FAA!