Sometimes this instrument stuff just doesn't make any intuitive sense, does it? You’ll see an
approach chart with minimums like these:
The LNAV MDA and visibility are lower than the LNAV/VNAV DA and vis! We “assume”
that because the LNAV/VNAV offers a glideslope, that it must be better than the
LNAV. “Better” is a subjective term of course, but in this case it doesn’t mean “lower”.
Why?
Well, let me tell you. Here’s where it gets a little
involved.
(Note: The vast majority of LNAV/VNAV procedures out there were
evaluated using the criteria in FAAO 8260.54A. While this has been replaced by
the 8260.58, the concepts and calculations are similar. I will use the 54A in
my examples below, since that’s what most current procedures are based on.)
It’s really all a matter of WHERE the most significant
obstacle in final is located. This is called the “controlling obstacle”, and is
the one which causes the highest MDA or DA.
For a non-vertically guided approach, like an LNAV,
Localizer, or VOR, the evaluation can be very simple. Find the highest obstacle
in final, and add 250 feet to it, then round up:
 |
| Yes, that's the Eiffel tower. Why not? Note: not to scale! |
That’s your MDA! (It’s not always as simple as this, but it
can be. I’m leaving out some details for brevity.)
But what about a vertically-guided approach? It’s different
for LPV and ILS than it is for LNAV/VNAV, and LNAV/VNAV has some serious
handicaps. LNAV/VNAV was originally designed for use with barometric altimetry –
meaning that the “glideslope” you would follow was calculated by your FMS using
barometric pressure – basically an internal altimeter – NOT an electronic
signal. For the most part, only business jets were ever equipped with this technology. Also, we know that altimeters have many errors as a result of non-standard
temperatures.
This was called “Baro-VNAV” and the formulas have
to account for varying temperature limits. That’s why you’ll often see in the
notes for an RNAV (GPS) approach procedure something like this:
The errors introduced here require a little more “cushion”
when it comes to obstacle clearance, so instead of something nice and simple like the LNAV evaluation,
the evaluated area is composed of two general regions:
That flat part of the dashed red line extends about a mile
from the runway threshold, dependent on altitude and how cold it gets at that airport during
the winter (yes, really). If there are no obstacles that penetrate that dashed
red line, then the LNAV/VNAV will get great minimums. But if an obstacle DOES
penetrate, then the DA is highly dependent on WHERE it penetrates and by how
much. An obstacle that penetrates that flat area has a comparatively small
effect. However, an obstacle that penetrates the sloped portion can have a
significant effect on DA.
The new DA is determined by placing the DA at a point on the
glidepath above where the obstacle clearance surface is at the same height as
the obstacle. Now that’s a mouthful, a picture hopefully is a little clearer:
Whatever the glidepath height is at that distance from the
runway, well, there’s your DA.
It works out that an obstacle that penetrates the surface within
a mile of the runway will usually not cause the LNAV/VNAV DA to be higher than the LNAV MDA.
But an obstacle that penetrates more than a mile out will! So when you see this
situation occur, you know there’s an obstacle maybe 1-2 miles from the runway. If the obstacle is further away than that, the DA gets really high!
Okay, clear as mud. But what about that visibility value?
Fortunately that’s a little easier to explain.
Visibility values are set so that the pilot has at least a
reasonable chance of seeing the runway from the missed approach point.
Hopefully sooner, of course, but at least by then. On any vertically-guided
approach, this is pretty straightforward – how far is the airplane from the
runway at the DA point? Convert that to statute miles, and there’s your visibility.
At approximately 318 feet per nautical mile for a 3 degree
glidepath, a Height Above Touchdown (HAT) of 688 ft as in the example above
gives a distance, and therefore visibility, of just shy of 2.50 sm. So it’s
rounded up to 2 1/2, and published.
Approach lighting systems, if installed, get figured into this too, essentially
by subtracting the length of the approach lights from the calculated visibility.
It’s all in a table that the procedure developers refer to.
For non-vertically guided procedures, however, there is no “DA”
point, and most often the MAP is either at the runway end or relatively close
to it (sometimes past it on a VOR procedure). For these procedures, the visibility is determined one of two ways. For
Cats C and D, the same table as for vertically-guided approaches is used, so
the visibility is the same for a given HAT.
For Cats A and B though, a different table is used, and is
greatly simplified. A basic visibility of 1 sm is used until HATs start
getting over 740 ft for Cat B and 880 for Cat A, at which point it starts increasing. So you will see many,
many LNAV (and LOC and VOR) approaches with 1 sm of visibility. Since many Cat A and B aircraft are capable of making a perfectly safe descent at steeper than 3 degree glidepaths, the lower visibility requirement actually gives them a little more flexibility than the faster aircraft.
Like before, approach lights can help here too. There
are some other limitations as well.
To briefly recap:
1. The LNAV/VNAV DA may be higher than the LNAV DA if the
obstacle is sufficiently far from the runway due to the geometry of the
evaluated areas. This is a result of the original design of Baro-VNAV.
2. The visibility values are calculated differently because the approaches are flown differently, and
therefore LNAV visibility for Cats A and B will often be less than the
LNAV/VNAV Cats A and B.
