Thursday, October 22, 2015

Why is the LNAV/VNAV DA sometimes higher than the LNAV MDA?

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”.


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 While this has been replaced by the 8260.58, the concepts and calculations are similar.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.

See HERE and HERE for more discussion on that topic.

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.

Simple, huh? I hope this answers some questions about this seemingly strange situation!