On 8 Jan 2004 11:41:53 -0800, (Michael) wrote:

How much protection do we have on an off-field VOR approach? I was
always under the impression that as long as the equipment was within
tolerances for an airborne VOR receiver check (+/- 6 degrees) and the
approach was flown to PTS tolerances (no more than 3/4 scale
deflection) it was impossible to hit anything. I now know this is not
necessarily true.

The approach in question is the VOR-B to LVJ; the URL for the plate is
http://www.myairplane.com/databases/...ral/LVJ_vB.pdf

The long and the short of it is this. The Cat-A altitude minimum on
the approach is 520 MSL; there is a 1200 ft MSL tower less than 4 nm
South of the airport, and the FAC is 260. The VOR is 25.8 nm from the
MAP, which is over the field. No matter how you slice it, that's less
than 9 degrees off the FAC. 3/4 scale deflecion on a VOR is more than
7 degrees in itself.

So what am I missing?

Michael

Just some discussion without going into the math of what happens if you are
flying to the maximum allowed PTS variance.

Assuming I remember my math and have read the rules correctly:

A line from the tower perpendicular to the FAC would intersect at about 25
miles from the VOR. At that point, the primary protected area should be
about 2.25 miles on either side of the centerline; and the secondary area
about 5/6 mile or a total of about 3.082NM.

My VFR chart shows that tower to be about 3.35 NM from the FAC centerline,
so it is outside the protected area. My VFR chart could be off, and one
should really be using a USGS topographical map, but I don't have one for
that area.

So the first conclusion is that that tower is, indeed, outside of the
protected area.

In the primary area, you have 250' obstacle clearance, and at the outer
edge of the secondary area, you have zero clearance.

From the VOR to the outer edge of the secondary area, at 25 miles from the
VOR, would be a difference (error) from the FAC of 7°.

So to hit the tower, you would have to be more than 7° off course. Again,
one should be using topographical maps, but if the VFR charts are accurate,
it looks like a 7.6° error would put you into the tower.

I believe the FAA assumes that total VOR system error will be no more than
±4.5° (including your VOR error and station errors) 95% of the time.

So -- fly safe!


Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)