How much protection on approach?

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

In article , 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.

Without going into my copy of TERPS, I believe the obstruction
clearance is a 20:1 slope for non-precision approaches.

In article ,
(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?

The tower, I hope!

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)

(Michael) wrote in message om...
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.
... So what am I missing?

An approach certified GPS

Sorry couldn't resist...but I recall we've had past
discussions where you opined that a GPS approach
didn't give you much in the way of capability and
this is one of the places where I feel the ability to
legally request and fly GPS approaches is more than
just an issue of whether there are other IAPs and
what their minima are.

Early on, I did the math on a bunch of approaches
around here, approaches where the VOR is seriously
off-field, and came to the similar conclusions. That
one could be w/in tolerances for a VOT check, be
just over half-scale deflection, and smack into
something. Heck you can center the needle on the
VOT check, center the needle on the approach,
and come out of a cloud with the airport over
THAR.

6 degrees and 3/4 scale deflection clearly aren't
good enough.

Cheers,
Sydney

"Michael" wrote in message
m...
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.

From my reading of TERPS para 513, the primary area is defined by a
trapezium that is 2 miles wide at the facility and 5 miles wide at 30 miles
from the facility (which is the furthest permitted). The secondary area is
a mile wider on each side at 30 miles. So at 30 miles the deviation against
which you are protected is 3.5 miles. As you say, that's about 7 degrees.
Scary stuff!

Julian Scarfe

My take on this is that such specifications exist, but we, as pilots,
don't have access to them. This is due to their complexity. However,
if we pilots fly reasonable approaches, and don't get "too far" off
course, we will be safe. In this case, the tower is 4 miles away from
the field. THAT is a long way. If you take a look at the approach to
ERIE, Colorado, there is a tower that is less than a mile off course,
and only 100' below where you are supposed to be. Very scary. All it
would take is a combination of bad altimeter setting, or being a
little low, and flying a couple of dots off. I always am very, very
cautious flying approaches in IMC if I have never flown in visual
conditions to acertain the radio tower and hill situation. There are
some frightenly close obstructions out there. Nevertheless, we as
pilots seem to be doing a fairly good job, as we are not plowing into
such obstacles at a very high rate. So the system seems to be working.
Certainly you bring up a good point. I would like to see the zone of
protection marked on the charts in some way. This would obviously be a
major change from current charting system.

(Michael) wrote in message om...
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

(Snowbird) wrote
An approach certified GPS

Sorry couldn't resist...but I recall we've had past
discussions where you opined that a GPS approach
didn't give you much in the way of capability

Relative to a VFR GPS. With a VFR GPS, this is a non-issue since you
would never be 3.5 miles from the airport in the first place. Sorry,
no sale. However, if the point you were trying to make is that it
would be safer to shoot a VOR appproach using an uncertified VFR GPS
rather than the legally usable VOR receiver, the point is made .

6 degrees and 3/4 scale deflection clearly aren't
good enough.

Clearly.

Michael

(Michael) wrote in message . com...
(Snowbird) wrote
An approach certified GPS

Sorry couldn't resist...but I recall we've had past
discussions where you opined that a GPS approach
didn't give you much in the way of capability

However, if the point you were trying to make is that it
would be safer to shoot a VOR appproach using an uncertified VFR GPS
rather than the legally usable VOR receiver, the point is made .

No, that wasn't my point, although it's certainly valid -- I always
felt much safer flying an NDB approach while following it on GPS.
It definately does improve the safety.

Here, let me restore the point I was trying to make:
I feel the ability to legally request and fly GPS approaches
is more than just an issue of whether there are other IAPs and
what their minima are.

Approaches where the navaid is off the field are of necessity
constrained by the location of the navaid. Stand-alone GPS
approaches IMO add significant capability vs. using VFR GPS
to fly an impromptu overlay approach.

You don't have to agree with my point, but please don't delete
it and then surmise that I must have been making a different
one.

Cheers,
Sydney

(Snowbird) wrote
Here, let me restore the point I was trying to make:
I feel the ability to legally request and fly GPS approaches
is more than just an issue of whether there are other IAPs and
what their minima are.

Approaches where the navaid is off the field are of necessity
constrained by the location of the navaid.

Actually, whether the navaid is on or off the field, there are still
plenty of constraints on the approach unless it's in the middle of
nowhere on flat ground. Check out the NDB and GPS approaches to my
home field of EYQ for a perfect example of what happens when you're
not in the middle of nowhere.

Stand-alone GPS
approaches IMO add significant capability vs. using VFR GPS
to fly an impromptu overlay approach.

I fail to see how this is relevant to the protection offered on
approach. The hazard on the approach into LVJ is a 1200 ft tower.
That tower is actually much CLOSER to the FAC on the GPS approach.

It's simply that the GPS (VFR or IFR) is so much more accurate than
the VOR, and thus the pilot can easily avoid the hazard without the
need to keep the needle perfectly centered. If a VFR GPS is used to
back up the VOR approach, then I would argue that safety is increased
over the standalone GPS approach, since (a) the hazard is
significantly farther from the FAC and (b) a second source of
navigation, relying on a completely different signal source, is
available as a sanity check.

You don't have to agree with my point, but please don't delete
it and then surmise that I must have been making a different
one.

It simply never occurred to me that your point was that having the IFR
rather than a VFR GPS was a safety issue, especially when flying into
an airport like LVJ. If that is your point, I see no support for it
whatsoever.

One thing you are failing to realize is that the reason for the use of
the MHF VOR/DME as the basis of the sole non-GPS approach to LVJ has
NOTHING to do with the lack of a suitable closer navaid. HUB VOR/DME
is much closer, and would allow an approach to 32 with straight-in
minima. In fact there was such an approach. It was decomissioned
because too many people were using it. There are several flight
schools on the field, and when the CFII's filed IFR they were
interfering with arrivals into Hobby. Once IFR GPS becomes
sufficiently popular, you can rest assured the GPS 32 approach into
LVJ will be decomissioned as well, and will probably be replaced with
an approach much like the VOR-B.

Michael