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

(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

(Michael) wrote:
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.

Why not call up the local tracon, ask to speak to the procedures guru,
and find out from somebody who is in a position to know for sure?

(Michael) wrote in message m...

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.

Granted. Let's do the math:

GPS approach: constrained by airspace, terrain, and obstructions

off field navaid approach: constrained by airspace, terrain, obstructions
*and* the location of the navaid.

It appears pretty clear to me that the latter is more constrained.
YMMV

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.

Yes, I know. We've had this discussion before. It seems
obvious to me that it's relevant. In the case you cite,
apparently the GPS approach doesn't avoid the tower. However,
with a GPS, the capability to establish a waypoint at a
convenient spot clearly allows the approach designer to work
around terrain and obstructions in a way that an approach
based on one or two ground navaids can not.

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.

This is definately a factor, but it's not the factor to which
I was referring.

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 may even be correct in this instance, but I don't think that
can be generalized, nor can it be generalized that the capabilies/
safety of IFR and VFR GPS are equal.

I understand some VFR GPS can be set to precise course guidance
(full deflection = 0.3 miles) and of course a VFR GPS can be
installed so as to have the same features of IFR GPS -- RAIM
prediction and monitoring, installed antenna etc. But many
are not installed or set up that way.

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

I apologize for failing to write with sufficient clarity, nor to
make clear that I was speaking to a generalized point not of your
specific airport.

If that is your point, I see no support for it whatsoever.

Yes, I know. As I said, we've had this discussion before.

Cheers,
Sydney