Why are non precision approaches not lined up?

"Matt Whiting" wrote in message
news

No, the VOR is not on the runway centerline. It is several hundred feet
off to the side of the runway. Why they didn't use the runway heading for
this approach I don't know, but it could be for noise abatement,
obstruction clearance, or other reasons.


If they did that the MAP would be several hundred feet off to the side of
the runway.

In article .net,
says...



"Matt Whiting" wrote in message
news

No, the VOR is not on the runway centerline. It is several hundred feet
off to the side of the runway. Why they didn't use the runway heading for
this approach I don't know, but it could be for noise abatement,
obstruction clearance, or other reasons.


If they did that the MAP would be several hundred feet off to the side of
the runway.


OK - the VOR is not exactly on the runway centerline - maybe two runway
widths off. Is that the reason then? Are we sure, or just guessing?
Is it better to break out with two runway widths of sidestep, or with the MAP
"on" the extended centerline, but the runway visible out the side window?

My question is why it's done that way. If we're sure that the answer is "to
place the MAP on the centerline, regardless of heading" then that answers my
question.

Regarding Norwegian, Icelandic or lunar ILS' 30° off the centerline, I don't
know. I always see ILS' (and by implication LOC) aligned with the runway,
whereas VOR/DME/NDB approaches are very frequently not aligned.

G Faris

Steven P. McNicoll wrote:
"Matt Whiting" wrote in message
news
No, the VOR is not on the runway centerline. It is several hundred feet
off to the side of the runway. Why they didn't use the runway heading for
this approach I don't know, but it could be for noise abatement,
obstruction clearance, or other reasons.



If they did that the MAP would be several hundred feet off to the side of
the runway.



True, but the MDA at ITH is something like 700' so you'd still be a long
way from the runway when you broke out. I doubt that a couple of
hundred feet of offset would be a big deal. That may be the reason, but
I'm guessing there are other factors in play as well.


Matt

"Greg Farris" wrote in message
...

Nope - Look at the VOR 14 approach to ITH (Ithaca New York - Just
grabbed the book and picked that one by chance). The VOR is on the
centerline, the runwya heading is 144.6° and the VOR approach is 133°.
When you break out, you have to turn 11.6° right to land. I don't see why
they
couldn't have published it right on the 145° radial.


You're apparently looking at the wrong airport. If you look at the plate
for the VOR RWY 14 approach at ITH you'll see that the VOR is not on the
extended runway centerline. Here's a link to the plate:

http://www.myairplane.com/databases/.../00779VG14.PDF

Sure are a lot of micrometers measuring something that is cut with a
chainsaw. If you consider the maximum allowable error that is allowed for
VOR receivers and then plot that allowable error plus some safety factor you
would most likely see an obstacle that the approach designer has to
consider. It is even worse for NDBs.

Paul

"Greg Farris" wrote in message
...
In article . net,
says...


Because the navaid is not on the extended runway centerline.



Nope - Look at the VOR 14 approach to ITH (Ithaca New York - Just
grabbed the book and picked that one by chance). The VOR is on the
centerline, the runwya heading is 144.6° and the VOR approach is 133°.
When
you break out, you have to turn 11.6° right to land. I don't see why they
couldn't have published it right on the 145° radial.

GF

"Steven P. McNicoll" wrote in message =
ink.net...
=20
"Greg Farris" wrote in message=20
...

The title just about says it - I've always wondered WHY many
non-precision approaches (not talking about LOC only here, but VOR,
VOR/DME, NDB etc) are not lined up with the runway heading. Sometimes =
it
looks as though the approach desginers have gone out of their way to
make sure the non-precision approach is just a few degrees off - as =
if
to say - "look stupid - this is not a precision approach . . ." But =
if
this were so, then we would have to wonder why SOME of these =
approaches
ARE lined up and straight-in. I'm surev there's a simple explanation
that will be pointed out here.

=20
Because the navaid is not on the extended runway centerline.=20
=20


Furthermore, those alignments usually are not to the airport reference =
point,
but instead cross that centerline somewhere near the approach end of the =
runway,
just far enough out to turn and land if the runway environment comes =
into view.

"Greg Farris" wrote in message
...
The title just about says it - I've always wondered WHY many
non-precision approaches (not talking about LOC only here, but VOR,
VOR/DME, NDB etc) are not lined up with the runway heading. Sometimes it
looks as though the approach desginers have gone out of their way to
make sure the non-precision approach is just a few degrees off - as if
to say - "look stupid - this is not a precision approach . . ." But if
this were so, then we would have to wonder why SOME of these approaches
ARE lined up and straight-in. I'm surev there's a simple explanation
that will be pointed out here.

thanks,
G Faris



There are numerous reasons. Off field navaids often serve serveral roles in
both the enroute and approach environment for several airports. On field
navaids may not be straight in as the approach coarse must lead to the
runway, and unless the navaid is at the end of a runway, the course would
either be parallel to the extended centerline of the runway, or offset to
intersect with the runway or runway threshold. Finally, obstructions along
the approach path or missed approach may be a fact in offsetting the
approach course. Lastly, air traffic considerations may be a factor,
especially with smaller airports situated close to busy airports.

On 10/6/2005 10:42, Greg Farris wrote:

The title just about says it - I've always wondered WHY many
non-precision approaches (not talking about LOC only here, but VOR,
VOR/DME, NDB etc) are not lined up with the runway heading. Sometimes it
looks as though the approach desginers have gone out of their way to
make sure the non-precision approach is just a few degrees off - as if
to say - "look stupid - this is not a precision approach . . ." But if
this were so, then we would have to wonder why SOME of these approaches
ARE lined up and straight-in. I'm surev there's a simple explanation
that will be pointed out here.

thanks,
G Faris


I can think of a couple reasons. First, as other have said, the VOR
will not always be lined-up with the runway.

Secondly, it may be that the approach path has to come in at an
angle to avoid something, like high terrain, housing communities,
etc. (so they may have placed the VOR off center to facilitate
that).

Also, you shouldn't think of the difference between precision and
non-precision approaches as being lined-up or not with the runway.

Precision approaches are those that provide vertical guidance. Non-
precision approaches do not. That's the difference.

The fact that the typical precision approach uses a localizer which
is lined up with the runway (because the antenna is situated at the
field) does not make the approach 'precision' - the fact that it
provides vertical guidance does.


--
Mark Hansen, PP-ASEL, Instrument Airplane
Sacramento, CA

Greg Farris wrote in
:

The title just about says it - I've always wondered WHY many
non-precision approaches (not talking about LOC only here, but VOR,
VOR/DME, NDB etc) are not lined up with the runway heading. Sometimes
it looks as though the approach desginers have gone out of their way
to make sure the non-precision approach is just a few degrees off - as
if to say - "look stupid - this is not a precision approach . . ." But
if this were so, then we would have to wonder why SOME of these
approaches ARE lined up and straight-in. I'm surev there's a simple
explanation that will be pointed out here.

The term 'precision approach' refers to having vertical guidance (a
glideslope), not to the runway alignment. A precision approach does
have to be aligned relatively closely, but not precisely. I can't
recall the exact number of degrees off the top of my head, but it's not
exact. Even if a non-precision approach is perfectly aligned with the
runway, it's still a non-precision approach, because there is no
glideslope. An ILS without a glideslope, which becomes a localizer
approach, is a non-precision approach.

--
Regards,

Stan

"They that can give up essential liberty to obtain a little temporary
safety deserve neither liberty nor safety." B. Franklin

Stan Gosnell wrote:
A precision approach does
have to be aligned relatively closely, but not precisely. I can't
recall the exact number of degrees off the top of my head, but it's not
exact.

Here's an example of a precision approach that's not aligned with the runway,
the LDA Rwy 6 at ROA, Roanoke, VA. Terrain appears to be the motivation for the
misalignment.

http://www.myairplane.com/databases/.../00349LDA6.PDF