Why are non precision approaches not lined up?

[Greg Farris]

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

[paul kgyy]

Many VORs are used for approaches to multiple airports (CGT for
example) - it would be a bit much to expect that every one will line up
perfectly. I'm surprised at how close they usually are.

[Greg Farris]

In article . com>,
says...
>
>
>Many VORs are used for approaches to multiple airports (CGT for
>example) - it would be a bit much to expect that every one will line up
>perfectly. I'm surprised at how close they usually are.
>

Nah - happens just as often with the VOR or NDB right on the field!

GF

[Steven P. McNicoll]

"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.
>

Because the navaid is not on the extended runway centerline.

[Greg Farris]

In article et>,
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

[Brad Zeigler]

"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.

[Mark Hansen]

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

[John R. Copeland]

"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.

[Brad Zeigler]

"Greg Farris" > wrote in message
...
> In article et>,
> 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

Because if they had you flying the 145 radial, you'd be flying parallel to
the extended centerline of runway 14 and would still have to sidestep to be
aligned.

[John R. Copeland]

"Greg Farris" > wrote in message =
...
> In article et>,=20
> says...
>=20
>>
>>Because the navaid is not on the extended runway centerline.=20
>>
>>
>=20
> Nope - Look at the VOR 14 approach to ITH (Ithaca New York - Just=20
> grabbed the book and picked that one by chance). The VOR is on the=20
> centerline, the runwya heading is 144.6=B0 and the VOR approach is =
133=B0. When=20
> you break out, you have to turn 11.6=B0 right to land. I don't see why =
they=20
> couldn't have published it right on the 145=B0 radial.
>=20
> GF
>

Don't just say "no", Greg.
Examine the airport surface chart, and see where the VOR 14 course must =
lie
in order to cross the approach end of Runway 14.
Looks like about 133=B0 to me.
Then check the VOR 32 approach to see the corresponding offset the =
opposite way.

[Stan Gosnell]

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

[Dave Butler]

Greg Farris wrote:

> 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.

The VOR is on the centerline? I gotta get my eyes examined.

[Paul Tomblin]

In a previous article, Greg Farris > said:
>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

The VOR most definitely is NOT on the centerline. Look at the airport
diagram on that approach - it's a hundred feet or more from the
centerline.

>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.

Because then you would never cross the extended centerline, but would be
that same distance (a hundred feet or more) to one side. I'm guessing
that in a case like this they want the course to cross the runway
centerline some specific distance from the runway. Even ROC's VOR/DME 4
(an approach which I've never heard used in 10 years of being based at
ROC) is offset a tiny bit.


--
Paul Tomblin > http://xcski.com/blogs/pt/
I read [.doc files] with "rm". All you lose is the microsoft-specific
font selections, the macro viruses and the luser babblings.
-- Gary "Wolf" Barnes

[Tauno Voipio]

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.

This has little to do with non-precision / precision classification.

If you can get hold of them, get some ILS approach plated for
Norway. There are ILS:es with approach path 60 degrees off
runway direction.

The approach path has to fulfill separation requirements for
both other flight procedures with protection zones and for
obstacle celearance.

--

Tauno Voipio
tauno voipio (at) iki fi

[Matt Whiting]

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.

It is due, at least in the cases I've seen, to the location of the
associated navaid, be it VOR or NDB. Sometimes they can't be sited on
the extended centerline of a runway, or possibly the navaid existed
prior to the airport. Aren't most GPS approaches lined up with a runway?


Matt

[Matt Whiting]

Greg Farris wrote:

> In article et>,
> 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.

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.


Matt

[Steven P. McNicoll]

"Matt Whiting" > wrote in message
...
>
> 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.

[Steven P. McNicoll]

"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/approach/pdfs/00779VG14.PDF

[Dave Butler]

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/approach/pdfs/00349LDA6.PDF

[Greg Farris]

In article . net>,
says...
>
>
>
>"Matt Whiting" > wrote in message
...
>>
>> 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

[John R. Copeland]

"Greg Farris" > wrote in message =
...
>=20
> OK - the VOR is not exactly on the runway centerline - maybe two =
runway=20
> widths off. Is that the reason then? Are we sure, or just guessing?=20
>=20
> G Faris
>

We are not guessing.

[Matt Whiting]

Steven P. McNicoll wrote:
> "Matt Whiting" > wrote in message
> ...
>
>>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

[Nathan Young]

On Thu, 06 Oct 2005 19:42:18 +0200, 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.

Off-field VOR's are often used. If the runway is 09/27, and the VOR
is North of the airport - you have no choice but to make it a circling
approach.

Obstacle or terrain clearance.

[JPH]

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
>

Alignment is not what defines precision vs nonprecision; vertical
guidance is the defining difference.
As for alignment, the only way to get an "on-airport" NAVAID to provide
a course right down the runway centerline is to place the NAVAID right
on the centerline of the runway, but then that would be a hazard to
airplanes as they had to swerve to avoid it as they were rolling out.
Seriously, though, the reason the course is not lined up is that the
TERPS criteria requires that it be aligned so as to cross the extended
runway centerline at a point optimally 3000' from the threshold. There
is some flexibility in this, as it can be aligned to cross the
centerline anywhere from over the threshold itself, out to 5200 ft from
the threshold, and in some cases can be aligned so it doesn't even cross
the threshold as long as it's within 500' of the centerline at the 3000'
point. Most on field NAVAIDS are a minimum of 500 ft from the runway
centerline. The further the NAVAID is from the edge of the runway, the
greater the difference between the course and the runway alignment. If
the procedure had the same course as the runway, then it would parallel
the centerline all the way down final, requiring an "S" turn rather than
one gentle turn to lign up.
At KITH, there's also a difference of 2 degrees between the airport
magnetic variation (12W) and the ITH VOR/DME magnetic variation (10W),
so even if the courses were parallel, the displayed headings would be 2
degrees apart.

JPH

[Greg Farris]

In article >,
says...
>
>
>"Greg Farris" > wrote in message
...
>>
>> 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?
>>
>> G Faris
>>
>
>We are not guessing.
>


That's good. I wrote here with my question because I was sure I would find an
informed response. So are you telling me, without a doubt, that the 12° offset
on this approach, as well as similar offsets on many non-precision approaches
is so that the MAP will fall on the runway extended centerline - and you can
point to documentation to support this?

I ask you in this manner because I agree with Matt Whiting that this does not
appear to be a plausible explanation, and there must be another.

G Faris

[Mark Hansen]

On 10/7/2005 00:23, Greg Farris wrote:

> In article >,
> says...
>>
>>
>>"Greg Farris" > wrote in message
> ...
>>>
>>> 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?
>>>
>>> G Faris
>>>
>>
>>We are not guessing.
>>
>
>
> That's good. I wrote here with my question because I was sure I would find an
> informed response. So are you telling me, without a doubt, that the 12° offset
> on this approach, as well as similar offsets on many non-precision approaches
> is so that the MAP will fall on the runway extended centerline - and you can
> point to documentation to support this?
>
> I ask you in this manner because I agree with Matt Whiting that this does not
> appear to be a plausible explanation, and there must be another.
>
> G Faris
>

Greg,

I think you're going about this all wrong. The design of the approaches,
etc. is based on a number of factors. Primarily TERPS, which is the 'rule
book' for designing approaches. These 'rules' must take into account several
factors, including terrain, local controls (politics), other runways, other
airports, availability of navaids, etc.

It's not up to the Pilot to determine *why* the approach was designed the
way it was, only to correctly interpret the procedure and fly it safely.



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

[Greg Farris]

In article >,
says...

>Greg,
>
> It's not up to the Pilot to determine *why* the approach was designed the
>way it was, only to correctly interpret the procedure and fly it safely.
>


Well, I have no objection to that, and certainly no intention of doing
otherwise - however I don't think there's anything wrong with wanting to know
why they look like this.

I'm guessing (I admit I'm guessing) it has to do with terrain avoidance - a
straight-in approach could be published for the ILS, because of the precision
vertical guidance, but the larger tolerance of the non-precision approach does
not allow this, even though a precise vertical profile is published. A VOR
approach, without FAF, or an NDB approach requires 300' of obstacle clearance
in the primary area, 6mi wide at 10nm out. Considering that aircraft can
descend to MDA at different rates within this area, this becomes a lot of
airspace to protect, compared to the ILS. I'm guessing this is why many of
them cannot be drawn straight-in, when an ILS to the same runway can.

That's just my guess though - I was hoping to find more precise information
here.

G Faris

[Bob Gardner]

The feds build VORs and place NDB antennas in locations where they can get
the land at a decent price and where the terrain is suitable. The feds build
runways for much the same reasons, plus allowing for terrain/obstacle
clearance. It's asking a lot to expect everything to fall nicely into place.

Bob Gardner

"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
>

[Paul Lynch]

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 et>,
> 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
>

[[email protected]]

Greg Farris wrote:

> 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.

You're arguing with the criteria. The criteria requires that the final approach
course intercept the runway centerline, extended, at 3,000 feet from the approach
end of the runway. There is a slight parallel option when that is not possible.
If you want to learn all about it, check the TERPs criteria, which is available
on Summit Aviation's Aviation Reference Library CD-ROM.

[[email protected]]

Stan Gosnell wrote:

> pl
>
> 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.

3 degrees

[[email protected]]

Dave Butler wrote:

> 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/approach/pdfs/00349LDA6.PDF

That is not really a precision approach so far as most of the world is concerned.
It is a FAA category of IAP that does not meet the ICAO definition of a precision
approach. The FAA calls these an approach with vertical guidance ("APV") and they
provide the definition in the AIM:

"b) Approach with Vertical Guidance (APV). An instrument approach based on a
navigation system that is not required to meet the precision approach standards of
ICAO Annex 10 but provides course and glidepath deviation information. For example,
Baro-VNAV, LDA with glidepath, LNAV/VNAV and LPV are APV approaches."