Why are non precision approaches not lined up?

"Greg Farris" wrote in message =
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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.

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

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.

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

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

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

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

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

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

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.