Why are non precision approaches not lined up?

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/.../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."

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

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

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

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

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