Unpublished GPS fixes on ILS approaches

With the Garmin 430W (and presumably the 530 as well) when one flies
certain approaches the Garmin has you fly to fixes that appear nowhere
on the published approach plate. For example, if you fly the ILS24
approach at ABE from STW (Stillwater) IAF, the Garmin will have you
fly from STW to D16.5 then 4 miles to MUDRE intersection. The fix
"D16.5" appears nowhere on the published approach plate. Is there any
discussion of this anywhere ? Why does the Garmin seem to need a fix
4 miles outside of another IAF ?

Thanks.

Evan Salant wrote:
With the Garmin 430W (and presumably the 530 as well) when one flies
certain approaches the Garmin has you fly to fixes that appear nowhere
on the published approach plate. For example, if you fly the ILS24
approach at ABE from STW (Stillwater) IAF, the Garmin will have you
fly from STW to D16.5 then 4 miles to MUDRE intersection. The fix
"D16.5" appears nowhere on the published approach plate. Is there any
discussion of this anywhere ? Why does the Garmin seem to need a fix
4 miles outside of another IAF ?

Thanks.

Go to http://www.pplir.org, and get the RNAV tutorial manual.
It explains clearly how the extra points are named and why.

The whole document is recommended reading. The database coding
part begins on page 68.

--

Tauno Voipio, (CPL, ME-IR, MSEE avionics)
tauno voipio (at) iki fi

That is a great manual. But, I fail to see where anything in it answers
the OP's question.

Jeppesen adds computer navigation fixes (CNFs) to many non-RNAV IAPs to
tie the procedure togeter for RNAV navigation. CNFs are charted. The
fix at issue, the STW 16.5 DME is not charted as a CNF.

Other fixes that will be added are (such as) turn-on CNFs for DME ARC
interception, as a common example.

But, the STW 16.5 DME doesn't serve that purpose either. In fact, it
doesn't serve any navigation or continuity purposes that I can see. Do
you have some additional insight?

Tauno Voipio wrote:
Evan Salant wrote:

With the Garmin 430W (and presumably the 530 as well) when one flies
certain approaches the Garmin has you fly to fixes that appear nowhere
on the published approach plate. For example, if you fly the ILS24
approach at ABE from STW (Stillwater) IAF, the Garmin will have you
fly from STW to D16.5 then 4 miles to MUDRE intersection. The fix
"D16.5" appears nowhere on the published approach plate. Is there any
discussion of this anywhere ? Why does the Garmin seem to need a fix
4 miles outside of another IAF ?

Thanks.


Go to http://www.pplir.org, and get the RNAV tutorial manual.
It explains clearly how the extra points are named and why.

The whole document is recommended reading. The database coding
part begins on page 68.

Evan Salant wrote:
With the Garmin 430W (and presumably the 530 as well) when one flies
certain approaches the Garmin has you fly to fixes that appear nowhere
on the published approach plate. For example, if you fly the ILS24
approach at ABE from STW (Stillwater) IAF, the Garmin will have you
fly from STW to D16.5 then 4 miles to MUDRE intersection. The fix
"D16.5" appears nowhere on the published approach plate. Is there any
discussion of this anywhere ? Why does the Garmin seem to need a fix
4 miles outside of another IAF ?

Thanks.

Here is the official word from Garmin, with my commentary at the end:

Notice the leg sequences for STW to MUDRE. The first leg is an FC
(course from fix) from STW for a distance of 16.5 nm on a course of 243.
This is followed by a CF (course to fix) leg to MUDRE for a distance
of 4 nm on a course of 243.

According to ARINC 424-19 Attachment 5 paragraph 6.3.6.5:

"When a CF leg is used as the ending leg of a transition to a
localizer-based procedure, the maximum leg distance will be witnin 8 NM
of the FACF or within the reception area of the localizer...

NOTE: For precision approaches relying on an electronic glide slopde,
the FC/CF is preferred over a TF...for those legs ending at a fix."


While we would definitely prefer a TF leg, ARINC 424 instructs Jeppesen
to use the FC/CF combination, probably because that is the way older
FMS preferred it.

We don’t like these combinations because there is often a gap at the
point where the FC leg ends and the CF leg begins, which causes the CDI
scale to snap to a new course. This also can cause confusion
on the flight plan page and the moving map as to what these represent.

Unfortunately, the FC/CF combination is quite common for initial
transitions for ILS approaches.

Bottom line (my words): The fix is useless in a modern platform like the
Garmin 430/530/430W/530W, and G-1000. But, if Garmin were to strip the
fix out of their database then they wouldn't be in compliance with the
ARINC type legs provided to them by Jeppesen (which is *the* database
source instead of the government).

Sam Spade wrote:
Evan Salant wrote:

With the Garmin 430W (and presumably the 530 as well) when one flies
certain approaches the Garmin has you fly to fixes that appear nowhere
on the published approach plate. For example, if you fly the ILS24
approach at ABE from STW (Stillwater) IAF, the Garmin will have you
fly from STW to D16.5 then 4 miles to MUDRE intersection. The fix
"D16.5" appears nowhere on the published approach plate. Is there any
discussion of this anywhere ? Why does the Garmin seem to need a fix
4 miles outside of another IAF ?

Thanks.


Here is the official word from Garmin, with my commentary at the end:

snip

Seems like I wasted my time going to the trouble of obtaining an
authoritative answer.

On 5/1/2010 2:02 AM, Sam Spade wrote:
Sam Spade wrote:
Here is the official word from Garmin, with my commentary at the end:

snip

Seems like I wasted my time going to the trouble of obtaining an
authoritative answer.

Why do you say that? I certainly appreciated the posting.

Mark Hansen wrote:
On 5/1/2010 2:02 AM, Sam Spade wrote:

Sam Spade wrote:

Here is the official word from Garmin, with my commentary at the end:

snip

Seems like I wasted my time going to the trouble of obtaining an
authoritative answer.


Why do you say that? I certainly appreciated the posting.

Thanks.

It was the OP and the expert commentator to which I was referring.

Mark Hansen wrote:
On 5/1/2010 2:02 AM, Sam Spade wrote:
Sam Spade wrote:
Here is the official word from Garmin, with my commentary at the end:
snip

Seems like I wasted my time going to the trouble of obtaining an
authoritative answer.

Why do you say that? I certainly appreciated the posting.

Me too.

BrianW