Garmin DME arc weidnress

I recently flew my first DME arc transition using my Garmin GNS-530.
It's always fun and challenging to figure out what the Garmin thinks
it's doing; see my list of "Garmin gotchas" for examples:

http://www-2.cs.cmu.edu/~dst/Garmin/

Below is the newly-added entry on my DME arc experience.

-- Dave Touretzky

================================================== ==============

DME Arc: the Garmin will guide you through a DME arc as part of an
approach, but the manual does not provide much guidance on how this
works. DME arcs are covered only briefly, on pp. 71-73, and the
example given in the manual does not look like what I experienced when
I tried to fly a DME arc in the airplane. Approaching Erie,
Pennsylvania (KERI) from the south, I requested the DME arc transition
for the ILS 24 approach. This is a 61 degree arc at a 19 mile radius
off the ERI VOR. The Garmin offered a choice of several IAFs for the
ILS 24. The correct one, OMIRY, is not shown by name on the government
plate for the ILS 24 approach! Fortunately it is shown on the VOR/DME
OR GPS 24 plate. So I selected OMIRY as my IAF, and the Garmin set up
the following curious flight plan:

OMIRY-ia 143 deg. 16.9 nm
dme arc ERI 19.0 nm
D066S 033 deg. 18.2 nm
CF24
WAILS-fa 242 deg. 7.5 nm
RW24-ma 243 deg. 4.5 nm

1. The first line was a mystery. At the time I hit the PROC button to
select the approach and transition, I was south of OMIRY, heading
north. Why was the course shown as 143 deg.? It turns out that my en
route flight plan had me flying direct to KERI. So when I selected the
approach and the OMIRY transition, the Garmin calculated the course
from KERI to OMIRY, not from my present position to OMIRY.

2. The second line indicated a DME arc segment flown at a radius of 19
miles from the ERI VOR.

3. The third line indicated the intersection of the DME arc with the
lead radial for the turn onto the localizer: D066S means a waypoint at
066 deg. and 19 miles (because S is the 19th letter of the alphabet)
from the VOR. But what were those two other values? Notice that 033 deg.
is 88 degrees counterclockwise from the radial that defines OMIRY. The
033 deg. course is the initial heading along the DME arc at OMIRY,
and has nothing to do with the lead radial that marks the end of this
segment. And eventually I figured out that the 18.2 nm distance is the
length of the DME arc segment from OMIRY to the lead radial
waypoint. You can verify this by calculating
(121-066)/360 * 2*pi * 19 = 18.2.

4. The fourth line, CF24, is apparently the waypoint marking the
beginning of the final approach segment, with a course of 242 deg.

5. The fifth line shows WAILS, the FAF. The segment from CF24 to WAILS
is 7.5 nm.

6. The sixth line shows the runway 24 threshold as the missed approach
point. The distance from the FAF to the threshold is 4.5 nm.

Is it possible that the "143 deg" in the first line is the radial, thus you
would fly the inverse of 323 deg to get to the station?

Kobra

"Dave Touretzky" wrote in message
...
I recently flew my first DME arc transition using my Garmin GNS-530.
It's always fun and challenging to figure out what the Garmin thinks
it's doing; see my list of "Garmin gotchas" for examples:

http://www-2.cs.cmu.edu/~dst/Garmin/

Below is the newly-added entry on my DME arc experience.

-- Dave Touretzky

================================================== ==============

DME Arc: the Garmin will guide you through a DME arc as part of an
approach, but the manual does not provide much guidance on how this
works. DME arcs are covered only briefly, on pp. 71-73, and the
example given in the manual does not look like what I experienced when
I tried to fly a DME arc in the airplane. Approaching Erie,
Pennsylvania (KERI) from the south, I requested the DME arc transition
for the ILS 24 approach. This is a 61 degree arc at a 19 mile radius
off the ERI VOR. The Garmin offered a choice of several IAFs for the
ILS 24. The correct one, OMIRY, is not shown by name on the government
plate for the ILS 24 approach! Fortunately it is shown on the VOR/DME
OR GPS 24 plate. So I selected OMIRY as my IAF, and the Garmin set up
the following curious flight plan:

OMIRY-ia 143 deg. 16.9 nm
dme arc ERI 19.0 nm
D066S 033 deg. 18.2 nm
CF24
WAILS-fa 242 deg. 7.5 nm
RW24-ma 243 deg. 4.5 nm

1. The first line was a mystery. At the time I hit the PROC button to
select the approach and transition, I was south of OMIRY, heading
north. Why was the course shown as 143 deg.? It turns out that my en
route flight plan had me flying direct to KERI. So when I selected the
approach and the OMIRY transition, the Garmin calculated the course
from KERI to OMIRY, not from my present position to OMIRY.

2. The second line indicated a DME arc segment flown at a radius of 19
miles from the ERI VOR.

3. The third line indicated the intersection of the DME arc with the
lead radial for the turn onto the localizer: D066S means a waypoint at
066 deg. and 19 miles (because S is the 19th letter of the alphabet)
from the VOR. But what were those two other values? Notice that 033 deg.
is 88 degrees counterclockwise from the radial that defines OMIRY. The
033 deg. course is the initial heading along the DME arc at OMIRY,
and has nothing to do with the lead radial that marks the end of this
segment. And eventually I figured out that the 18.2 nm distance is the
length of the DME arc segment from OMIRY to the lead radial
waypoint. You can verify this by calculating
(121-066)/360 * 2*pi * 19 = 18.2.

4. The fourth line, CF24, is apparently the waypoint marking the
beginning of the final approach segment, with a course of 242 deg.

5. The fifth line shows WAILS, the FAF. The segment from CF24 to WAILS
is 7.5 nm.

6. The sixth line shows the runway 24 threshold as the missed approach
point. The distance from the FAF to the threshold is 4.5 nm.

In article ,
Kobra wrote:
Is it possible that the "143 deg" in the first line is the radial, thus you
would fly the inverse of 323 deg to get to the station?

No. I double checked this after the flight. The 143 deg./16.9nm is the
course from KERI to OMIRY.

Having had a chance to think carefully about what the Garmin is
displaying on each line, it does make sense. But it wasn't at all
intuitive when I first saw this screen in the air, and the manual's
skimpy coverage of DME arcs did not prepare me to decode this sequence
"on the fly".

-- Dave Touretzky

Dave Touretzky wrote:

Having had a chance to think carefully about what the Garmin is
displaying on each line, it does make sense. But it wasn't at all
intuitive when I first saw this screen in the air, and the manual's
skimpy coverage of DME arcs did not prepare me to decode this sequence
"on the fly".

Then again, a DME ARC is not an RNAV procedure. If the Garmin engineers' view
of how to approximate an ARC results in bad human-factors, the pilot is
ultimately respnsible for remaining in protected airspace. Sometimes, it is
simply easier and safer to do the ARC the "old fashion" way.

You have the patience of Job.

I now understand better why I've never felt comfortable using my GPS155XL
(with similar logic) for instrument procedures.

Thanks for the insights.

wrote in message
...


Then again, a DME ARC is not an RNAV procedure. If the Garmin engineers'
view
of how to approximate an ARC results in bad human-factors, the pilot is
ultimately respnsible for remaining in protected airspace. Sometimes, it
is
simply easier and safer to do the ARC the "old fashion" way.

I hear that.