Re: NACO charts - why have a reference circle?

[Bob Gardner]

Their format allows them to depict navaids that would not be visible if the
whole thing was to scale....might have an IAF 20 miles out that would be off
the chart.

Bob Gardner

"Phil Verghese" > wrote in message
.44...
> I don't understand the point of the reference circle on the NACO
> instrument charts. According to the FAA Instrument Flying Handbook:
> "The majority of NACO charts contain a reference or distance circle
> with a 10 NM radius. Normally, approach features within the plan view
> are shown to scale; however, only the data within the reference circle
> is always drawn to scale. The circle is centered on an approach fix
> and has a radius of 10 NM, unless otherwise indicated. When a route
> segment, outside of the circle, is drawn to scale, the symbol [wiggly
> lines] interrupts the segment."
>
> I understand the definition above, but how does the reference circle
> help me at all? I think the charts would be easier to read without the
> reference circle. Almost all charts I've seen are drawn to scale, and
> when they are not the wiggly lines going through a route segment makes
> it clear that something is not to scale. I feel like I'm missing
> something about the reference circle, so please enlighten me.
>
> I started flipping through some charts and noticed a few curious
> things related to the reference circle. I guess the reference circle
> would be less confusing to me if I understood when it would be there
> or not, why it's centered in a certain fix, and why it's sometimes not
> 10 NM.
>
> 1) I could not find any standalone GPS approaches that had a reference
> circle (unless there were enroute facilities that could not be shown
> to scale). I certainly didn't miss the reference circle when it wasn't
> there. Does anyone know of an example of this? I wonder why standalone
> GPS approaches don't have reference circles.
>
> 2) Most of the time the reference circle is centered around the
> procedure turn fix and the same size as the maximum PT size (or
> centered on some other fix if there is no PT for the approach).
> Exceptions:
> a) Arcata, CA (ACV) ILS 32. The PT is w/in 10 NM of KNEES. The
> reference circle is 15 NM centered on the ACV VOR.
> b) Buffalo, WY (BYG) VOR/DME or GPS 30. The PT is w/in 10 NM of
> INJUX. The reference circle is 10 NM centered on MIZNR.
> c) Jackson, WY (JAC) VOR or GPS-A has a 10 NM reference circle and
> the PT requires you to remain within 15 NM.
> d) Hoquiam, WA (HQM) ILS/DME 24 has a 15 NM reference circle and
> the PT requires you to remain within 10 NM.
>
> If I just ignore the reference circle, will I be missing something
> important? I don't see how it helps me interpret the chart or fly the
> approach. I guess Jeppesen agrees with me, since they don't bother with
> a reference circle on their charts.
>
> Phil
> www.pfactor.com

[Phil Verghese]

"Bob Gardner" > wrote in
news:dmT5b.359099$uu5.70294@sccrnsc04:

> Their format allows them to depict navaids that would not be visible
> if the whole thing was to scale....might have an IAF 20 miles out that
> would be off the chart.

Oh I understand that. The chart can't always be to scale, that's why they
have the wiggly line symbol. If an route segment is not drawn to scale they
put the wiggly line symbol on the route to highlight that fact. That works
fine, and would still work without the reference circle. I believe Jeppesen
has something similar.

I still don't understand how the reference circle helps anything.

Phil

[Bob Gardner]

It just divides things that are to scale from things that are not.

Bob

"Phil Verghese" > wrote in message
. 3.44...
> "Bob Gardner" > wrote in
> news:dmT5b.359099$uu5.70294@sccrnsc04:
>
> > Their format allows them to depict navaids that would not be visible
> > if the whole thing was to scale....might have an IAF 20 miles out that
> > would be off the chart.
>
> Oh I understand that. The chart can't always be to scale, that's why they
> have the wiggly line symbol. If an route segment is not drawn to scale
they
> put the wiggly line symbol on the route to highlight that fact. That works
> fine, and would still work without the reference circle. I believe
Jeppesen
> has something similar.
>
> I still don't understand how the reference circle helps anything.
>
> Phil

[Mark Kolber]

On Fri, 05 Sep 2003 03:41:50 -0000, Phil Verghese
> wrote:

>Oh I understand that. The chart can't always be to scale, that's why they
>have the wiggly line symbol.

The wiggly line only tells you that the feeder route you're looking at
is substantially longer than depicted.

When we look at a map, our minds generally expect it to be to scale.
If the final approach course is 10 NM long, we will tend to (actually
should) use that segment as a "ruler" for situational awareness. That
2000' AGL tower 5 NM left of the FAC "looks" 5 NM away. Unfortunately,
the similar tower on the missed that takes us 10 NM away from the
airport might be closer than it appears on the chart.

The circle is =visually= telling us the limits. A simple statement
that "Not to scale beyond 10 miles" may be enough for you, but some
people want a picture. Ever notice that DPs and STARS are done both in
text and graphically?

Jepp does it a little different, but also uses some visuals. Charts in
mountainous terrain often have "Not to scale boxes". Jepp and NACO do
a number of things a little different. That's why some people like
Jepp and other NACO.

Mark Kolber
APA/Denver, Colorado
www.midlifeflight.com
======================
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[Phil Verghese]

Mark Kolber > wrote in
:

> When we look at a map, our minds generally expect it to be to scale.
> If the final approach course is 10 NM long, we will tend to (actually
> should) use that segment as a "ruler" for situational awareness. That
> 2000' AGL tower 5 NM left of the FAC "looks" 5 NM away. Unfortunately,
> the similar tower on the missed that takes us 10 NM away from the
> airport might be closer than it appears on the chart.

Thanks Mark, that makes sense.

> The circle is =visually= telling us the limits. A simple statement
> that "Not to scale beyond 10 miles" may be enough for you, but some
> people want a picture. Ever notice that DPs and STARS are done both in
> text and graphically?

Here's where I get confused, and maybe the problem is that I'm thinking
about it too much. Every NACO chart I've looked at is to scale, even
outside the reference circle. The exceptions to that are when the wiggly
lines are shown on the route segments, or the conentric dashed circles
indicating enroute & feeder facilities. Does anyone know of a NACO chart
that is not to scale and does not show either the wiggly lines or the
concentric dashed circles?

Your point about obstructions outside the reference circle is good.
That's what got me measuring distances on the charts and discovering
that things even outside the reference circle are actually to scale. By
the way, NACO charts are the same scale as a sectional (when the
reference circle is 10 NM).

By way of example, take a look at the approaches to Boise, ID (BOI).
Most of them show a 7310' obstacle 12.5 NM NNE of the airport. That
obstacle is outside the reference circle, but it's to scale on every
chart. Additionally all the feeder routes are to scale unless they have
the wiggly lines on them (or there are the concentric dashed circles).

Compare the Boise ILS 10R approach to the RNAV (GPS) 10L. Both show
feeder routes from the RENOL intersection, and RENOL is not shown to
scale on either plate as indicated by the wiggly line. I find the GPS
plate easier to read since it doesn't have the reference circle
distracting me. If the whole point of the reference cirlce is to show me
the boundary of where the "to scale" part of the map ends, then the
wiggly lines from RENOL are redundent on the ILS plate.

Notice on the ILS 10R approach, there's a feeder route from SALLA
without wiggly lines on the route. SALLA is drawn to scale at 14.4 NM
out (4.4 NM outside the reference circle).

So my contention is the reference circle doesn't really show anything,
and I find it distracting. I think I can safely ignore it. Please
correct me if you feel that is unsafe to just ignore the reference
circle.

Phil
www.pfactor.com

[Mark Kolber]

On Sat, 06 Sep 2003 00:39:13 -0000, Phil Verghese
> wrote:

>That's what got me measuring distances on the charts and discovering
>that things even outside the reference circle are actually to scale. By
>the way, NACO charts are the same scale as a sectional (when the
>reference circle is 10 NM).

[snip]

>So my contention is the reference circle doesn't really show anything,
>and I find it distracting. I think I can safely ignore it. Please
>correct me if you feel that is unsafe to just ignore the reference
>circle.

I was never interested enough to measure myself, but it may be a
"guarantee" issue. The word isn't really used but I think what the
circle is telling you is that NACO vouches for the accuracy of the
terrain/obstruction information within the circle and not outside of
it.

Can you safely ignore it? The reality is that most of the obstruction
information is "nice to know stuff." For flying the approach, all you
really need are the frequency, course, altitude, and distance/time
numbers. So, the obstruction/terrain information probably doesn't
really matter much.

It's when you get into trouble (engine problem that requires that you
get on the ground now) that you want to know where the terrain and
obstructions are. so, based on pilot input, you get charts with
terrain in as much detail as Jepp. Less for flying the approach than
for general situational awareness and emergency use. Can you safely
ignore it then? I guess that depends on the nature of the problem and
your personal analysis of whether the risk of the information being
wrong is worth it.



Mark Kolber
APA/Denver, Colorado
www.midlifeflight.com
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