Why The Hell... (random rant)

Ron Natalie writes:

No you are confused. Magnetic SOUTH is the exact opposite of magentic
north you fool. The fact that the MAGNETIC SOUTH POLE is not exactly
opposite the MAGNETIC NORTH POLE is a different issue.

I assumed that others would understand this, but I often get into trouble when
I assume others will understand things.

The fact that
the poles aren't aligned with the arbitrary "true" datum just means that
the variation equations aren't as simple as they might be.

They are more than just misaligned: they are not at opposite points in terms
of longitude and latitude, either. A line drawn between them does not
intersect the center of the Earth.

Really, like what?

GPS and inertial reference platforms, VORs, NDBs, you name it.

Compasses are still damned reliable compared to
anything else. Cheap too. The only thing that presumes to do better
perhaps is GPS. And it's easier to program that to deal with the
magnetic measurements than to try to force everybody else the other
way.

So how often do you fly using just your magnetic compass alone?

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Ron Natalie writes:

Because we don't have a TRUE NORTH indicator in the cockit.

You do if you have systems like GPS or INS.

Just a magnetic compass and instruments derived from it. Real flying
ain't a stupid-assed computer simulation.

Maybe you should take a tranquilizer; you seem very upset.

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On Apr 4, 9:06 am, "Kev" wrote:
On Apr 4, 2:32 am, Tauno Voipio wrote:
It's a nuisance for maintenance, but it frees
the pilot from calculating the variation at
the operative time. Just to off-load the pilot.

A good answer. Same for winds in flight, etc. Everything is based on
the magnetic to make it easier for the pilot. [..]

Oops! I can't believe I wrote that about winds in flight. Of course
they're true, so they can be used over a wider area. Winds at the
_airport_ are magnetic, is what I meant to say... so the pilot doesn't
have to convert while landing.

Hmm. So if we switched to true North navigation, then runways would
all need repainting... but at least they and the VORs wouldn't have to
be changed ever again ;-) Seems like something the government would
come up with to save money!

Kev

writes:

Would be simple to have DGs slaved to true north automatically, given
a GPS or Inertial platform.

This is switch-selectable in some aircraft, I think because true north is much
easier to deal with when flying over long routes without navaid coverage
during which latitudes and longitudes must be dealt with directly (e.g.,
transatlantic flights).

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On Wed, 04 Apr 2007 16:28:41 GMT, "Steven P. McNicoll"
wrote:


"Ron Natalie" wrote in message
om...

Huh? What does the wind have to do with VOR's. If the needle is
centered, you're within the tolerance of the VOR. The tolerance
is primarily 4-6 degrees. This keeps you inside the airway at
moderate distances from the VORs.


What about at more than moderate distances from the VOR? At 40 miles from
the VOR 6 degrees is 4 miles.

Under the heading of "Enquiring Minds Want to Know:

In defining an intersection, what's the maximum distance from a VOR
that's allowed?

Does an intersection in a GPS database represent the actual
intersection of VOR radials, or is it defined in LAT/LON terms?
(I.e., can there be two locations for an intersection, depending on
whether you're using VORs or GPS?

Apart from shifting declination, what is the mechanism by which VORs
drift out of calibration? Isn't the phase shift for each antenna in
the array set by the length of coax between it and the transmitter?
Does Velocity Factor change significantly as the coax ages? If it is a
matter of aging cable, does that mean the error is uniform and the
pattern is shifted consistently around the compass rose, or are the
longer cables affected more? And in the latter case, how would that
affect the error? (I would think that the higher the radial, the
greater the error, if this were the case.)

(Or do I not understand how a VOR array is phased?)

Don

Steven P. McNicoll writes:

"The magnetic variation of the earth changes at a rate of 50.27 seconds of
arc per year." That seems a bit low to me. At that rate six degrees of
change would take 430 years.

It changes at a variable rate, and once it approaches a degree, charts and
navaids have to change. It's a tremendous amount of extra work, an additional
source of error, and an imprecise and fickle basis for navigation.

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Steven P. McNicoll writes:

That would be true if VORs were set to local magnetic variation. They
rarely are.

Which variation are they set to, and how useful can they be if their
orientation doesn't match the local magnetic variation?

That doesn't happen until they differ by 6 degrees with local variation.

Where did you get this figure?

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Ron Natalie writes:

So that they line up with the compass. All in-flight
navigation is magnetic based.

Some transoceanic flights might disagree.

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Some of you guys are a *lot* more trusting than I am. I was really hoping
for some of the current airmen to say this, but most have only nibbled
around the edges--so here goes:

The magnetic compass has exactly one thing in its favor, and that is just
plain old Brute Reliability.

It requires no power from the aircraft's systems, it is not subject to
happenstance or whim concerning any transmitting stations, and wide spread
interference with (the) signal is unimaginable.

Learn to love it, because it is here to stay!

Peter

Well, Peter, in 40 some years and 5000 some hours in everything from
no-electric system gliders to modern jet fighters, I've never had to
resort to only a whiskey compass to navigate. Sure I cross check it,
but I view it as an emergency system. And I've actually seen more
inoperative whiskey compasses than other bits of more complicated
navigation equipment.

If you are worried about system failure, then carry a handheld backup
GPS and radio. About the same cost as a good mag compass, by the way
- but will tell you where you are, or let you ask someone where you
are if the GPS gods take a nap. All that mag compass will tell you is
which way you are kinda going.

Anyone out there who (honestly) uses only his mag compass as his
primary navigation equipment, let's hear from you. And using the
whiskey compass to set the DG doesn't count - we're talking navigation
by charted heading and mag compass. Tried rolling out on and holding
an accurate heading using only your whiskey compass lately? Fun,
isn't it...That's why they invented the DG.

Don't have a fancy nav system that figures out true north? Gee, apply
local variation to your mag heading and presto, true north! Easy to
set a DG to true that way.

Bottom line, if the mag compass was demoted to emergency heading
reference, we could accept the mag var problem and use true heading
for day to day use. Heck, most whiskey compasses swing as much as the
local variation inflight, anyway!

But as has been stated; inertia, old habits, and existing equipment
all work against moving to the use of True instead of Magnetic.

Kirk

Peter Dohm writes:

Some of you guys are a *lot* more trusting than I am. I was really hoping
for some of the current airmen to say this, but most have only nibbled
around the edges--so here goes:

The magnetic compass has exactly one thing in its favor, and that is just
plain old Brute Reliability.

It requires no power from the aircraft's systems, it is not subject to
happenstance or whim concerning any transmitting stations, and wide spread
interference with (the) signal is unimaginable.

It's already so inaccurate without interference that that's bad enough. There
are plenty of spots on charts where the compass will be 6-8 degrees off even
from the already irrgular declination over larger areas.

Anyway, if you push this concept to its limit, you should be able to complete
a trip without an engine, since engines are not 100% reliable. Obviously,
that's not a practical reality, and at some point you have to recognize that a
compass alone, no matter how reliable in the sense of always working to some
extent, may simply not be enough to get you home.

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