Compass trouble

Compass calibration is somewhat of a black magic art, but my suspicion is
that some steel parts of your aircraft have become magnetized and are a very
good source of compass errors.

You said that you replaced a radio. Remove the radio from the rack. Take
an unmagnetized steel screwdriver and see if any of the steel screws holding
the rack to the airframe are little magnets. Do the same with any steel
that you can find in the general vicinity of the compass. Try the engine
mounts as well.

Go to a TV repair shop that has been around for a while. In general, the
dingier, dirtier the store the better chance you have for finding what you
need. Ask the nice TV fixit guy if he has an old degaussing coil and ask
him if he would rent it to you.

Remove all instruments with moving coil/magnet meters (VOR/ILS indicators,
analog ammeters/voltmeters, etc.) from the panel.

Turn the airplane to the compass heading with the WORST error. Now remove
the compass. Run that degaussing coil over the whole instrument panel
slowly. When you think you are done, do NOT switch the degaussing coil off
until you are at least a foot or two away from the aircraft. If you screw
this up and switch the coil off close to the panel you will have more
problems than when you started.

Now put the compass back in its mount. Did it clear up the error? If not,
remove the compass again and run that degaussing coil S L O W L Y over the
engine mount, switching it off again when you get it a foot or two away from
the mount. Did that clear up the error?

If not, Sacramento Sky Ranch has a degausser on steroids that they rent out
for problems just like this.

Mumetal shields are a patch for the real problem ... which is that you are
carrying a magnet around with you that is sucking the compass towards it on
all headings. Solve the problem; don't patch it.

Jim

--
"If you think you can, or think you can't, you're right."
--Henry Ford

"Road Dog" wrote in message
m...

I took my Piper Warrior in for its annual and the mechanic
said that the compass card was missing and that he'd have
to swing the plane to generate a new one.

After swinging it, he claimed that he couldn't get the
E-W deviation within the minimum. So he suggested we
try "mu" shield (or something) to block the source of
magnetism which he claims is coming from one of the
instruments. (He says the compass works fine outside
the plane.)

In article ,
"RST Engineering" wrote:

Mumetal shields are a patch for the real problem ... which is that you are
carrying a magnet around with you that is sucking the compass towards it on
all headings. Solve the problem; don't patch it.

Don't forget that people cary magnets into the plane with them too. Steel
tools. Electronic gizmos. Wris****ches. Not to mention sunglasses:

http://www.framesdirect.com/magnetic...r-gst-lgp.html

The solution to that is a fluxgate out on the wingtip, but that's a few
more AMU's than most people are willing to spend.

"RST Engineering" wrote in message
m...
Go to a TV repair shop that has been around for a while. In general, the
dingier, dirtier the store the better chance you have for finding what you
need. Ask the nice TV fixit guy if he has an old degaussing coil and ask him
if he would rent it to you.

Or watch EBay.
http://cgi.ebay.com/Heavy-Duty-Degau...ViewItemVaughn

Get a campers compass with visible pointer and use it to locate sources of
magnetism...


"RST Engineering" wrote in message
m...
Compass calibration is somewhat of a black magic art, but my suspicion is
that some steel parts of your aircraft have become magnetized and are a
very good source of compass errors.

You said that you replaced a radio. Remove the radio from the rack. Take
an unmagnetized steel screwdriver and see if any of the steel screws
holding the rack to the airframe are little magnets. Do the same with any
steel that you can find in the general vicinity of the compass. Try the
engine mounts as well.

Go to a TV repair shop that has been around for a while. In general, the
dingier, dirtier the store the better chance you have for finding what you
need. Ask the nice TV fixit guy if he has an old degaussing coil and ask
him if he would rent it to you.

Remove all instruments with moving coil/magnet meters (VOR/ILS indicators,
analog ammeters/voltmeters, etc.) from the panel.

Turn the airplane to the compass heading with the WORST error. Now remove
the compass. Run that degaussing coil over the whole instrument panel
slowly. When you think you are done, do NOT switch the degaussing coil
off until you are at least a foot or two away from the aircraft. If you
screw this up and switch the coil off close to the panel you will have
more problems than when you started.

Now put the compass back in its mount. Did it clear up the error? If
not, remove the compass again and run that degaussing coil S L O W L Y
over the engine mount, switching it off again when you get it a foot or
two away from the mount. Did that clear up the error?

If not, Sacramento Sky Ranch has a degausser on steroids that they rent
out for problems just like this.

Mumetal shields are a patch for the real problem ... which is that you are
carrying a magnet around with you that is sucking the compass towards it
on all headings. Solve the problem; don't patch it.

Jim

--
"If you think you can, or think you can't, you're right."
--Henry Ford

"Road Dog" wrote in message
m...

I took my Piper Warrior in for its annual and the mechanic
said that the compass card was missing and that he'd have
to swing the plane to generate a new one.

After swinging it, he claimed that he couldn't get the
E-W deviation within the minimum. So he suggested we
try "mu" shield (or something) to block the source of
magnetism which he claims is coming from one of the
instruments. (He says the compass works fine outside
the plane.)

On Tue, 15 Apr 2008 21:53:31 +0800, Stealth Pilot
wrote:

On Sat, 12 Apr 2008 16:11:59 GMT, (Drew
Dalgleish) wrote:


the problem doesnt seem to be the compass but some stray magnetism in
the aircraft.
swinging a compass takes about 15 minutes.

Swinging a compass in 15 minutes may be possible if no corrections are
needed but to create a new card properly requires 8 points instead of
just 4. chasing stray magnetism can take up a lot of time. Unless you
have other reasons to believe that your mechanic is incompetent or
trying to screw you then I'd be inclinded to trust him.

your first one will take lots longer.
you need two people.
one pilot in the aircraft another outside with an accurate land
compass.

in a location well away from metal objects like hangars line up the
aircraft with magnetic north using the land compass to align.
(in my tailwind I just lift the tailwheel to reposition so I suppose
that would be a sizeable time aid)
take the cover off the compass front. usually just one screw is
removed, the other loosened and the cover swings away.
under the cover are two screws. one labeled NS the other EW.
with the engine running at idle, all electrics turned on, you adjust
the NS screw with a bronze or austinitic stainless screwdriver, until
the card is reading N. (both screwdrivers mentioned are non magnetic)

turn the aircraft and line it up to magnetic east using the land
compass. adjust the EW screw until the compass reads E.

using the land compass to align, turn the aircraft to face due
magnetic south.
here is the important bit. using the NS screw, adjust out *HALF* the
error.

turn the aircraft to face west using the land compass (and standing
well back from the aircraft)
here is the other important bit. using the EW screw, adjust out *HALF*
the error.

now turn the aircraft back to mag north using the land compass to
align. record on the chart the error seen on the aircraft compass.
turn to 30 degrees magnetic aligning with the land compass. again
record the error on the correction chart.
repeat this for every 30 degree increment around the compass card.

now provided that all your corrections are under 5 degrees you have a
newly swung and adjusted compass.
lifting the tailwheel to position it all takes me 15 minutes.

at the end of it all the engine (idling all this time) and oil are
quite hot so that is the time to drain the oil for the oil change.

Stealth Pilot



My home airport has a compass rose and I use that. Taxiing aroud and
getting lined up on each heading takes me quite a bit longer than
that. Having a helper would speed things I'm sure but if we consider
man hours since the OP was about mechanics time then double your
15minutes. I was going to ask how do you know if it's an accurate land
compass but it occurs to me that a GPS would do the job and then you
could do it by yourself . I still don't think I could do it as fast as
you can though. It takes me that long to find my brass screwdriver.

In article (Drew Dalgleish) writes:

My home airport has a compass rose and I use that. Taxiing aroud and
getting lined up on each heading takes me quite a bit longer than
that. Having a helper would speed things I'm sure but if we consider
man hours since the OP was about mechanics time then double your
15minutes.

It is easier to shut down and just turn the plane from heading to
heading manually.

I was going to ask how do you know if it's an accurate land
compass but it occurs to me that a GPS would do the job and then you
could do it by yourself .

How?

A GPS knows where it is, not which way it is facing.


Alan

On Wed, 16 Apr 2008 02:31:50 GMT, (Drew
Dalgleish) wrote:

On Tue, 15 Apr 2008 21:53:31 +0800, Stealth Pilot
wrote:

On Sat, 12 Apr 2008 16:11:59 GMT, (Drew
Dalgleish) wrote:


. I was going to ask how do you know if it's an accurate land
compass

the compass is supremely well made but not anything better than an
army prismatic compass.

think about how a compass functions. it cannot be in error if made
accurately. you use the compass about 20 feet back from the aircraft
to remove any influence of the fuselage.

Stealth Pilot

On Wed, 16 Apr 2008 05:19:27 +0000 (UTC),
(Alan) wrote:

In article (Drew Dalgleish) writes:

My home airport has a compass rose and I use that. Taxiing aroud and
getting lined up on each heading takes me quite a bit longer than
that. Having a helper would speed things I'm sure but if we consider
man hours since the OP was about mechanics time then double your
15minutes.

It is easier to shut down and just turn the plane from heading to
heading manually.

I was going to ask how do you know if it's an accurate land
compass but it occurs to me that a GPS would do the job and then you
could do it by yourself .

How?

A GPS knows where it is, not which way it is facing.


Alan
Do it while flying. The GPS know which way it's going.

On Wed, 16 Apr 2008 21:46:41 +0800, Stealth Pilot
wrote:

On Wed, 16 Apr 2008 02:31:50 GMT, (Drew
Dalgleish) wrote:

On Tue, 15 Apr 2008 21:53:31 +0800, Stealth Pilot
wrote:

On Sat, 12 Apr 2008 16:11:59 GMT, (Drew
Dalgleish) wrote:


. I was going to ask how do you know if it's an accurate land
compass

the compass is supremely well made but not anything better than an
army prismatic compass.

think about how a compass functions. it cannot be in error if made
accurately. you use the compass about 20 feet back from the aircraft
to remove any influence of the fuselage.

Stealth Pilot

That makes sense Thanks.

A GPS only can measure your ground track, not aircraft heading.