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How can a compass suddenly go out by 20 degrees?



 
 
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  #1  
Old February 15th 04, 04:41 AM
Bob Chilcoat
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Default How can a compass suddenly go out by 20 degrees?

Recently one of my partners reported that the compass (standard whiskey
compass, not a vertical card) on our Archer was reading 20-25 degrees off of
runway heading at departure. Today a new partner was up with an instructor
for his sign off and reported the same thing. I hadn't noticed, but we seem
to have a definite problem. Nothing has changed in the plane for a long
time. We recently replaced the old Garmin GPS with a Lowrance AirMap 300
(which has its antenna on the top of the glare shield near the compass), but
the first incident was before that substitution was made. Any idea how
something like this can happen?

--
Bob (Chief Pilot, White Knuckle Airways)



  #2  
Old February 15th 04, 06:08 AM
Jay Smith
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Default

Bob Chilcoat wrote:
Recently one of my partners reported that the compass (standard whiskey
compass, not a vertical card) on our Archer was reading 20-25 degrees off of
runway heading at departure. Today a new partner was up with an instructor
for his sign off and reported the same thing. I hadn't noticed, but we seem
to have a definite problem. Nothing has changed in the plane for a long
time. We recently replaced the old Garmin GPS with a Lowrance AirMap 300
(which has its antenna on the top of the glare shield near the compass), but
the first incident was before that substitution was made. Any idea how
something like this can happen?



Move the antenna and you will observe the compass swing. Although the
antenna cable is theoretically shielded (?), the leakage of the electric
field affects the adjacent magnetic field, thereby affecting the
magnetic compass.

  #3  
Old February 15th 04, 06:41 AM
Don Tuite
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Default

On Sun, 15 Feb 2004 05:08:30 GMT, Jay Smith
wrote:

Bob Chilcoat wrote:
Recently one of my partners reported that the compass (standard whiskey
compass, not a vertical card) on our Archer was reading 20-25 degrees off of
runway heading at departure. Today a new partner was up with an instructor
for his sign off and reported the same thing. I hadn't noticed, but we seem
to have a definite problem. Nothing has changed in the plane for a long
time. We recently replaced the old Garmin GPS with a Lowrance AirMap 300
(which has its antenna on the top of the glare shield near the compass), but
the first incident was before that substitution was made. Any idea how
something like this can happen?



Move the antenna and you will observe the compass swing. Although the
antenna cable is theoretically shielded (?), the leakage of the electric
field affects the adjacent magnetic field, thereby affecting the
magnetic compass.


Doubtful. No DC. A screwdriver in the glove box is more likely.

Don
  #4  
Old February 15th 04, 08:39 AM
Mark Mallory
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Default


Bob Chilcoat wrote:

Recently one of my partners reported that the compass (standard whiskey
compass, not a vertical card) on our Archer was reading 20-25 degrees off of
runway heading at departure. Today a new partner was up with an instructor
for his sign off and reported the same thing. I hadn't noticed, but we seem
to have a definite problem. Nothing has changed in the plane for a long
time. We recently replaced the old Garmin GPS with a Lowrance AirMap 300
(which has its antenna on the top of the glare shield near the compass), but
the first incident was before that substitution was made. Any idea how
something like this can happen?



I had a problem with the Airpath compass in my '182, which at first seemed very
similar to what you describe. Turns out it wasn't a magnetic-field-related
problem; rather, the compass was simply "sticking". The compass has a flexible
diaphragm at the rear of the housing to allow for fluid expansion; the diaphragm
is separated from the main volume of the housing by a thin (non-magnetic) metal
plate. Apparently, pressure changes had caused the diaphragm to force the plate
forward, such that it would occasionally come in contact with the compass card,
causing the card to stick. The sticking would only happen on certain headings
with certain pitch attitudes; it worked fine most of the time.

I replaced the original compass with the same model Airpath, but after about two
years the replacement compass developed the same problem! I finally replaced IT
with one of these:

http://www.vargaair.com/navigator.htm

which works great (looks cool at night too.)

Mark/C182L


  #5  
Old February 15th 04, 03:04 PM
Roy Smith
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Default

In article ,
"Bob Chilcoat" wrote:

Recently one of my partners reported that the compass (standard whiskey
compass, not a vertical card) on our Archer was reading 20-25 degrees off of
runway heading at departure. Today a new partner was up with an instructor
for his sign off and reported the same thing. I hadn't noticed, but we seem
to have a definite problem. Nothing has changed in the plane for a long
time. We recently replaced the old Garmin GPS with a Lowrance AirMap 300
(which has its antenna on the top of the glare shield near the compass), but
the first incident was before that substitution was made. Any idea how
something like this can happen?

--
Bob (Chief Pilot, White Knuckle Airways)




There are two basic possibilities:

1) Something is wrong with the compass. Things that can go wrong with
compasses include leaking fluid, worn or damaged bearings, or somebody
twiddling with the conmpensation magnets.

2) Something changed in the environment the compass operates in. The
aircraft has its own magnetic field. What the compass senses is the
(vector) sum of the Earth's field and the aircraft's field (this is what
the compensating magnets compensate for). If you've had equipment
installed or removed, or wires re-routed, this could cause the problem.

Note that "installed" doesn't mean it's got to be bolted in. If it's in
the plane, it'll affect things. Maybe your partner is carrying
something in his flight bag that might affect things?
  #6  
Old February 15th 04, 03:56 PM
rip
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Default

Actually, I think most GPS antennae do receive DC through the coax to
power the preamplifier in the antenna.

Rip

Don Tuite wrote:
On Sun, 15 Feb 2004 05:08:30 GMT, Jay Smith
wrote:


Bob Chilcoat wrote:

Recently one of my partners reported that the compass (standard whiskey
compass, not a vertical card) on our Archer was reading 20-25 degrees off of
runway heading at departure. Today a new partner was up with an instructor
for his sign off and reported the same thing. I hadn't noticed, but we seem
to have a definite problem. Nothing has changed in the plane for a long
time. We recently replaced the old Garmin GPS with a Lowrance AirMap 300
(which has its antenna on the top of the glare shield near the compass), but
the first incident was before that substitution was made. Any idea how
something like this can happen?



Move the antenna and you will observe the compass swing. Although the
antenna cable is theoretically shielded (?), the leakage of the electric
field affects the adjacent magnetic field, thereby affecting the
magnetic compass.



Doubtful. No DC. A screwdriver in the glove box is more likely.

Don


  #7  
Old February 15th 04, 04:10 PM
Dave Stadt
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Posts: n/a
Default


"rip" wrote in message
. com...
Actually, I think most GPS antennae do receive DC through the coax to
power the preamplifier in the antenna.

Rip


You are correct. My Garmins do exacly that.


Don Tuite wrote:
On Sun, 15 Feb 2004 05:08:30 GMT, Jay Smith
wrote:


Bob Chilcoat wrote:

Recently one of my partners reported that the compass (standard whiskey
compass, not a vertical card) on our Archer was reading 20-25 degrees

off of
runway heading at departure. Today a new partner was up with an

instructor
for his sign off and reported the same thing. I hadn't noticed, but we

seem
to have a definite problem. Nothing has changed in the plane for a

long
time. We recently replaced the old Garmin GPS with a Lowrance AirMap

300
(which has its antenna on the top of the glare shield near the

compass), but
the first incident was before that substitution was made. Any idea how
something like this can happen?


Move the antenna and you will observe the compass swing. Although the
antenna cable is theoretically shielded (?), the leakage of the electric
field affects the adjacent magnetic field, thereby affecting the
magnetic compass.



Doubtful. No DC. A screwdriver in the glove box is more likely.

Don




  #8  
Old February 15th 04, 05:21 PM
RJ
external usenet poster
 
Posts: n/a
Default

Bob,

This happens with precision when I turn on the airconditioning in my
Bonanza. (It's described in the POH.) While I realize you're probably
not running AC in the winter, it does show how equipment can affect
the compass. Have you noticed a difference with and without the master
turned on?

Rob


Mark Mallory wrote in message ...
Bob Chilcoat wrote:

Recently one of my partners reported that the compass (standard whiskey
compass, not a vertical card) on our Archer was reading 20-25 degrees off of
runway heading at departure. Today a new partner was up with an instructor
for his sign off and reported the same thing. I hadn't noticed, but we seem
to have a definite problem. Nothing has changed in the plane for a long
time. We recently replaced the old Garmin GPS with a Lowrance AirMap 300
(which has its antenna on the top of the glare shield near the compass), but
the first incident was before that substitution was made. Any idea how
something like this can happen?



I had a problem with the Airpath compass in my '182, which at first seemed very
similar to what you describe. Turns out it wasn't a magnetic-field-related
problem; rather, the compass was simply "sticking". The compass has a flexible
diaphragm at the rear of the housing to allow for fluid expansion; the diaphragm
is separated from the main volume of the housing by a thin (non-magnetic) metal
plate. Apparently, pressure changes had caused the diaphragm to force the plate
forward, such that it would occasionally come in contact with the compass card,
causing the card to stick. The sticking would only happen on certain headings
with certain pitch attitudes; it worked fine most of the time.

I replaced the original compass with the same model Airpath, but after about two
years the replacement compass developed the same problem! I finally replaced IT
with one of these:

http://www.vargaair.com/navigator.htm

which works great (looks cool at night too.)

Mark/C182L

  #9  
Old February 15th 04, 08:21 PM
Jim Weir
external usenet poster
 
Posts: n/a
Default

Oh, fer corn sake. Not likely. The cable isn't theoretically shielded, it is
shielded. The leakage of the electric field? You mean the e-field of the
sub-microvolt GPS signal or the e-field of the three or four milliamps into the
antenna amplifier? Not likely. Not through a tinned copper braid shield.

More likely that the antenna has a steel mounting plate. Move the antenna or
remove the mounting plate.

Jim



Jay Smith
shared these priceless pearls of wisdom:


-
-Move the antenna and you will observe the compass swing. Although the
-antenna cable is theoretically shielded (?), the leakage of the electric
-field affects the adjacent magnetic field, thereby affecting the
-magnetic compass.





Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com
  #10  
Old February 15th 04, 08:30 PM
Don Tuite
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Posts: n/a
Default

Ok. But he should check the glovebox anyway.

Don

On Sun, 15 Feb 2004 14:56:03 GMT, rip
wrote:

Actually, I think most GPS antennae do receive DC through the coax to
power the preamplifier in the antenna.

Rip

Don Tuite wrote:
On Sun, 15 Feb 2004 05:08:30 GMT, Jay Smith
wrote:


Bob Chilcoat wrote:

Recently one of my partners reported that the compass (standard whiskey
compass, not a vertical card) on our Archer was reading 20-25 degrees off of
runway heading at departure. Today a new partner was up with an instructor
for his sign off and reported the same thing. I hadn't noticed, but we seem
to have a definite problem. Nothing has changed in the plane for a long
time. We recently replaced the old Garmin GPS with a Lowrance AirMap 300
(which has its antenna on the top of the glare shield near the compass), but
the first incident was before that substitution was made. Any idea how
something like this can happen?


Move the antenna and you will observe the compass swing. Although the
antenna cable is theoretically shielded (?), the leakage of the electric
field affects the adjacent magnetic field, thereby affecting the
magnetic compass.



Doubtful. No DC. A screwdriver in the glove box is more likely.

Don


 




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