How can a compass suddenly go out by 20 degrees?

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.

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

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

"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

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

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