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#1
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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
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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
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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
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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
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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
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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
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"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
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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
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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
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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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