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Strange compass behavior



 
 
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  #1  
Old February 13th 04, 01:27 AM
me
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Default Strange compass behavior

Help, can someone help explain these compass symptoms? I know that the
compass should be about the most reliable piece of equipment in the plane,
but ours is acting strange.

We have a Precision PAI-700 vertical card compass mounted in a 1966 Cessna
182. We’ve swung the compass on the ground with the engine running at 1800
RPM and all electronics on. All readings are within 5 degrees; most have 3
degrees or less error.

In the air is another story. On a day with very light winds aloft we
checked the compass indication against the GPS (and the GPS against a chart)
and we had errors between plus 12 degrees and minus 19 degrees when in
un-accelerated straight and level flight, roughly on the four cardinal
directions as shown below.

GPS Compass Error
163 166 +3
272 253 -19
327 339 +12
067 062 -5

While on a long trip with the autopilot slaved to the GPS, we noticed that
the Compass does not swing smoothly in the air, (although it seems to on the
ground) but acts as though it has detents and moves in steps of more than 10
degrees. We took a reading on the GPS and the compass in un-accelerated
straight and level flight, then turned approximately 3 degrees to the right
and repeated to get the following set of readings:

GPS Compass Error
246 220 -26
249 230 -19
252 231 -21
255 232 -23
258 233 -25
261 243 -18
264 244 -20
267 254 -13

We also repeated the above measurements on a different heading, but with the
avionics master off and compared against the heading indicator.

HI Compass Error
135 135 0
130 125 -5
125 121 -4
120 120 0
115 109 -6
110 73 -37
105 60 -45
100 51 -49
95 48 -47

All of the above measurements seem very repeatable. The compass is
relatively new. At first glance it might seem that we should replace the
compass, but it swings fine on the ground, and seems to move freely on the
ground with no funny behavior. We just don’t want to replace the compass
and find we still have this problem (although that is probably our next
step).

Any ideas?

Thanks,
--Michael A.



  #2  
Old February 13th 04, 01:53 AM
John Harlow
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Default

Any ideas?

Maybe you can borrow another aircraft compass or even a handheld one (know
someone with scuba gear?) and set it beside it to see if it gives the same
errors - if it does, look for a localized disturbance.



  #3  
Old February 13th 04, 02:52 AM
external usenet poster
 
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Default

me wrote:
Help, can someone help explain these compass symptoms? I know that the
compass should be about the most reliable piece of equipment in the plane,
but ours is acting strange.


We have a Precision PAI-700 vertical card compass mounted in a 1966 Cessna
182. We?ve swung the compass on the ground with the engine running at 1800
RPM and all electronics on. All readings are within 5 degrees; most have 3
degrees or less error.


In the air is another story. On a day with very light winds aloft we
checked the compass indication against the GPS (and the GPS against a chart)
and we had errors between plus 12 degrees and minus 19 degrees when in
un-accelerated straight and level flight, roughly on the four cardinal
directions as shown below.


GPS Compass Error
163 166 +3
272 253 -19
327 339 +12
067 062 -5


While on a long trip with the autopilot slaved to the GPS, we noticed that
the Compass does not swing smoothly in the air, (although it seems to on the
ground) but acts as though it has detents and moves in steps of more than 10
degrees. We took a reading on the GPS and the compass in un-accelerated
straight and level flight, then turned approximately 3 degrees to the right
and repeated to get the following set of readings:


GPS Compass Error
246 220 -26
249 230 -19
252 231 -21
255 232 -23
258 233 -25
261 243 -18
264 244 -20
267 254 -13


We also repeated the above measurements on a different heading, but with the
avionics master off and compared against the heading indicator.


HI Compass Error
135 135 0
130 125 -5
125 121 -4
120 120 0
115 109 -6
110 73 -37
105 60 -45
100 51 -49
95 48 -47


All of the above measurements seem very repeatable. The compass is
relatively new. At first glance it might seem that we should replace the
compass, but it swings fine on the ground, and seems to move freely on the
ground with no funny behavior. We just don?t want to replace the compass
and find we still have this problem (although that is probably our next
step).


Any ideas?


Thanks,
--Michael A.


Most airplanes tend to shake a lot more at high RPM on the ground than in
the air.

Since you say the compass does not swing smoothly in the air, try tapping
it with your finger and see if the thing is sticky.

--
Jim Pennino

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  #4  
Old February 13th 04, 05:15 AM
Victor J. Osborne, Jr.
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Default

Get a new flashlight? Mine had a big magnet in the bottom I didn't know
about. The flashlight now lives in my garage.

Thx, {|;-)

Victor J. (Jim) Osborne, Jr.



take off my shoes to reply


  #5  
Old February 13th 04, 03:32 PM
Dan Thomas
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Default

Might have to move the compass up away from the panel. There are a lot
of electrics under the glareshield that could be interfering with it.
I had the same problem with a vertical card compass in an Ercoupe, and
had to put it way up on the windshield bow.
Those vertical card units are very sensitive to vibration. You HAVE
to use the really soft, floppy mount they provide for them.
Remember that GPS give you track, not heading, and if there's any
appreciable wind you won't get anywhere near accurate heading
indications for compass calibration purposes. Use the heading
indicator instead.

Dan
  #6  
Old February 13th 04, 04:20 PM
Dave Butler
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Default


Dan Thomas wrote:

Remember that GPS give you track, not heading, and if there's any
appreciable wind you won't get anywhere near accurate heading
indications for compass calibration purposes. Use the heading
indicator instead.


Yep, forget the GPS. Use this method instead:

Line up on a taxiway or runway with a known, published heading and carefully set
the HI. Go fly on gyro headings 360, 030, 060, etc. and carefully record the
magnetic compass reading. You might want to try it with and without lights,
pitot heat, any other major power consumers. Return to your known taxiway or
runway and verify the HI hasn't drifted.

Dave
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  #7  
Old February 13th 04, 04:46 PM
Chris Hoffmann
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Default

You don't live near Bermuda, do you? I'm half serious....

Then again, I'm guessing that at one time your compass was working fine....



  #8  
Old February 13th 04, 07:55 PM
me
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Default

Thank you for the numerous suggestions. I think we'll try taking another
compass with us in flight for comparison, we still have the whiskey compass
that we replaced with the vertical card compass (and might be going back
to). Also isolating the compass better from vibration sounds like a good
thing to test.

I should have mentioned that the compass is mounted on the windshield in a
1966 Cessna 182 so it is well above the electronics. We did some of the
tests with the avionics master off and we switched off all strobes, lights;
it did NOT make a measurable difference. We're in the Sacramento Valley in
California and have repeated the tests on different flights so that we're
convinced it is not a local disturbance, nor have we seen the lost squadron
near Bermuda, but those are good points to bring up. And as I mentioned we
also accounted for wind on all tests.

Thanks to the group for the ideas, we're still listening if anyone has
suggestions.

--Michael A.

================================================== ==

Here is a summary of the suggestions we received so far ...

Maybe you can borrow another aircraft compass or even a handheld one (know
someone with scuba gear?) and set it beside it to see if it gives the same
errors - if it does, look for a localized disturbance.

Most airplanes tend to shake a lot more at high RPM on the ground than in
the air.
Since you say the compass does not swing smoothly in the air, try tapping it
with your finger and see if the thing is sticky.

Get a new flashlight? Mine had a big magnet in the bottom I didn't know
about. The flashlight now lives in my garage.

Might have to move the compass up away from the panel. There are a lot of
electrics under the glareshield that could be interfering with it. I had the
same problem with a vertical card compass in an Ercoupe, and had to put it
way up on the windshield bow.
Those vertical card units are very sensitive to vibration. You HAVE to use
the really soft, floppy mount they provide for them.
Remember that GPS give you track, not heading, and if there's any
appreciable wind you won't get anywhere near accurate heading indications
for compass calibration purposes. Use the heading indicator instead.

Remember that GPS give you track, not heading, and if there's any
appreciable wind you won't get anywhere near accurate heading
indications for compass calibration purposes. Use the heading
indicator instead.

Yep, forget the GPS. Use this method instead:
Line up on a taxiway or runway with a known, published heading and carefully
set the HI. Go fly on gyro headings 360, 030, 060, etc. and carefully record
the magnetic compass reading. You might want to try it with and without
lights, pitot heat, any other major power consumers. Return to your known
taxiway or runway and verify the HI hasn't drifted.

You don't live near Bermuda, do you? I'm half serious....
Then again, I'm guessing that at one time your compass was working fine....



  #9  
Old February 13th 04, 09:48 PM
MikeM
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Posts: n/a
Default

In my experience, in-panel, vertical card compasses aren't worth a ****.
Go back to a standard wiskey compass.

MikeM


  #10  
Old February 14th 04, 02:00 AM
Max T, CFI
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Default

There used to be an aricle on www.avionicswest.com (perhaps it's still there)
that talked about how the vertical compasses they tested were ruined by placing
a magnetic object a single time next to the compass. Things as innocent as
a Jepps binder (with it's steel spine). Sounds like they're great when they work,
but that you have to be very careful with them.
Max T, MCFI

Victor J. Osborne, Jr. wrote in message ...
Get a new flashlight? Mine had a big magnet in the bottom I didn't know
about. The flashlight now lives in my garage.

Thx, {|;-)

Victor J. (Jim) Osborne, Jr.



take off my shoes to reply




 




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