Strange Compas

Strange Compas

<HTML>I recently bought a 1969 172K model.  On 2 occasions the compass has gone completley crazy. It starts spinning and then just spins for about a minute.  The first time it happened I had just finished my run up and was taxing onto the runwa, it started spinning as I turned.  Then about 2 weeks later it did this just after I started the engine.  It stopped and functioned propery through the duration of the flight.  I ask the local avionics shop about it and they said they have never heard of such a thing.
1. Has anyone else experianced this?
2. Does anyone have any ideas on how to trouble shoot this?

Thanks for any help or suggestions,

Dan</HTML>

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Re: Strange Compas

<HTML>The magnetic compass or the Directional Gyro that is usually driven by a engine driven vac. system.????</HTML>

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Re: Strange Compas

<HTML>Dirty air filters will sometimes fail and allow a peice of grit to enter the gyro, which can settle into the caging bearing.  If this locks the bearing, the card will spin wildly.  I've seen this happen a few times, in which it never re-manifested itself, but usually it is the sign of a gyro in need of overhaul, and a vacuum system in need of new hoses and filter.  The hoses themselves can deteriorate internally and allow rubber particles to enter the gyro, even if a filter is fine.</HTML>

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Re: Strange Compas

<HTML>This is happening with the magnetic compas.  I think it is indicative of some sort of electromagnetic field being created by something on the airplane,  I just don't know what or how to find it.</HTML>

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Re: Strange Compas

<HTML>Have you had any Major Avionics work done lately?</HTML>

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Re: Strange Compas

<HTML>No nothing.</HTML>

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Re: Strange Compas

<HTML>Has your compass lost it's fluid?  It's possible for a compass to lose it's fluid and do this.  It's usually the result of the rubber gasket failing at the back of the compass.  You should send your compass out to an instrument shop, or order a compass overhaul kit from Aircraft Spruce (877/477-7823) and fix it yourself. (It's easy. Instructions are included.)</HTML>

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Re: Strange Compas

<HTML>Rebuilding is a good idea, however I've been thining about replacing the old compass with a verticle card compass.  Do you know if I have to have an A&P install or can I do it myself?

Dan</HTML>

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Re: Strange Compas

<HTML>You'll have to get it signed off by a licensed A&P.  I should warn you that the vertical-card compasses are not as reliable as the originals.  Some examples have a nasty habit of "hunting" for their headings, and all of them are easily damaged.  Many times the mere act of installing them causes them to malfunction, when they are tilted or "jostled" during the installation their internals sometimes mis-align.  I once thought I wanted one myself, until I flew behind a couple of them.  (Also, remember that many of the characteristics of magnetic compasses are dampened out by the mechanism with the result that the vertical card compass is not as capable of being used accurately in a "no-gyro" situation.)  My 2 cents.</HTML>

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Re: Strange Compas

<HTML>George ,

Thanks for the input,  I think the rebuild kit is in order.

Dan</HTML>

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Re: Strange Compas

<HTML>George, in Aug I bought a 172K which has a vertical card and I assumed it was accurate. So every time it got 20 to 30 degrees off from my DG, I reset my DG.  I felt I'd need to replace the DG since it was precessing so much.

Finally while tracking on a VOR radial with a heading which matched my GPS and my DG, I learned my my problems was with the vertical card   -- not my DG.
I was about ready to spend some big bucks for a new DG.

So, you are correct about your assessment of the vertical card -- at least as far as I can tell.</HTML>

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Re: Strange Compas

<HTML>I put a SIRS Navigator compass in my 172K mounted on the windshield (stick on) and I am incredibly happy with it -
Its lighted for night - I also put on a padded glare shield with built in flood lights - they adjust with the overhead rheostat - the combination makes night flying much more enjoyable - and I enjoy flying at night anyway.
anyway - I highly recomend the compass. I believe I found it at aircraft spruce, but I have seen them several places.
- Ken.</HTML>

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