Mag Compass Spinning

[Bruce Fenstermacher]

I've got a mystery I've been trying to figure out for more than two years. On occasion my mag compass will spin or take up a heading with no regard for the actual direction we are heading. The compass was mounted from the windshield center rib down near the instrument panel. It had been in this location since I bought the plane which was at least 8 years with no trouble. In this time I've removed and installed radios and various other things. Then one flight about 2 years ago it started acting strange. Leaning to one side then turning to the wrong heading and or spinning.

Of course I know this is some type of magnetic field. The question is were is it coming from. It does not happen all the time. Yet I fly the aircraft with the same equipment turned on for every flight. I've tried moving my handheld GPS with no change, I've turned the entire electrical system OFF with no change.

Finally after the 08 convention I gave up and moved the compass to the top of the windshield center rib and all seem to be OK. Flew this way for about a year with no problems as far as interference goes. Problem was I couldn't read the compass at that level so I decided to move it a bit down the windshield to it's current location shown in the picture. This has been fine till today.

170interior.jpg

Today for the first leg of our flight, about 1 hour long, the compass spun like a top. About one complete revolution every 5 seconds. It did this almost none stop for the entire one hour flight no matter what I turned off or moved. But on our return flight with the aircraft in the exact same configuration and electrical load the compass worked perfectly.

I need ideas what could be causing this. It's got to be some sort of motor I'd think to spin the compass. The closest motor are those in the turn and bank which is mounted center of the panel below the compass. But turning the turn and bank off does not stop the compass from spinning.

Could anything in the engine compartment like the generator or mags cause this?

[jrenwick]

Vibration?

[johneeb]

Cell Phone?????????????

[GAHorn]

If you hadn't mentioned that it spun 1-rpm per 5-seconds I would have suggested your compass is out of fluid. (A compass low or out of fluid is subject to vibratory occiliations that can result in spinning.)

From a US Patent office letter regarding compass design: "Virtually all compass cards spin under some horizontal vibration frequencies, which are encountered when compasses are mounted on powered vehicles, such as automobiles or motorized boats. The difference in inertia between the compass card and fluid within which the compass card is positioned causes relative movement. The relative movement in turn causes contact at the pivot point that leads to friction that drags the compass card in a circular path. Ultimately, the compass card will spin resonantly at some vibration frequencies. "

I would first look into the actual fluid within the compass to determine that it is indeed "compass fluid" and not alcohol or some other less viscous fluid. I would secondly examine the mounting bracket to determine if it isn't of a design or length which magnifies the normal vibrations encountered in your airplane.

Try this: Next time it spins...place your hand upon it to steady it and thereby alter the vibrations. If that significantly affects the spinning (and the compass has proper fluid), then you should remount the compass either in a different location and/or with a different/altered mount to change the resonant-frequency of the mount. If that doesn't work, then try painting the airplane green, which will at least slow the spinning down somewhat .

[Bruce Fenstermacher]

Damn you guys are good.

Johneeb, how did you know I had a new cell phone I was using to get weather and FSS i.e. another recent thread.

But seriously. John R and George you guys are probably on to something. First though I didn't mention it because I didn't think it had any relevance is the compass is low on fluid. About a third of the way down the sit glass. The card is still covered. The compass had recently discharged fluid which we suspect was the oily substance we found coating the flap handle below. So this is a recent change and perhaps what brought on this round of the spins. Curious that it didn't spin on the way home.

As for what fluid is inside this compass I believe it had the official compass card fluid stamp. The bracket being used is the same bracket I've used for the 10 years I've had the plane. It could be part of the problem as it is just flat aluminum bent into a bracket and is very springy.

----

BTW as compass fluid goes I had an interesting conversation with technicians at a local instrument repair shop who have been in the business since kthe '40s. When asked what exactly is in a compass they say today we use only a fluid that has "Compass Fluid" stamped on it. Keeps the conversations with the FAA to a minimum.

But that in their combined experience there is several fluids they've seen used and used themselves over their careers that will work just fine. They said lots of stuff was used during the war and all depended on what was on hand. While not recommended today a white kerosene is one of them. Kerosene it seems yellows with time. A clear lamp oil might be another choice. Stoddard fluid might work as well. The key they say is to know the fluid won't dissolve the painted numbers and won't effect the gaskets. As far as viscosity they said that if the compass is over reactive it is to thin and if it is to slow it is to thick. When asked what the common name of "Compass Fluid" might be they said they had no idea what it really was but they knew it would work.

[GAHorn]

Compass fluid is an "odorless mineral spirit".

Here's the MSDS:

Compass Fluid MSDS.doc

[1SeventyZ]

This is a solution that I like:

http://www.pai700.com/

i know it's sort of modern and unoriginal, but I've never liked whiskey-style compasses.

[jrenwick]

This is a solution that I like:

http://www.pai700.com/

i know it's sort of modern and unoriginal, but I've never liked whiskey-style compasses.

My 170 had one of these in it when I bought it five years ago. I never could swing the deviation down to less than five degrees anywhere around the compass, but it worked OK until it finally gave up the ghost this year. I replaced it with a whiskey compass, and swung it down to 3 degrees max deviation. It looks much nicer on top of my Schroers glare-shield (IMHO), and it works just fine -- I know I can trust it!

The vertical card compass is certainly easier to read, and it has a very simple mechanism. (If I can remember to do it, I'll take some pictures of the innards and post them here.) But instead of the single bearing surface in a whiskey compass, it has four bearings and a pair of gears (if I remember right) to translate the vertical axis of the magnet to the horizontal axis of the card. These parts will eventually wear out, and you've got to watch for that if the compass is getting old.

BTW, I don't use whiskey in a compass. I use WD-40. I would have tried MMO, but I couldn't figure out how to get the red out.

[GAHorn]

...BTW, I don't use whiskey in a compass. I use WD-40. I would have tried MMO, but I couldn't figure out how to get the red out.

What are you going to do if you crash and end up in a survival situation? As much as I like the stuff to keep the ticks and fleas off, ... you can't drink WD-40!

I believe the reason most fans of the vertical-card compasses like it is because they are ADD and cannot remember which way to turn to a heading.
The horizontal-card compass has several features which can be very useful if one learns to utilize them, such as the pronounced lead/lag (which despite their claims to the contrary, the diminished-effect of which was actually one of the early selling-points of the PAI product) as well as the Easterly/Westerly turning "error" which can be very useful in a case of partial-panel/failures.

But I've given up trying to teach people that in order to turn to a particular heading using a horizontal-card compass, all one has to do is turn the direction you wish the numbers on the card to move. (People who become mesmerized by the vertical card compass are thinking in only two-dimensions. Imagine yourself in three-dimensions as you do in all other aspects of flying airplanes and it'll make perfect sense.)

Of course, I use my Garmin 196 so much with it's nifty HSI presentation down in the lower-right corner of the display that perhaps I've forgotten how confusing it might be to some folks.

Long Live the Whiskey Compass, Leather jackets, the .45 ACP, and nylons with a seam-down-the-back!

[jrenwick]

....But I've given up trying to teach people that in order to turn to a particular heading using a horizontal-card compass, all one has to do is turn the direction you wish the numbers on the card to move. (People who become mesmerized by the vertical card compass are thinking in only two-dimensions. Imagine yourself in three-dimensions as you do in all other aspects of flying airplanes and it'll make perfect sense.)

I always think of turning the airplane as moving the compass's lubber line. I never get confused!

Of course, I use my Garmin 196 so much with it's nifty HSI presentation down in the lower-right corner of the display that perhaps I've forgotten how confusing it might be to some folks.

Experienced airmen will point out that the "HSI" in your Garmin is not the same as a compass, George! It's giving you the track you used to be following -- not your present heading!

John

[Bruce Fenstermacher]

Oh now you've gone and done it John. Had to bring up heading and track in the same sentence.

[jrenwick]

Oh now you've gone and done it John. Had to bring up heading and track in the same sentence.

I can do that because I live in a part of the country where the magnetic variation is almost zero!!!!

[johneeb]

...........Long Live the Whiskey Compass, Leather jackets, the .45 ACP, and nylons with a seam-down-the-back!

And WD-40

[1SeventyZ]

I believe the reason most fans of the vertical-card compasses like it is because they are ADD and cannot remember which way to turn to a heading.
The horizontal-card compass has several features which can be very useful if one learns to utilize them, such as the pronounced lead/lag (which despite their claims to the contrary, the diminished-effect of which was actually one of the early selling-points of the PAI product) as well as the Easterly/Westerly turning "error" which can be very useful in a case of partial-panel/failures.

The funny thing is, I have a horizontal card DG.

I admit that the vertical card in my head isn't very high performance. I often look at my vertical card compass before announcing my position on the CTAF.

[GAHorn]

...Experienced airmen will point out that the "HSI" in your Garmin is not the same as a compass, George! It's giving you the track you used to be following -- not your present heading!

John

I did not refer to it as a "compass".

(Nor do I use it for heading info.)

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