Compass deviation from yoke?

[Flyfshr]

So I just got off the phone with my mechanic who was walking me through the items I still need to address for my owner assisted annual. One of the surprises he came up with was the fact that my compass indication has a deviation from the yoke position. Yoke full forward gives reading X, yoke full aft gives reading of X+30 degrees. What the.... !?

The compass is still mounted in the panel roughly where it is pictured in the owners manual for my bird (S/N 26398). Roughly right in front of the somewhat large mass of metal that makes up the T column I suppose.

I've noticed that most upgraded panels have the compass either on the brow or above the windscreen, far away from radios, gyros, and magnetized T-columns (I'm guessing on that last one). Does this mean I'm stuck with relocating the compass or is there some other solution out there? A compass correction card with yoke position seems like it would be a bit much....

[n2582d]

Do a search here under dega* to find information on degaussing. Here's a good lead-just a short flight away:

Sacramento sky Ranch has some good info on this subject and rents a degaussing probe.
http://www.sacskyranch.com/degauss.htm

[Bruce Fenstermacher]

Gary hit it.

Think about degaussing. Depends what is magnetized. I'm thinking it might be the yoke tubes, not the T. Probably from years of sliding back and forth. They would be easy to remove and degauss. Remove two AN-3 bolts at the universal and remove both yokes. Move the T back and forth and see if the compass swings. If not it's the yoke tubes. Degauss them.

If you find it is the control T you might try to degauss it in position. I'd probably relocate the compass before removing the control T with all the cables if that is what it take to degauss it.

[Flyfshr]

Great tip. Ill give that a try...

[GAHorn]

Any ferrous metal will become magnetized if it is vibrated, hammered, or electrically welded. There's no telling what portion (or ALL) of your yoke/controls are affecting your compass. It could be the CHAINS running on the sprockets...or any/all the rest of that control "T".

You can try to degauss the "T" as well as the shafts and chain.... or you can put your yoke in the position it spends the majority of it's time in-flight .....(right in the middle where your elevator is nuetral)....and develop your compass card from that location.

(The amount of time your yoke is in any other position while flying is extremely unlikely to be when you will be consulting your magnetic compass. Check/set your gyro's for takeoff in the nuetral position as well.)

[FredMa]

If you can find a mechanic with an Oersted meter finding out exactly what is magnetized should be easy. Engine shops should have one also. Anyone that does mag particle inspection will also have a coil for demagnetizing. Not sure what all is readily available in your area. If they let you do it yourself, a word of caution, Hang on for dear life.

[Bruce Fenstermacher]

George does have a point. Your are unlikely to be consulting the compass with full up elevator. And you could have two correction cards (at annual time at least) if someone felt that necessary. I've flown a few helicopters with at least two correction cards depending on the equipment installed or in use.

[FredMa]

Hopefully I am wrong but I have serious doubts that you will be able to perform an accurate compass swing with yoke in any position with a 30 deg + deviation when yoke is moved as you described. My experience however is on twin engine aircraft with wing mounted flux detectors. I have never been able to compensate for strongly magnetized components like we are talking about here. CRJ's are notorious for this type of problem after lightining strikes due to magnetized flap actuators or leading edge screws. I have seen actuators de-magnetized on the aircraft but it isn't easy. Good luck

[Flyfshr]

George, I was pretty much in your camp on this since I use this plane only for fair weather flying. But it's pretty clear that my A&P's strong preference is for me to relocate the compass. Since he's really helping me out with this owner assisted annual I'm willing to just go along with him on this one.

Maybe it's time for a vertical card?

[Bruce Fenstermacher]

At first blush a vertical card seems like a neat idea. But I've heard of people with problems with them. They would be effected by the magnetism same as your whisky compass.

Where are you going to move the the compass? About the only place to move it further away is to mount it on the top of the instrument panel. This isn't that far away though the magnetic field effecting the compass may be blocked enough by the aluminum to make this a workable position.

The next furthest place is to mount it somewhere on the windshield. This isn't that hard if you have the center strip on the windshield. Remove a screw and replace it with a longer one holding the bracket. This is where mine was and to be honest I'd rather it hadn't been there. Is your compass internally lit? If down low running the wire to it on the windshield might not be so bad but up higher is another consideration.

If you have no metal center windshield strip then your only option is to buy a plexi mount and glue it to the windshield. Then how do you run the light wires?

Before I did any of this I'd identify the source of the magnetism and try to eliminate it or reduce it's effect.

[GAHorn]

George, I was pretty much in your camp on this since I use this plane only for fair weather flying. But it's pretty clear that my A&P's strong preference is for me to relocate the compass. Since he's really helping me out with this owner assisted annual I'm willing to just go along with him on this one.

Maybe it's time for a vertical card?

What Bruce said.
Plus....why does your mechanic believe magnetic fields aren't all over
your airplane? (and why does he suggest killing the dog instead of the fleas?)

[Flyfshr]

George, I was pretty much in your camp on this since I use this plane only for fair weather flying. But it's pretty clear that my A&P's strong preference is for me to relocate the compass. Since he's really helping me out with this owner assisted annual I'm willing to just go along with him on this one.

Maybe it's time for a vertical card?

What Bruce said.
Plus....why does your mechanic believe magnetic fields aren't all over
your airplane? (and why does he suggest killing the dog instead of the fleas?)

The world may never know...

[Bruce Fenstermacher]

Yes I forgot to mention but should be obvious, before I bought anything to move the compass I'd test the proposed new position with a compass by either temporarily moving yours or using another one. Would be a waste to install the compass somewhere else and find that location doesn't work either.

Removing the yokes and yoke shafts takes about 15 minutes. That is what I'd do first then test.

BTW did the mechanic test the compass with all electrical consumers in the panel turned on? If not perhaps one of those produces a magnetic field that cancels the problem one associated with the control yoke position.

[Flyfshr]

At first blush a vertical card seems like a neat idea. But I've heard of people with problems with them. They would be effected by the magnetism same as your whisky compass.

Where are you going to move the the compass? About the only place to move it further away is to mount it on the top of the instrument panel. This isn't that far away though the magnetic field effecting the compass may be blocked enough by the aluminum to make this a workable position.

The next furthest place is to mount it somewhere on the windshield. This isn't that hard if you have the center strip on the windshield. Remove a screw and replace it with a longer one holding the bracket. This is where mine was and to be honest I'd rather it hadn't been there. Is your compass internally lit? If down low running the wire to it on the windshield might not be so bad but up higher is another consideration.

If you have no metal center windshield strip then your only option is to buy a plexi mount and glue it to the windshield. Then how do you run the light wires?

Before I did any of this I'd identify the source of the magnetism and try to eliminate it or reduce it's effect.

Thanks for the advice Bruce. Unfortunately (for this purpose anyway) I don't have the center strip on the windshield.

It makes total sense to try and figure out where the magnetic fields are the strongest and fluctuate the most. But to also do this when everything is on and running. So I may talk to my mechanic to defer this until after we get the rest of the punch list done and it's time to run in the new cylinders.

Then I can figure where the problems are, fix the areas that can be fixed and find the optimal spot.

[jrenwick]

My 170 had a vertical card compass when I bought the airplane, and it worked pretty well for a while. Never could get the deviation down below about 10 degrees, though. It got worse, I replaced it with a wet compass, and now I have much lower deviation. I learned that while a wet compass may have to be overhauled after a great long while, a vertical card compass will have to be thrown away and replaced after a few years, and might not work as well. I'm done with 'em!