You can probably google up some compass-swinging instructions. The two screws are for adjusting compass- one's for morth/south and the other's for east/west as I recall.
The *right* way is to go to a compass rose, adjust the compass at each of the cardinal compass headings-- with the engine running, radio's on, and the tail up in flying attitude. Good luck.I swung mine in-flight, flying GPS headings on a calm wind day. I've also heard of people swinging them in-flight by flying section lines, again, on a calm day. Probably best to do the in-flight adjustment with one person flying the airplane and another doing the compass work. I did mine by myself but then I'm a master pilot.
You adjust the screws to get the best compromise of correct headings for all four directions. This will probably involve more than one adjustment at each heading in turn. I got mine pretty well sqaured away within just a few minutes, but then again I wasn't shooting for perfect, just close enough.

Eric

definately sounds like you could use a compass swing. when was the last time you had one done?

I assume your aircraft is a cub. There is so much steel tubing a around and behind the instrument panel that it is extremely difficult to calibrate exactly. Add electrinics, more of a pain.I use my GPS to set and update my DG.
Do you know where the whiskey compass is on the DC9 and MD80 is located? Douglas located the compass behind the copilot high on the circuit panel. The engineers decided it was easier to calibrate in that location even though it had be read backwards thru a mirror. There are two retractible small mirrors above the dashboard for the CA and FO use in the event everything else is lost for direction.
Good luck!


Idaho Supercub

If your airframe has been repaired, welded on, it may not swing without degaussing the area around the airframe. 50 degrees is more than I've seen before.
The rwo screws turn small magnets inside the compass. You are supposed to use a non-magnetic screwdriver when swinging a compass, but a brass writing pen cartridge works. Just take the pen apart and bite the open end flat.
Compensating balls can be bought to allow a compass to swing if it can't be swung anyother way, but I've never used them so I'm no help there.

Ok I guess the avionics guy better step up to the plate and give you the low down.

First try to find a real compass rose on an airport. If you can't find one call a local avionics shop and ask if they know of one.

I don't think having the airplane in a three point stance will effect the compass much. Not having the engine at cruising RPM where the generator isn't giving full voltage will.

Ok, so now set the airplane pointing north centered ontop of the compass rose. Turn on the generator, all avionics and lights that you normally have on. (you may have to make two compass correction cards for lights on & off). Now run the engine at cruising RPM and take a brass screwdriver and move the "N-S" screw so the compass reads perfect north.

Now reposition the plane so it is centered on the rose pointing East. Again run up everything and move the "E-W" screw so it is perfect East.

reposition the plane again pointing South. Now for the "skill" part. Take the error and half it. This is how much you adjust the N-S screw. ie If the compass reads 190deg adjust the N-S screw to read 185deg.

Reposition the the plane again to the West and do the same thing. If the compass reads 265 adjust it to read 267.5.

You should be now done adjusting the compass.

Now it's time to make a correction card. The maximum correction should be +/- 10 deg.

Reposition and run-up the plane the same you did during the adjustments and write down the corrections. Make up a correction card and put it in the plane and your done!

Why is having an accurate whiskey compass/correction card important? It's because the more accurate you know which way your plane is pointed the more accurate the wind aloft results your E6B (on the GPS) is going to be.

If this doesn't work I would suspect some magnetic screws are <1' of the compass or the steel structure is magnetized. We can cross that bridge if need be.

Try this before spending any money on a compass.

BTW- Get a brass rod at a hardware store and file it into a small screwdriver. Also make sure you keep your headsets away from the compass when you are working/reading it. The headbands do effect it even when on your head.

Ask any questions you may have.

-Todd Giencke

A big deal is always made about using a non-magnetic (brass, etc) screwdriver to adjust the compass screws. Why not just adjust the screws with a steel screwdriver, then move the screwdriver away and let the needle settle down for a minute?

Eric

zero.one.victor wrote:A big deal is always made about using a non-magnetic (brass, etc) screwdriver to adjust the compass screws. Why not just adjust the screws with a steel screwdriver, then move the screwdriver away and let the needle settle down for a minute?

Eric

Because you have the engine at cruise RPM to make sure the generator/alternator is making full voltage (the field is at full strength). I want to get the adjustments and readings done as quickly as possible.

How hard is it to go buy a small piece of brass?

-Todd Giencke

I promise biting the end of a cheap pen insert will work just as well. Having swung a bunch of compasses myself, the only thing I would add to tgiencke's procedure is if you have a DG, use it to "cheat" because lining up the leading edge of the wing every time on the compass rose will be a PIA.

I have one of those new little vertical card compasses from Precision. I put it on the rose yesterday, and N was dead-on, but W was about 282. When I can get my dad up in the plane with me and a GPS, we'll swing it in with at cruise with all those ponies dragging us through the sky at mach loonie.

Stupid question, but I can't remember: A GPS gives you a magnetic heading, or true? I assume magnetic so that you can maintain your track with your DG. There's prob a setting somewhere on the unit to choose.

zane wrote:I have one of those new little vertical card compasses from Precision. I put it on the rose yesterday, and N was dead-on, but W was about 282. When I can get my dad up in the plane with me and a GPS, we'll swing it in with at cruise with all those ponies dragging us through the sky at mach loonie.

Stupid question, but I can't remember: A GPS gives you a magnetic heading, or true? I assume magnetic so that you can maintain your track with your DG. There's prob a setting somewhere on the unit to choose.

Neither.

A GPS gives your track not your heading. You can have a 20 knot cross wind and have a heading of 180 but a track of 150. You can set your track to be true or or compensated for magnetic deviation.

If you want to do the adjustment in the air do it with section lines. But I don't recommend it.

Hope this helps.

-Todd Giencke

Swinging the compass on the ground on a compass rose is no doubt the correct way to do it, but doing it in the air via section lines and/or GPS is a quick and dirty way to get it "close enough". Worked for me.

Eric

tgiencke wrote:

zane wrote:I have one of those new little vertical card compasses from Precision. I put it on the rose yesterday, and N was dead-on, but W was about 282. When I can get my dad up in the plane with me and a GPS, we'll swing it in with at cruise with all those ponies dragging us through the sky at mach loonie.

Stupid question, but I can't remember: A GPS gives you a magnetic heading, or true? I assume magnetic so that you can maintain your track with your DG. There's prob a setting somewhere on the unit to choose.

Neither.

A GPS gives your track not your heading.

Ummm...I knew that. A GPS only gives differential data calculated from point A to point B, and nothing about the orientation of the aircraft. On my little handheld unit (non-flying specific) you can choose between whether you want "heading"/bearing/track values to be expressed as true or magnetic, as it's simply a matter of applying the declination.

I don't understand why the engine has to be at full throttle. As long as the battery is fully charged my alternator makes all the required power at 1000 rpm, remember the field strength of an alternator is varied by the voltage regulator so that the output is only what is required, not sure about generators, but they are regulated as well. I only turn on the normal things that are on continously while flying when I swing a compass. I don't xmit on the radio or turn on landing lights.

Zane,

Good luck with your vertical card compass. Mine was possessed. At some engine rpm, it would spin around and around. I finally gave up on it. I know some guys who swear by them, though. I solved all of my mag compass problems with a Navigator compass from S.I.R.S. http://www.sirs.co.uk/. It is amazing.

CAVU

What's so amazing about the Navogator compass? I know a guy who installed one, I don't care for the way it looks. Kinda toylike somehow. So unless it outperforms a standard whiskey compass I wouldn't be too interested. I like the visual representation of a vertical card compass but have heard too many bad things about them so never bothered. The headings I fly are usually something like "N" or "W" anyway.

Eric

a64pilot wrote:I don't understand why the engine has to be at full throttle. As long as the battery is fully charged my alternator makes all the required power at 1000 rpm, remember the field strength of an alternator is varied by the voltage regulator so that the output is only what is required, not sure about generators, but they are regulated as well. I only turn on the normal things that are on continously while flying when I swing a compass. I don't xmit on the radio or turn on landing lights.

I said cruise RPM which might take full throttle on a fixed prop because I want to simulate in flight conditions. True there might be no difference between idle and cruise RPM but I didn't want to assume that.

I have a vertical card mag compass in my plane and it works very well. They might be slightly less stable then a normal compass because I think they have no dampening fluid.

I like visulating runway and wind direction on the vertical compass because my plane still has the old, big, heavy, AN style DG compass which I like because it has a heavy, slow (RPM), gyro that doesn't precess hardly at all.

CAVU

-Todd Giencke

So unless it outperforms a standard whiskey compass

Eric, the short answer is, it does, by a lot.

There are several differences between the S.I.R.S and the typical whiskey compass: 4 magnets, silicone damping fluid, metal bellows to compensate for temp and pressure changes and a smaller barrel. All of these things combine to reduce the typical turning errors to the point where I can barely detect them--no comparison to the old Airpath.

The S.I.R.S. uses a magnifying lens to make it easier to read the numbers on the smaller barrel, and it is well-illuminated at night. I flew with an Airpath whiskey compass for years, then I had my carnival card compass. The S.I.R.S. is an order of magnitude better in performance, and it hasn't leaked a drop of fluid.

A good card compass would be nice to have. It seems that more guys have had bad experiences like mine with them than have had good experiences, but if you're one of the lucky ones, then you have the best solution. It seems to depend a great deal on luck of the draw and the particular airplane. My A&P says he likes vertical card compasses because he gets paid to install them, and then again to remove them! I'll stick with the S.I.R.S.

CAVU