The International Cessna^® 170 Association

Two years after installing my Atlee-Dodge windshield V-brace, I suddenly have a problem whereby the compass always indicates North or pretty close to that.

The compass is on an aluminum bracket just a few inches above the center of the windshield V-brace, which of course is attached there with 3 AN4 bolts to the top of the panel.

We installed a new compass today, but that did not help. We therefore removed all 3 center V-brace attach bolts, and the compass reads correctly, so the problem is the bolts. The steel bolts we removed were not magnetized -- wouldn't even pick up a paper clip.

We installed new AN4C stainless bolts, and the new compass is off by 30 degrees, which is still well beyond what compass adjustment magnets can handle (20 degrees) per the new compass installation instructions.

My IA thinks there may be some sort of special "non-magnetic" flavor of stainless bolts, but nobody seems to even have a part number for them, or even have heard of such hardware.

What's REALLY strange is that I flew this airplane last summer on floats with the V-brace and steel bolts, with no compass problems. Suddenly, on the first flight of this season (last week), it only wanted to say "North." The ONLY things that were done to the airplane over the winter were to rewire the airframe and panel and install a new engine, but, mind you, this problem shows itself on the ramp with nothing turned on.

Our only "Plan B" is to remove the cabin speaker from the ceiling (isn't even wired up) to get rid of its 12" radius magnet effect, and glue the compass to the top of the windshield.

If anybody has some insight into this mystery, or knows of a source for special "non-magnetic" AN4 bolts ... please speak up.

It takes much less magnetism than what's required to pick up a paper clip to affect a compass. Sometimes even dropping a bolt can induce enough magnetism to have an effect. If you have another set of bolts, you might give them a try. If not you might try de-gausing the ones you have.

Miles

That's interesting.

I just got a callback from the fastener guru at Aerocenter/Spencer Aircraft near Seattle, which claims to have found special non-magnetic bolts on the East Coast. I should receive them here in AK next Tuesday.

Try holding each of the removed bolts up near the compass to see which of them affects it. Stainless does not ordinarily affect compass indications, but you may have a low-grade that includes an unusually large amount of ferrous mat'.
Miles is absolutely correct, ...one method of creating a magnet is to simply hold a piece of ferrous material (steel, iron, etc.) in alignment with the earth's magnetic field and vibrate or shock it repeatedly. It will assume a N/S magnetism. The V-brace it'self may have become magnetized subsequent to it's original installation due to engine vibrations, etc.
Whenever checking compass systems it's also wise to consider nearby buildings and REINFORCEMENT RODS and other steel items which may be embedded nearby. (Heavily reinforced concrete can influence a compass, although usually it does not because it is usually reinforced in a large-scale, regular pattern. But locally concentrated reinforcments may influence a compass. This is a consideration when laying out an airport compass calibration rose.)

I've held the compass next to the V-brace along the entire length of each leg and there is no effect on the compass. The problem seems to be the bolts. We'll see for sure when the special ones arrive next week.

From other dialog on this site, it seems that what Atley Dodge sells is stainless braces, not aluminum, so I suppose a brace could get magnetized.

My last flight last season took me down one side of Clear AFB's R-2206, which says on the chart "CAUTION: Possible damage and/or interference to airborne radio due to high level radio energy vicinity R-2206." I wonder if our government had something to do with this ...

Stainless steels can exhibit magnetism from next to none, to strong depending on the alloy, how it's been formed, and/or heat treated. 300 series CRES that's been cold worked can be weakly magnetic. 400 series CRES can be strongly magnetic. 400 series can also rust. CRES actually forms a surface film much like aluminum that protects the metal from oxidation. Some alloys are more sensitive to damage of that film and subsequent oxidation. 400 series being one. We had pits that went all the way through a radioactive liquid waste storage tank made of 1/2" thick 400 series.

One theoretical method of demagnetizing a piece is striking it. The shock is supposed to disorient the molecular magnetic domains. In practical terms it's useless, besides being nearly impossible to measure outside a lab the practice usually damages the part beyond use. A more practical means of demagnetizing is using an AC coil and passing the part through several times rotating between passes.

tye

Well, the new magic (and expensive) bolts didn't help at all.

After partial disassembly of the panel looking for the culprit, we determined that the problem the WELDING between the two members of the seaplane brace and the large sleeve in the middle. A pencil-tip magnet sticks to the weld line and nowhere else.

Why it took 2 years for this to become a problem is anybody's guess. We don't really have a good way to determine if the weld line is actually magnetized, but it apparently contains iron or whatever magnets like to stick to. Even if it were magnetized and we could demagnetize it, I have no assurance that it wouldn't become magnetized again, since I don't know how it got this way.

My IA talked to someone at Atlee Dodge and apparently this is not the first time this has happened with their V-braces.

So, I removed the (unused) speaker from the ceiling and ordered from a Cessna dealer all the hardware to move the compass to the top of the windshield.

Lakehood, that's interesting. I've just noticed the same problem on mine, and I'm having a bit of a time tracking it down.

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

Had something similar happen while flying a 39 Piper J5 to Alaska. Got to Fort Nelson, where we actually needed the compass for the first time, and noticed that the compass always read 264 degrees. Took the compass out of the plane and over to the compass rose and it was OK. Turned out that the similar "V" braces had been magnetized on the left side where we huing our head seats. Could not get it de-gausses till Anchorage. Have seen the "V" braces in a 185 do some similar stuff even on a vertical card compass. Might want to be carefull where ya hang your ears.

I once took a lightning hit in a Duke while on a research flight, then had to shoot an ILS to mins with my HSI convinced that my left wingtip was N. interesting time.

The local avionics shop had a handheld degausser (not too fancy Radio Shack stuff) which I used on the various pieces of steel braces after I disassembled the equipment mounted below the wing. checked each piece with a compass until I got it demagnetized (some peices took several tries) then reassembled everything with new hardware and all was well again.

so, try your local avionics shop or Radio Shack, and you should be able to demag the braces.

I found the following in my files from bush flying days. Thought it might be of some use to those with magnetized V braces or control yoke shafts.

Hey! Where's the FAA approval on that document?

(If you use a "Radio Shack" or similar tape-eraser, as you finish degaussing the item, slowly move the degausser away from the item before releasing the activation switch. This is why an AC generator is used in the above illustration....so that the degaussing field may be reduced to nil before stopping the action. Ordinary house current will only work if a large capacity rheostat or some other method is used to reduce current to nil prior to stoppage.)
It should not be necessary to remove the compass in order to degauss the item.

Hey George if you were paying attention Gary mentioned it was fro his HELO DAYS.

Helo stuff don't need no stinken approvals cause we don't really fly the earth repels our ugly machines.

(an alternate ending would be, we beat the air into submission)

Bruce,
Actually, it was from my Helio--as in Helio Courier--days. The local tribesmen in Irian Jaya viewed these as "male" aircraft. The smaller C-185's MAF flew were "female" -- probably because of the belly pods MAF had on them made them look "pregnant". They became a bit confused when we started installing belly pods on the Helios!