Good evening folks, I'd like to pick your brains again. Since we bought our aircraft we have noticed that the compass seems to always be out of line, by 25 to 30 degrees. I have no experience of this, so I was wandering what the causes of this could be? Any info would be great.
I have just recently been through this myself. A few things you should check: 1st, take the compass out of the aeroplane and check it on its own. 2nd, have you noticed whether it is influenced by electrical consumers in the a/c? Ttry switching things on one by one and monitor the reading. My pitot heater will cause a 20° swing! 3rd, try doing a compass swing, there are plenty of how-to guides on the net but you'll need to borrow a landing compass. 4th. try to find a professional who will sort it out for you. I tried and failed at this bit.....
I don't know why, but that degree of wandering leaves me wondering whether there is a more fundamental problem than simply the effects of systems affecting its accuracy.
FOK
Last edited by FlyingOfficerKite; 14th Oct 2012 at 18:52.
Has the airframe been stored for a long time in the same position? Or in the last year or so suffered a (near-) lightning strike?
Those are two reasons why the airframe itself (well, the ferro metal bits of it) may have become magnetized. You can check this by taking a simple hand-held compass and moving it about the airframe, close to the various metal bits. If the metal bits give a big error on that compass your airframe might need to be degaussed, especially if the errors can not be corrected with the built-in adjustable magnets in the compass.
Other than that, SNRMs list is a good place to start.
Cheers Stick, all points make sense. FOK I would do it, but the rest of the lads in the share want to try and do anything and everything bar buy a new one!! Although I have just remembered there is a bent aircraft in one of the hangers that we could probably borrow the compass from to check whether it is indeed just a knackered compass.
every airliner taking off from New York's Laguardia airport is warned of magnetic compass problems due to steel holding up the runway...
any thing like that around you?
it is interesting if activiating electrical items make the compass worse...have you checked the internal lighting system of the compass for a short?
The aircraft is a Piper Archer, no work has been done recently apart from the annual and a problem with our Mode S transponder which just whips out of the radio stack, as you all know! It hasn't been stood for ages either, it just has me baffled as I don't know enough to be able to have a go at diagnosing it.
First thing - do a 4-point compass swing using a known good external compass to compare with the aircraft. Is the 25-30 degree error constant on all 4 cardinals or does it reverse on some? There are North-South and East-West adjusters within the compass and if the error is constant on all headings, you shim the mount with brass washers to impose a fixed correction on all headings. Any good mIntenance company and particularly a good avionic tech should be easily able to sort this.
Hey, just ask any CAA ATPL holder. I seem to recall being 'awarded' the privilege of being able to swing a compass on getting the Green Book. Now, of course, I didn't just dump that invaluable knowledge and the ridiculous formulae in favour of something far more useful but, alas, I no longer reside in Blighty and have taken my brass screwdriver with me
We had a really odd problem once. I thought the DI was packing up as it would "drift" 45 degrees on N-S headings. Actually it wasn't the DI at all but the compass, and if you were flying N-S and set the DI then turned E-W you'd be off by 45 degrees.
We only figured this out by cross referencing to the GPS ground track. When setting GT on the DI it was fine!
So we borrowed an external calibrated compass from our maintenance place and swung the aeroplane which indeed showed that the compass was out in the N-S headings. We adjusted the N-S heading adjustment screw to correct and it was right as rain ever since.
I used to do swings as part of my job as an avionics chappie. They really aren't that hard to do, it's all available on the net these days, all you need to do is borrow a landing compass and tripod.
It would seem a bit odd that it's always out by the same amount though, they are usually more 'out' on certain headings than others. Having said that, a dash mounted E2 will go out of true depending on how far forward or back you have the stick...
I had one recently that wouldn't come true and I noticed the E-W compensator made no difference.
It turned out that someone had stripped the threads on that bit of the compensator by trying to turn it beyond the limit.
It may be worth sliding out the compensator to check.
I've seen advertised an external compensator that can pull in compasses that won't otherwise correct.
My sequence for compensating (from memory):
Line up on mag N Turn on all normal services Zero error on N using N-S screw Line up on mag S adjust error to half initial value using N-S screw Repeat process on E-W using E-W screw.
Then check error every 30 degrees for compass card.
If the compensator can't cope with the errors then you may need to de-gauss, re-position the compass, look for ferrous objects causing the deviation or use an external compensator.
The compass itself can't go wrong. The compensator possibly can as I described. The only other source of errors are stray magnetic fields due to ferrous objects or electrical currents.
I've also seen a Garmin GPS put on the coaming pull a compass out by 30 degrees.
I might be missing something here but as Castleford is in the UK and a Piper Archer is a C of A aircraft. It should be maintained under Part M subpart F arrangement. Private owner maintenance doesn't allow for adjusting or changing compasses. Further to this the maintenance organisation who have certified the aircraft are entitled to some consideration as they have a legal responsibilty for the aircraft.
I woulds suggest that you should ignore some of the so called advice being given here as it suggests you should indulge in illegal activity..
However nothing to stop you doing some fault finding providing you do not adjust or remove the offending item.
We had a Robin which had a magnetized canopy latch, every time you closed the canopy the compass went out. Took a while to figure that out as we were swinging in the summer with the canopy open due to the heat. Then it went out in flight.
Helicopter newly imported had a main rotor drive shaft that had not been demagnetized following a maganaflux inspection. That came from the states and direct from the manufacturer with a valid swing card. Often wondered who penned that one off.
I would suggest trying to get hold of a flux meter or a simple hand held compass and move it around within the cockpit looking for areas that attract the needle. Do it power off then if no joy do it power on.
Nothing to stop you doing a compass check swing. Go on to the interweb and look at
airpathcompass.com
they have instructions on how to conduct a swing.
The usual cause of compass errors is the crap that pilots put on top of the instrument panel i.e knee boards or other items with steel fittings.
I would think that the compass is probably an Airpath and they are not expensive. Probably £150. Lets face it if that is too much for a group then you are too poor to own an aircraft. Remember it could affect your entitlement to benefit!!!!!
Last edited by ericferret; 16th Oct 2012 at 11:32.
Ok, well my advice would be to go to your Maintenance outfit and ask to borrow their compass. If they are any good they will say "sure". You then swing the aeroplane, and write down your results. Then go back to the engineer and say "here, can I ask your advice" and ask him to adjust your screws for you, or you adjust them under his (her) supervision. Bung him 20 quid or a bottle of scotch for his trouble and then you are done legally.
