geoffreywh Posted May 16, 2008 Share Posted May 16, 2008 here is a question. I was returning to base recently and turned on the GPS (only just fitted). I discovered that my coming mounted compass did not match the numbers on the GPS (Avmap) by 10 degrees. This is a LOT...The aeroplane is not new and has been flown all around OZ ..BUT, I am inclined to believe the GPS first. Is that necessarily so? Maybe, just maybe, I have introduced the error myself by panel mounting the Micro air instead of the "Radiogram" on the parcel shelf the week previously. Also how much use is the gyro compass these days? I was correcting it about every five minutes...I have seen lovely digital compasses to use as backup....What does the team think?...Geoff Link to comment Share on other sites More sharing options...
Captain Posted May 16, 2008 Share Posted May 16, 2008 GPS "track" is not the same as "heading" if there is any breeze (and there usually is). Did you have a cross breeze at the time? If so they can often be that different. Link to comment Share on other sites More sharing options...
facthunter Posted May 16, 2008 Share Posted May 16, 2008 difference. Could this be VARIATION (as in magnetic)? Nev.. Link to comment Share on other sites More sharing options...
geoffreywh Posted May 17, 2008 Author Share Posted May 17, 2008 Yes there was a cross wind, I never thought of that. The GPS shows the track (across the ground) I am flying, while the compass shows the actual direction the aeroplane is heading to counteract the cross wind. Yes it was about 15 knots from the side. Would that be enough to account for the discrepancy? Thanks...Geoff Link to comment Share on other sites More sharing options...
Guest Macnoz Posted May 17, 2008 Share Posted May 17, 2008 One other consideration might be changed magnetic fields around the compass. this could be from the device itself or any wiring harness that was installed My bean bag mounted Garmin 296 will swing the compass +/- 5 degrees depending or where I mount it on the dash Link to comment Share on other sites More sharing options...
Guest Andys@coffs Posted May 17, 2008 Share Posted May 17, 2008 I reckon NEV is on the money. Compass obviously shows Magnetic north. GPS shows True north with the difference around 7-8 degrees. Add in a little cross wind and 10 degrees is perfectly acceptable and expected. That said most GPS's have the ability to have a configuration change so that any compass direction is shown as true or Mag, however pretty sure the default is true. Andy Link to comment Share on other sites More sharing options...
facthunter Posted May 17, 2008 Share Posted May 17, 2008 Numbers. The Variation is 10 degress (OTA) around these parts. Nev.. Link to comment Share on other sites More sharing options...
geoffreywh Posted May 17, 2008 Author Share Posted May 17, 2008 Thanks People..... I see it all clearly now...The GPS showed west... the Compass included the magnetic var. for the area (-13deg). The extra 3 degrees would be the wind factor. Giving me 10 degree variation between the two...I worked it out on my whizz wheel.. Comes in at a cross wind speed of 10 knots... So I don't need to change the compass at all.................BUT how can I use the GPS to check the compass?...Standing still on the runway? Link to comment Share on other sites More sharing options...
Guest Andys@coffs Posted May 17, 2008 Share Posted May 17, 2008 The only way is during taxi when wheels on ground ensure no cross wind impact. Compass = at any time I point towards mag north GPS= at any time I will tell you in what direction you are traveling over the last quantum of time that the GPS uses. When sat still, GPS will say no movement = no direction. Like I said in the previous post, if this is important to you, change the GPS configuration so that it displays mag based directions. The GPS pilot operators handbook will cover how this is done. Andy P.S I 'll make a generalised statement, of all the instruments you have available the compass is the one I'd be least concerned about providing that a few rules are adhered to. Firstly that comment is true while nothing changes. Any work on the avionics, or an addition or subtraction from the avionics load will generally mean that the compass should be swung. Terminology that means the compass is adjusted for the magnetic field variations that the added instruments generate. Something the average owner will not be capable of doing due to the fact that they wont have surveyed reference points and directions available to them. Secondly the magnetic fields around the earth that a compass reacts to are relatively weak and as such any magentic material that you, or any other passenger brings on board has the potential to completely upset the compass and your resultant navigation. For example one of those wind up torches in cargo pants pocket with its stong magnet may completely overpower the earths fields and make the compass useless. Probably not worst case because a pilot should quickly notice a compass that doesnt swing at all, however weaker magnetic material that just offsets the heading by 30-40 degrees, that may be more insidious depending on where you are in Australia. Link to comment Share on other sites More sharing options...
facthunter Posted May 18, 2008 Share Posted May 18, 2008 Pre take-off compass check. Compass swings are done with all electrics on and engine(s) running, well away from steel buildings and other aircraft, then the figures are recorded on the little card near the compass. These indicate the corrections required when on various headings. If the compass was able to be perfectly adjusted ( never happens) no correction would be required, on any heading. Unless something is altered in the aircrafts equipment there should be no need to suspect an unreliable indication. A lightning strike is likely to cause major changes of a permanent nature with a magnetic compass, requiring a re-swing. This would be the least of your worries in an ultralight, if you were to be struck by lightning The most likely cause of a significant indication error, is ( as has been pointed out by andys@gawler,) is where ferrous or magnetic material is placed near the compass. Runway designators are derived from the first 2 digits of the magnetic alignment of the runway, rounded to the nearest 10.(ie. 296 becomes 30 so the runway becomes designated as Rwy 30/12. If you are taxiing at a steady speed on a relatively smooth taxiway, on ,say the runway reciprocal, this gives you an opportunity to check your compass to an accuracy of less than 5 degrees. You set your gyro compass ( if you are lucky enough to have one) at the same time. This prevents you tearing off in the wrong direction when you are in unfamiliar territory, and don't have the luxury of a GPS. Which should not be relied on solely anyhow.. Nev.. Link to comment Share on other sites More sharing options...
rick-p Posted May 18, 2008 Share Posted May 18, 2008 Nev, "A lightning strike is likely to cause major changes of a permanent nature" Mate this is the quote of quotes it would get the Oscar. The mind boggles, the imagination runs rife when one gives your quote an analytical evaluation. Did you receive my PM? Rick-p Link to comment Share on other sites More sharing options...
facthunter Posted May 18, 2008 Share Posted May 18, 2008 Attention. rick-p, got your attention. Sometimes I try to be funny, but it's true about the lightning. Q2 Yes, thanks. Nev.. Link to comment Share on other sites More sharing options...
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