please explain this one
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All this talk of earth rate and lat nut is irrelevant, since the op has clearly stated that both compass and DI were showing 210, so he would have reset any error from the gyro.
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Having given this a bit more thought I would like to retract my previous smart ass comment.
If the a/c flew North for 1 hour on the compass. If there was any wind the a/c will NOT be due North of home at this time. Therefore home will NOT be South at 180 & the Garmin should not read 180.
If there was no wind on the way out & he was actually due North, then the wind must have suddenly increased as he turned South, for the compass to read 210 would need a hell of a wind to need 30deg drift.
On the other hand.
If he flew North on the GPS for 1 hour then turning round, home WOULD read 180 on the GPS. And the compass could read whatever.
The question therefore is: Was the outbound flown on the wet compass or the GPS?
It certainly has buggerall to do with Lat nuts or transport drift nor Coriolis nor Isaac Newton.
If the a/c flew North for 1 hour on the compass. If there was any wind the a/c will NOT be due North of home at this time. Therefore home will NOT be South at 180 & the Garmin should not read 180.
If there was no wind on the way out & he was actually due North, then the wind must have suddenly increased as he turned South, for the compass to read 210 would need a hell of a wind to need 30deg drift.
On the other hand.
If he flew North on the GPS for 1 hour then turning round, home WOULD read 180 on the GPS. And the compass could read whatever.
The question therefore is: Was the outbound flown on the wet compass or the GPS?
It certainly has buggerall to do with Lat nuts or transport drift nor Coriolis nor Isaac Newton.
RTN11
I'm not suggesting that the problem in the Op was caused by gyro wander.
What I am saying is that Sycamore's assertion that the latitude nut compensates for Transport Wander is just plain wrong.
One of the problems with these threads is that if untrue statements are left unchallenged they often become "accepted truth" in the minds of some readers.
I'm not suggesting that the problem in the Op was caused by gyro wander.
What I am saying is that Sycamore's assertion that the latitude nut compensates for Transport Wander is just plain wrong.
One of the problems with these threads is that if untrue statements are left unchallenged they often become "accepted truth" in the minds of some readers.
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The other consideration is that while the compass may read correctly on a heading of 270, any uncalibrated objects close to it may mean it doesn't do so on a northerly heading.
In this case, since compass and DI agreed when he turned onto north after takeoff, that doesn't apply.
In this case, since compass and DI agreed when he turned onto north after takeoff, that doesn't apply.
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The fact that the compass And DI agreed only proves that he reset the DI to the compass as per FREDA periodically. So amount to the same thing.
Piperboy84 are you sure you aren't winding us up here?
Piperboy84 are you sure you aren't winding us up here?
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Piperboy84 are you sure you aren't winding us up here?
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I googled the OP's "Glens o'Angus" location. Having seen the pics, I think it is worth asking the OP whether he was near a mountain when he noticed the 180/210 difference?
High crosswinds near mountain tops are perfectly consistent with "light and variable" winds in valley bottoms.
High crosswinds near mountain tops are perfectly consistent with "light and variable" winds in valley bottoms.
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Nothing unusual about this.
Likely you have some combination of the wind pointing your airplane at an angle to your track - as calculated with your wizz-wheel; your wet compass being out of adjustment; your compass adjust card missing or you're not making the adjustment and possibly some transient interference with the wet compass (metal glasses case on the glare shield caught me out once).
Also your GPS could be then issue - if your magic magenta line is showing 'true' rather than 'magnetic' course.
Remember also that the 'numbers' for the runways were probably set years ago - the do change with drift but rarely get repainted with the new ones. For example mt home airport, TBPB, has a single runway 09/27 - but 09 is actually about 75 degrees - the airport I learned to fly at KHWO also had 09 runways but in the past year or so due to drift these are now 10L and 10R. So while setting or checking ones altimeter by reference to the field altitude before take off may be a good idea, verifying/checking the set of ones wet compass is not.
Have all or a number of these effects all acting in the same direction and it would explain the issue; either way getting the wet compass checked would be a good idea.
A further potential cause - acceleration/deceleration dip and swing - another effect to add to the others - not an issue if one is flying straight and level at a constant speed.
The wet compass, the DI and GPS will never all agree but big differences should be investigated.
Likely you have some combination of the wind pointing your airplane at an angle to your track - as calculated with your wizz-wheel; your wet compass being out of adjustment; your compass adjust card missing or you're not making the adjustment and possibly some transient interference with the wet compass (metal glasses case on the glare shield caught me out once).
Also your GPS could be then issue - if your magic magenta line is showing 'true' rather than 'magnetic' course.
Remember also that the 'numbers' for the runways were probably set years ago - the do change with drift but rarely get repainted with the new ones. For example mt home airport, TBPB, has a single runway 09/27 - but 09 is actually about 75 degrees - the airport I learned to fly at KHWO also had 09 runways but in the past year or so due to drift these are now 10L and 10R. So while setting or checking ones altimeter by reference to the field altitude before take off may be a good idea, verifying/checking the set of ones wet compass is not.
Have all or a number of these effects all acting in the same direction and it would explain the issue; either way getting the wet compass checked would be a good idea.
A further potential cause - acceleration/deceleration dip and swing - another effect to add to the others - not an issue if one is flying straight and level at a constant speed.
The wet compass, the DI and GPS will never all agree but big differences should be investigated.
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Right then, since my last flight detailed in my OP here I went on holiday and took a FAA flying manual with me and boned up on basic NAV. Still can’t figure out compass and Garmin 30 degree difference thing.
So I flew from Forfar to Plockton today and upon the return leg I plugged in “direct” to Forfar and this time the compass and DG sat at 090 while the Garmin gave me a course and track of around 120 consistently (which is a 30 degree swing in the opposite direction from the compass/garman differential on the OP trip) Again there were very little winds if any, and I verified that the Garmin is set for Magnetic Variation Auto”
So I'm buggered if I know what it is.
So I flew from Forfar to Plockton today and upon the return leg I plugged in “direct” to Forfar and this time the compass and DG sat at 090 while the Garmin gave me a course and track of around 120 consistently (which is a 30 degree swing in the opposite direction from the compass/garman differential on the OP trip) Again there were very little winds if any, and I verified that the Garmin is set for Magnetic Variation Auto”
So I'm buggered if I know what it is.
Last edited by piperboy84; 27th Feb 2013 at 17:06.
If you are flying a taildragger, was the tail raised when the compass was last swung?
If it was not, it will be accurate with the tail down but inaccurate in the flying attitude, which appears to be the case flying from Plocton to Forfar, which on a chart looks like a track of about 120deg, not 090!
If it was not, it will be accurate with the tail down but inaccurate in the flying attitude, which appears to be the case flying from Plocton to Forfar, which on a chart looks like a track of about 120deg, not 090!
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If you are flying a taildragger
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I never thought of that, My compass isn't too good either & I know I swung it tail down (9deg tail down)
Must pay more attention
Thinking out the box here. Did you check the wet compass in level flight northbound?
Does your wet compass read differently in the level attitude compared with the more extreme 3 point attitude you have with the bigger wheels?
D.O.
Does your wet compass read differently in the level attitude compared with the more extreme 3 point attitude you have with the bigger wheels?
D.O.
Last edited by Crash one; 27th Feb 2013 at 19:54.
