please explain this one

[piperboy84]

Is there something wrong or am I just being a dummy.

Line up on 27, wet compass shows 27 set HI to 27 and set off in very light variable wind. Fly about an hour north then turn around to head home, set the Garmin 496 for "go direct" back to my home field and follow the pointer that extends from the wee plane on the screen. The Garmin shows my Course, Bearing and Track all 180 BUT the wet compass and HI show I am flying 210 !

What am I missing here?

[fisbangwollop]

Maybe a strong westerly wind, tracking 180 but heading 210? :cool:

[piperboy84]

I understand that bearing doesn't but course and track must use either magnetic or true as a reference, or am I completely off the reservation here?

Maybe a strong westerly wind, tracking 180 but heading 210

If that was the case then wouldn't my course and track be different to reflect that?

[fisbangwollop]

Back to basics for you Piperboy....

http://en.wikipedia.org/wiki/Course_(navigation)

[piperarcher]

Do you have anything metal placed close to the Compass. I put my Zaon MRX up on top of the instrument panel and it was at the time a little too close to the compass and was knocking it off by about 20 degrees. Other than I can only guess one of the two screws that calibrate it have moved and your compass needs recalibrating. I would agree its a bit stange but I often find the wet compass is never as precise as a GPS reading.

[rans6andrew]

If you get the ball back in the centre, does it all swing back into agreement?;)

Rans6...

[piperboy84]

i have a Zaon XRX up there also

[piperboy84]

Back to basics for you Piperboy....

Looks like I may have to scurry away and do some reading (or re-reading) before I dig myself in deeper !!!

[Johnm]

Hmmmm magnetic heading is where the aeroplane is pointing. Track is the actual direction of travel over planet earth, the difference is the correction for drift due to wind. 30 degrees is a lot of drift so it's likely that the compass is subject to some sort of interference too.

You need to revise your basic navigation and make sure that there's nothing within a foot of the compass that might have magnetic properties.

[dublinpilot]

Is your GPS track set to true rather than magnetic?

What is the magnetic variation in your area?

[phiggsbroadband]

Hi, if you assume that magnetic = true for the purpose of an easy calculation, then you can use a velocity triangle to determine the wind velocity. The Hypotenuse side is your airspeed, and the Opposite side is the wind speed. Remember OHMS? Opp/Hyp means Sin.

So for 30 degrees, sin 30 = 0.5

Which means your cross wind is 0.5 of your airspeed... Say 45 knots if you are in a 90 knot cessna.

(Even allowing 3 deg magnetic variation, its still 41 knots.)

[dont overfil]

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.

[rans6andrew]

and don't forget, the HI is a gyro instrument and will drift off by 15 degrees per hour, give or take.

[UV]

the HI is a gyro instrument and will drift off by 15 degrees per hour, give or take.
Not if the Latitude nut is set correctly (which it should be).

[RTN11]

The GPS has no idea of your heading or how much drift you are offsetting, it only shows track, so your track is 180, heading is 210, where's the issue?

[Shaggy Sheep Driver]

This reminds me of a chap (non pilot) I used to take flying in the Chippy (him in rear cockpit) occasionally. One day we had a good ground speed and explained that was because of a healthy tail wind.

"Oh, I could tell that", he replied.... "by the wind blowing in under the back of the canopy!". :eek:

[sycamore]

UV, the latitude nut caters for `transport drift` ,not Earth rotation.The Di must be reset to the best `compass` every 15 minutes,as it will still drift,due Earth rotation..

[znww5]

Perhaps worth mentioning; the winds might be 270 and light on/near the ground, but what about the winds aloft from F214?

[keith williams]

Sycamore

the latitude nut caters for `transport drift` ,not Earth rotation.

The latitude nut is simply a metal nut mounted on a threaded stud which protrudes from the inner gimbal. Altering the position of the nut creates an imbalance of the gimbal. This in turn creates a wander rate which compensates for (part of) the wander of the gyro.

The latitude nut is not accessible to the pilot, so it can be adjusted only in the instrument workshop........................1

Earth rate wander (in degrees per hour) = 15 x Sin latitude. So for a given latitude it is constant.

Transport wander = West - East Ground speed x Tan Latitude / 60.

The west-east ground speed can be adjusted (within limits) at will by the pilot................2

If you look at statements 1 and 2 you should see that the (constant) position of the latitude nut cannot compensate for the (non constant) transport wander, but it can compensate for the (constant) earth rate wander.

[Crash one]

Piperboy, If you flew north for an hour was that on the wet compass or the GPS?
Frinstance in a westerly of 15knots you will be 15nm east of where you thought in a hour if yr on the wet compass. If you then turn round & fly south on the wet compass fr another hour yr gonna be 30nm East of home, or thereabouts, aren't you? So the GPS go to is telling you where home really is innit? Never mind all this wind triangle deviation gobbledegook.:ok:

[RTN11]

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.

[Crash one]

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.

[keith williams]

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.

[Keef]

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.

[Crash one]

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?

[RTN11]

Piperboy84 are you sure you aren't winding us up here?

I think we need him to take photos of the compass, DI and GPS display next time he's up so we're sure he's not just dreaming it all.

[24Carrot]

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.

[Ebbie 2003]

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.

[piperboy84]

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.

[bingofuel]

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!

[piperboy84]

If you are flying a taildragger

Ahh sugar !!! that is probably it I have big tundra tires on it which makes it sit very proud on the ground

[Crash one]

I never thought of that, My compass isn't too good either & I know I swung it tail down (9deg tail down)


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.


Must pay more attention:ugh: