Heading readings do not agree.

[01438820193]

Leaving Exeter Airport the ATC told me to go to a heading of 080 degrees. I set the heading bug to 80 and set the autopilot to Heading. when it had settled down. The heading dial was reading 79 the bug knob read 80 and the garmin 500 was reading 85. I pressed the 'D' ket to alighn them but it made no difference. can anyone help me please.

[SteveFSX/P3D]

The heading indicator is showing you the direction the nose is pointing, the GPS is showing track (trk) the way the aircraft is moving. Imagine a boat floating down a river with its nose 'not' pointed in the direction it's going. The direction it's nose is pointing is shown on the heading indicator, whilst the direction of travel down the river is shown on the GPS.

So you draw a line on a map between two towns to plan a flight. The direction of that line on the map gives you the track of say 180. If there is no wind this will also be your heading, as the nose of the aircraft will point right down that 180 track that you made on the map.

Now you add some wind coming from your left. That wind will push you off the track line between the towns if you don't point your nose toward the wind a bit. So you push the nose toward the wind if you want to stay on that line. Now you have a heading of 175 on your heading indicator because that's where your nose is pointed. Your GPS is showing you on 180 track because you are still on the 180 track line as your aircraft moves through the air. The only time the Trk and heading will match is when there is no wind.

If the ATC gives you a heading you use the heading indicator not the GPS track.

If you are in heading mode and trying to follow the GPS line, the wind adjustment to keep on trk will have to be worked out by you. If you select NAV mode the GPS will figure that out for you, and adjust your heading to keep on the autopilot Trk. Whilst heading mode just points the nose toward a certain direction, leaving the wind to possibly blow you off the GPS track.

[pirateinparadise]

Nice explanation.

[SteveFSX/P3D]

Originally posted by pirateinparadise:

Nice explanation.

Thanks, I still find this whole thing confusing myself. So if anyone spots an error in my description then let me know :-)

[01438820193]

Thank for the help very enlightening.

[De Kaashaas]

You're spot on Steve! The correct naviagtional terms are 'True Heading' (or TH, the actual heading when you would draw your trajectory on a map) and 'Magnetic Heading' (the heading you set on the heading bug).

And even that's a simplification of the real life stuff: there's also magnetic variation (the magnetich north differs from the true north of the planet and even the magnetic north is fluctuating every day. The anual magnetic variation is usually announced on maps use for navigation) and of course the fact that magnetic readings of the compass are different to the directional indicator, which uses gyro's. (The latter needs to be corrected with three degrees every 15 minutes).

Interesting stuff, if you want to go as real as it gets. Take a day off and google it. ;-)

[SteveFSX/P3D]

Originally posted by De Kaashaas:

You're spot on Steve! The correct naviagtional terms are 'True Heading' (or TH, the actual heading when you would draw your trajectory on a map) and 'Magnetic Heading' (the heading you set on the heading bug).

And even that's a simplification of the real life stuff: there's also magnetic variation (the magnetich north differs from the true north of the planet and even the magnetic north is fluctuating every day. The anual magnetic variation is usually announced on maps use for navigation) and of course the fact that magnetic readings of the compass are different to the directional indicator, which uses gyro's. (The latter needs to be corrected with three degrees every 15 minutes).

Interesting stuff, if you want to go as real as it gets. Take a day off and google it. ;-)

Thanks for this, very interesting - although your main paragraph has made my head hurt :-)

So if I use VOR from the charts I can avoid all this magnetic variation I suppose? .... lol

So charts are alinged with true north - and then I add or subtract the variation from magnetic to find my desired aircraft heading - because the aircraft only deals in magnetic headings?