Question about a variance between headings on the PFD and Nav displays

[Michael Radford]

Hi all, I noticed this a while ago but on a flight today there was quite a pronounced difference between the reported heading on the nav display and the one on the compass rose of the PFD. On the image attached the heading bug has been set to the same heading as reported by the nav display as 116, on the PFD though it is reporting the heading as around 124 (which interestingly is what the IRS reports too). The variance is quite large and you can see it quite clearly on the PFD heading bug on the compass.

So what causes this, which one of the two reported headings is accurate and is there a way to correct for it? Going at a complete guess I'd suspect it has something to do with magnetic variation deflecting the compass needle but any sort of clarification would be greatly appreciated.

[Andreas Fuchs]

Compare "heading" and "track", that's the answer to your question.

[Kirk Christie]

Heading is where your nose is ponting.

Track is the direction your plane is traveling.

On your nav display in the top left you have wind information, I can see that you have wind from your 5 o'clock position at 68 knots, this would be pushing on the rear of the aircraft causing the nose to point to the right of your track.

[William Teale]

9 hours ago, Kirk Christie said:

On your nav display in the top left you have wind information, I can see that you have wind from your 5 o'clock position at 68 knots, this would be pushing on the rear of the aircraft causing the nose to point to the right of your track.

Technically, this is pushing on the nose of the aircraft as much as it is pushing on the rear of the aircraft. Simple fact, all forces acting on an aircraft act through the centre of gravity, anything else is a moment. The wind is not pushing the nose to the right of track, it is pushing the ground track to the left of the nose. Correspondingly, your aircraft will (assuming an FMC or an autopilot smart enough - or possibly even a pilot smart enough) will react to the wind changing their ground track, and offset the nose (and thus the heading) into the wind, correcting the ground track for the effect of wind.

A steady wind has zero effect on the nose, or any other part of the aircraft attitude. A CHANGING wind, such as gusts, will cause keel effects, but that is another story again.

In summary - if you are flying along heading 360, and a wind from 090 slowly picks up from zero knots to 60 knots, your heading will remain 360 the whole time, but your ground track will deviate from 360 to the left, based on the vector sum of the wind velocity and your true airspeed (velocity).