View Full Version : HDG Info on A320

25th Feb 2002, 21:54
Wonder all A320 and Airbus drivers in general if someone could tell me where the IRU's get the heading info to be displayed on PFD and ND. I know how it gets its position, but how does it know which way its pointing??? <img src="confused.gif" border="0">

25th Feb 2002, 22:56
When you first switch on the units they go into automatic 'Align' phase don't they...... .Well, that's exactly what they are doing, aligning with the earths 'North-South' axis i.e.Longitude.. .They also remember in which hemisphere they were shut down.. .All earth accelerations about the 3 axes (x,y,z) are then sensed,and from the relative proportions of each, the heading can be calculated.

This is the 'simple' pilots explanation.If you want the full 'Avionics Engineers' version,just wait, someone will be bound to put finger to keyboard!! <img src="wink.gif" border="0">

[ 25 February 2002: Message edited by: moleslayer ]</p>

26th Feb 2002, 06:46
Saves us from applying the variation on the magnetic heading from the compass overhead :)

26th Feb 2002, 07:38
Here's an easy way to think of it:

If I draw a quarter of a circle on a piece of paper, you could look at that quarter arc, complete the rest of the circle, and point out the centre pretty easily.

When you turn on the IRS, and the aircraft is stationary on the ground, then the only movenment the aircraft senses is the rotation of the earth. So as the IRS's plot their position, they start to "draw" a cirlce. Once they have drawn enough of a circle, the computer can complete the rest.

The size of the circle tells the computer the latitude of the aircraft, while the centre of the circle is true north.

26th Feb 2002, 16:27
Checkboard, that's the best explanation I've ever seen on the subject!


Max Angle
26th Feb 2002, 21:13
Quite agree, a lovely simple explanation. It took my CPL ground instructor most of the morning to say that and I still don't think I understood it.

27th Feb 2002, 22:59
Have to agree with the others, checkboard, that was brilliant explanation. Only one question, when is the 4 volume series on the idiots guide to the ATPL out ?? <img src="smile.gif" border="0">

27th Feb 2002, 23:29
An interesting link about the earth's rotation.

<a href="http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970401c.html" target="_blank">http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970401c.html</a>

This link says that to calc the speed of rotation at a specific latitude, mult cos(latitude) * equator rotation speed (approx 1040 mph).

How is that formula derived ? I assume there is a relation between the cos(latitude) and the decreasing radius of the earth as the latitude increases.


28th Feb 2002, 07:27
Checkboard, Thanks a lot for your explanation!!!. .INS navigation is certainly a whole new bear for me, but I see it's one you can easily tame.