# How is the the turning error of an aircraft magnetic compass calculated?

I have a major exam in school tomorrow and I still don't get how to get the turning error of an aircraft magnetic compass. For example, if given an initial heading of 120 degrees and final heading of the Northern and Southern hemisphere, how to calculate the overshoot and the level off? I know the UNOS and ONUS but still don't get the concept of it. Please help me.

• Do you mind if I ask what the exam is for? It's just it seems to me like you're after more precision than what is expected on the basic pilot exams.
– Ben
Sep 17, 2015 at 9:06
• Just made some little grammar edits for you. If I made a mistake with my fixes than you can change it back. Sep 17, 2015 at 9:23
• @Ben Agreed - in the UK we're just expected to be aware of it
– Dan
Sep 17, 2015 at 10:12

First of all, in real life the overshoot will depend on the bank angle, so it is not really practical to do calculations unless you are doing some kind of special, precision flying or acrobatics. Normally you just get a feel for the overshoot and compensate intuitively. Exact calculations can be useful if you are doing precise figure-8's or something like that.

To actually do the calculation (which is based on a shallow turn 15-degrees to 20-degrees):

270 or 090 no correction necessary
northerly -- need to overshoot
southerly -- need to undershoot

Amount of correction (as variation from East/West parallel):

90-degree difference (eg heading 000) -- use 30-degree correction
60-degree difference (eg heading 330 or 030) -- use 20-degree correction
30-degree difference (eg heading 300 or 060) -- use 10-degree correction

Example 1:

You are on a 110 heading and want to turn to 030. You need a 20-degree overshoot. Turn left and roll out when you reach heading 010.

Example 2:

You are on a 335 heading and want to turn to 240. You need a 10-degree undershoot. Turn left and roll out when you reach heading 250.

• The over/under on compass headings will also vary with the rate of turn, particularly in fluid-damped compasses (this is the real effect you're capturing when you describe "bank angle": you can have a slow turn in a 45-degree slip and a coordinated turn in a 5-10 degree bank with similar error characteristics). The compass design also affects the error: Compasses with designs that promote eddy-current damping will have less pronounced overshoot/undershoot than ones without this effect. Sep 17, 2015 at 18:14