If you've ever wandered through the woods with a compass in-hand; you've figured out that it only reads accurately, when you're either standind still (or walking at a steady pace), and it is help perfectly level. Same deal for compasses in airplanes.

I won't get into the techy mush about how, bank/pitch/accelleration skew compasses. All you need to know for simming, is that anything other than level, constant-airspeed flight, can make a compass read erroneously. The only variables worth noting; are that banking has more of an effect on the north/south readings.. and acceleration (speeding up OR slowing down) has more of an effect on east/west readings.. and of course those two factors overlap  

You can see this in the sim. Load up the C172.. get into level, stable flight at a heading of 180. Start a steep turn and whatch what happens (immediately) to the compass. Then, fly stable heading 270 and start that turn. You'll see that the westerly headings are less altered by bank. You can try the reciprocal too. You'll see that a 270 compass reading will change as you increase or reduce power.. but a 180 heading will not  

ANYway.. Our prime concern is the banking.. because that's where compass-turns come into play. Like other aspects of sim training, exactness is more about understanding it all, than it is about execution.

Let's say that again, you've lost vacuum.. and your heading indicator is useless. You now need to make a heading change and don't want to bother with timing the turn via the turn-coordinator. Decide what heading you want to roll-out onto, and then apply this rule..


If your new heading is 090 or 270  (or near them), you actually  CAN  use the compass.. because THOSE headings aren't effected by bank. If your new heading is 360 or 180, you'll use this reference (valid only in the northern hemisphere, for the bulk of the 48, continental states)

U.N.O.S. "Undershoot North, Overshoot South"...

Meaning.. for a north heading, you watch the compass and start rolling out of the turn BEFORE it reaches 360 (undershoot). And for a south heading, watch the compass and start rolling out of the turn AFTER it passes 180 (overshoot).

How many degrees before or after, depends on the target heading. For pure north/south (180/360) it's aproximately 30 degrees and gets proportionally less, the further away form pure north/south.. eventually reaching zero (obvioulsy), for east/west headings.

Of all the bugs and quirks in the MSFS flight model.. this stuff is replicated PERFECTLY.. and one of many reasons I consider it an excellent training tool.  

Now.. go fly around in the C172 and practice this stuff. Pick headings at random, and see how closely you can turn to them, by compass alone (cover up or fail the heading indicator)