Originally Posted by Outlaw2097

OK, we have all had the rule of precession regarding the directional gyro/heading indicator/whatever you want to call it beat into our skulls:

3 degrees in 15 minutes. Outside of that it is not reliable.

My question: is this merely a suggestion or a black and white IFR regulation?

Everything I have scoured makes note of this rule (of thumb) but no reference to an actual rule. Anyone have the ultimate answer?

(side note, I was going to say 'f it,' but decided to APF it instead...)