HI/DG - Reg On Precession?
 

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...)

Im pretty sure it is not an actual Reg, but just a recommendation.

I was actually thinking this the other day while on a long IFR cross country. It seemed like we had to reset the thing every few minutes, and I can't even count how many times ATC asked if we were on the correct heading. It really made for a long trip. I haven't found any actual FAR on determining if it needs to be fixed or not though.

I hate flying planes without a HSI ...

a HSI is not necessarily a slaved compass card. precession exists is all gyros, a slaved gyro constantly corrects for precession. an unslaved HSI will need regular resetting

I'd still hate flying without one, slaved or not.

The 15 min DG check is a recommendation, because gyros precess over time.

As other said, it's not a reg. I'm not even sure I'd necessarily classify it as a recommendation.

What I think of it as is information: a normally operating DG will precess about 3 degrees over 15 minutes. If it precesses substantially more than that, the pilot is increasing the risk of having it go inop in flight.

The regulatory issue, I guess, comes into play if you take off into the soup with a DG that precesses a lot and end up having a problem as a result.