In article ,
Roger Long om wrote:
A club member has asked me why we spent money to have a 14 degree error
removed from our compass since it is just a back up instrument if everything
else quits.

Not for an NDB approach! Oh, wait, an NDB approach is a backup if
everything else quits. ;-)

A couple of 360 in our 172 to look at something on the ground will
put our DG 15 -20 degrees off and it drifts about that much each hour.

I've flown behind DGs that bad, but most hold heading better than that.

An A&P I asked in another forum said he hopes his customers don't expect him
to get the compass closer than about 10 degrees. Our shop says 10 degrees
is what is allowed.

I asked about this on rec.aviation.ifr and not many people said they
used the correction card. I've only used it on the ground to assure
myself that it was mostly small 1-2 degree errors so I could ignore it
in flight.

--
Ben Jackson

http://www.ben.com/