On Fri, 14 Mar 2008 06:08:16 -0400, "Roger Long"
wrote:

"Bruce in Bangkok" wrote
I used to swing aircraft compasses as part
of my job and never saw one that was 100% accurate on all headings.

You'll like this story then. Back when I was Maintenance Officer of the
flying club we put in a new windshield and some equipment that required
reswinging the compass. The shop said I couldn't do it because I wasn't an
AP but I was welcome to take the plane out and adjust it before they did the
official checks to save time and money.

I built a sight of a board with two nails in it and taped it to the
stabilizer carefully aligned with the aircraft center line. I then spent a
couple hours dragging the plane all over the ramp turning it 180 degrees and
tweaking out half the differences just as I used to do on boats. I did this
on 20 degree headings and got it damn near perfect, way within the limits
you can read while it's jiggling around in flight. I then put the plane on
the cardinal headings and ran the engine up to full power and turned all the
electronic equipment on and off and made radio checks to be sure nothing was
upsetting it. It was perfect.

The AP then came out with a little 5 dollar compass on a stick and had me
run the engine up to full power on six headings while he sighted down the
fuselage. The compass was shaking about 5 degrees each way from the
vibration of full power with locked brakes. I took his results and did a
polar graph as you are supposed to do with the data and said, "Look there is
no way you can have a 10 degree bulge on one heading sticking out like this.
You've got to recheck or do some smoothing of the data. Compass's don't act
like this. "

The shop said, "That's the FAA way. Here's your compass card." I had to
tell the club members to just ignore the compass care which would have lead
them 10 off in some directions but I could only do it verbally because, if
someone got lost and hit a mountain of course, it would have been my fault.

That's the way so much of the world works.


I worked for a company named Southern Airways at the time and we had a
"compass Rose". Put one wheel on a mark in the middle of the circle
and drag the airplane around until the tail wheel sat on another mark
and read the compass. At the time I was a lowly AP (actually A & E)
working on the flight line so just filled out a form with the compass
readings and turned it into the office. In due time an official
Compass Deviation Card was issued that we stuck in a little clip on
the instrument panel.

I used to listen to the instructors briefing the cadets for their
cross countries and I honestly think they could have flown them
without a magnetic compass as I used to hear the Instructors talk a
lot about the various railroad tracks in the area.


Bruce-in-Bangkok
(correct email address for reply)