wool@oracle.UUCP (Chris Wooldridge) (10/29/85)
In reponse to my ad offering a Hamilton compass for sale I received
several messages - one of which, from Dave Fielder @ Infopro, inquired
how I liked it, was it true it replaced a DG, etc. I thought I'd
publish this response for those of you on the net who are thinking about
getting one.
The Vertical Card compass is made by Hamilton Industries in Dallas. It's
a dry compass (as opposed to the "wet" compass usually found in aircraft)
and utilizes something called "eddy damping" to minimize the usual north/south
turning error. I don't know what effect the damping has on acceleration
error. Anyway, the claim is that this compass can be panel mounted and
flown like a DG. It looks and behaves like a DG, too; i.e., you turn the
airplane TOWARDS the heading you want. It costs about $250 (from Sporty's,
etc.).
Mine was installed in the aircraft when I bought it in Houston. It never
occurred to me to worry about compass calibration until one dark stormy
IFR night when ATC noticed my headings weren't good (they gave me a no-gyro
approach at my destination ... whew!). I had the thing investigated by
my radio shop. The compass was about five years old, mounted on the
windscreen without shock mounting, and its gimbles were shot. I had it sent
to Hamilton where they rebuilt it ($100) and returned it with several shock
mounts and the advice: ALWAYS shock mount it. Advertising aside, do not
mount it in the panel. Not only for vibration reasons, but because there
are such magnetic fields in there that the compass may not be compensatable.
My real problem though, on getting the compass back it proved unusable in
my airplane anyway. I own a Mooney, one of the rare airplanes with a
tubular steel frame. A dry compass like the Hamilton has an extra-large
magnet which is drasticly affected by outside magnetic fields. The FAA
says any aircraft compass has to be correct within 10 degrees on any one
heading ... I had errors of 20-30, after swinging it. In addition the
switching on of any high amperage equipment (pitot heat, landing light, etc.)
was good for another 10-15 degrees. The wet compass which came as original
equipment on the Mooney has some errors after swinging but is still within
FAA specs.
So, in summary, I can recommend the Hamilton with some qualifications,
to wit:
a) don't buy it if you have a Mooney, or are going to mount it in
an area with steel near it.
b) don't mount it in the instrument panel (even though it can be).
c) do mount with the recommended shock mounting. The Hamilton
people will even custom build, free, a mount for your airplane.
The gimbals won't last long without this (says the factory guys).
d) do buy their optional "balancing balls" kit; chances are good
you'll need it.
e) before you mount it, check the location for the normal deviation
tolerances - then switch on accessaries, one at a time, and watch
for excessive effect. Sometimes a move of only an inch or two
will eliminate the problem.
Happy aviating ...
(overheard one day over Dulles International ...)
Controller: "TWA 517, fly heading zero-niner-zero, vectors runway 1L
visual approach, say altitude passing."
TWA 517: "Altitude passing."
Controller: "TWA 517, please say altitude."
TWA 517 (with parrot screech): "Altitude! Altitude!"
Controller: "Very funny. Now, TWA 517, please say, cancel IFR".
TWA 517: "TWA 517 is passing through eight for three thousand".
Controller: "Thank you."
--
Chris Wooldridge
Oracle Corporation
1100 206th Avenue, N.E.
Redmond, Wa. 98053
(206) 868-1985 {ihnp4!muuxl,hplabs}!oracle!wool