Sender: <marv@lancair.net> (Marvin Kaye)
To: lml
Date: Tue, 23 Apr 2002 16:17:37 -0400
Message-ID: <redirect-1180373@logan.com>
From: "Charles R. Patton" <charles.r.patton@ieee.org>
Reply-To: charles.r.patton@ieee.org
MIME-Version: 1.0
Subject: Re: [LML] 40 degree compass error
References: <list-1180293@logan.com>
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Carl Cadwell wrote:
> The solution to the compass error is to start the engine.  It goes away
> when the standby alternator is turning.  

Carl,
I would have bad feelings about this solution if, as likely, it is due
to field excitation current effecting the compass.  What would be
happening then is that as the engine is turning, and the alternator
output comes up, the regulator kicks in and reduces the field excitation
current  -- but the key word here is "reduces."  It does not eliminate
it.  The current will go up with low engine speed or higher system
demand current, such as night flight with all lights on.  What this
comes down to is that the error now is a variable, and to me, the worst
of all scenarios.  I can compensate for a known, fixed offset, which is
the point of a compass error card.  But that is impossible when the
error amount is not guaranteed.

Ed and I have continued to have direct email conversations about his
problem, and I currently believe it is some error induced by the field
current in his case, but we haven't resolved it yet.  It may be that
Carl's problem has a different source, but I'd want to pin it down
before ignoring it.  I used to have a partner who said, "Ignore it, and
it will come back and bite you in the ass; take the time to figure it
out, and it will turn out not to have mattered."  Kind of a variant on
Murphy's law.  But when it comes to flying, I tend to be a bit more
cautious, and really want to know root cause, before I would dismiss
it.  My two cents worth.

Regards,
Charles R. Patton
N360JM