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Date: Mon, 18 Dec 2006 15:56:59 -0500
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From: Douglas Brunner <douglasbrunner@earthlink.net>
Reply-To: Douglas Brunner <douglasbrunner@earthlink.net>
Subject: Re: [LML] Re: electric attitude indicators
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I have had a failure of a vacuum driven attitude indicator during flight - fortunately in VFR conditions.  This was a 
failure of the instrument, not the vacuum pump.  

I fail to understand how an experimental plane that has a dual Cheltons, a backup TruTrak ADI and a TruTrak
autopilot with its own independent GPS and altimeter can be less safe (from an instrument failure perspective)
than a certified plane with a single vacuum A/I and a single vacuum pump - a very common setup.

The probability of two things failing together is the product of the probability of each failing separately.  If
we assume that the Chelton has a failure rate of much less than 1% and the TruTrak ADI has a failure rate
of much less than 1%, then the probability that they will both fail is much, much, much less than 1%.  And even if
they both fail the autopilot is still capable of keeping the plane straight and level.  This is a level of redundancy
much greater than many certified planes.

I plan to use a TruTrak ADI as a backup to my Cheltons and I feel confident that this is not likely to be the
"Achilles heel" of my plane.

D. Brunner
Legacy N241DB

"50% done and 80% to go"