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Date: Sat, 19 Jun 2010 15:01:32 -0400
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From: Hamid Wasti <hwasti@lm50.com>
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Subject: Re: [LML] Re: Legacy Glass Panel Suggestions?
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Stan Fields wrote:
> I like the idea of the Garmin 396 for backup.  Have you ever had to use it?
>   
Whether you are using a Garmin 396 or a classic 6-pack, you need to 
practice with it under a hood in VMC with a safety pilot to make sure 
that you can control the airplane with it. Being in IMC with a 
malfunctioning PFD and on the verge of going into an unusual attitude is 
no time to test this.

A backup system, whether it is vacuum gyros or a 396 is useless unless 
you practice using it on a regular basis. Also, if a Garmin 396 is you 
backup, it needs to be powered up and preferably on the appropriate page 
before you get IMC. Being in IMC with a dead PFD and on the verge of 
going into an unusual attitude is no time to be fiddling with the Garmin 
396 or waiting for it to power up and acquire satellites.

> Are your two GRT EFIS on different busses? It seems like that would make it
> tough to loose them both.
Not really. Two busses, if designed and wired properly can protect 
against some adverse events. But even the best designed system would not 
quality as "tough to loose them both." If you are thinking 
battery/alternator failure or a short on the bus, a buss can die leaving 
the other buss unharmed.  However, depending upon how the system is 
designed and wired, there are failure modes where a dying buss can take 
the other one with it.

As Jeff already pointed out, you need to worry about other things 
besides a dead battery & alternator. You can not address the issue of 
proximity on an airplane the size of a Lancair. Even completely 
independent busses will feel the effects from a lightning event or HIRF 
almost equally and if one fails, the other is likely to fail as well. If 
you are using identical systems, they likely have the same code and 
hence the same bugs.

Regards,

Hamid