From: "Marvin Kaye" <marv@lancaironline.net>
Subject: Re: [LML] Re: electric attitude indicators
To: lml
Date: Tue, 19 Dec 2006 17:53:14 -0500
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In-Reply-To: <45881F2B.1080401@starband.net>
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 <45881F2B.1080401@starband.net>
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Posted for "Hamid A. Wasti" <hwasti@starband.net>:

"Douglas Brunner" <douglasbrunner@earthlink.net> wrote:
>   In the first place the TruTrak ADI does have an option for battery 
> backup.
> (from the website:
>  "Backup battery option - $100" )  I intend to get the backup battery 
> option.
  The internal battery option in any instrument is a great backup to many
failures of the electrical system.  However, it is unlikely to help you in all
failure modes.  A severe over-voltage on the power bus can take out the power
supply of a device and depending upon the failure mode, the dying power supply
may send a surge of energy into the rest of the device that would kill some or
all electronics.  A battery will not help you run charred electronics.
  
>
>  Second, I plan to have a dual alternator, dual battery setup with the
> avionics bus feeding
>  from either.  And even if I lose both alternators, the batteries 
> should give
> me a good hour of electricity.
  If you started the flight with two working alternators and something 
happened
that caused you to loose both of them simultaneously, that "something" must
have been big.  What else did it take out?
  
  In your dual bus system, how are you doing the crossfeed?  How reliable is
the device that accomplishes the crossfeed?  How vulnerable is that device to
the event that took out the alternator?  Can you test that device to make sure
that it is working?
  
  Even if the batteries and electronics survived the event, did you ever
actually test the system under full load to see how long they would really
last?  One little detail that most people are not aware of is that a 35Ah
battery is rated to produce that much energy if it is drained over 20 hours.
  If you drain it at 35A, it will not last one full hour, likely somewhere
around 45 minutes or less at room temperature, much less if cold.  If you
drain it at 70A, you will end up well short of 30 minutes, likely under 15
minutes, possibly under 10 minutes if cold.  If you decide to do the test on
the ground (I would not recommend doing it in the air) cut the time in half.
  This is because the apparent battery capacity at high currents decreases
dramatically as the temperature decreases.  To be fair, a battery that starts
out at -15C in flight is not going to stay  at -15C for too long when
subjected to a heavy discharge.
  
  All that is assuming a new and fully charged battery of "typical" capacity.
  When was the last time you actually tested the batteries to see what their
real health is?  Maybe you should cut the number in half once more to account
for battery health and add a safety factor.  At that point, a dual alternator
failure in IMC may start looking more like a time to declare an emergency and
get on the ground ASAP.
  
  Regards,
  
  Hamid