Re: RAT for CVR

Date:         05 Oct 98 00:26:13 
From:         <ranck@joesbar.cc.vt.edu>
Organization: Virginia Tech, Blacksburg, Virginia, USA
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S.L. <look@the.sig> wrote:
> David Lesher wrote:
>> There are very valid reason for not putting the CVR onto the
>> last-chance emergency battery bus, but how about on its own
>> deployable ram air turbine?

> No, it shouldn't be on a bus at all, nor on a RAT. It *should* have an
> internal lithium battery backup so that it will continue operating even
> if you rip it out of the aircraft, IMHO :-) So should the DFDR- even if
> its inputs lose power and fall off-line, the recorder itself shouldn't
> be the weak link.

I seem to recall that FDR's and CVR's record over old data after 1/2 hour
or some other amount of time.  How do you propose to have them stop
recording after the crash?  If they are internally powered they would
keep on recording over the information with null inputs for a few days
till the recovery crew found them?  What good would that be?

Getting them to keep running through a total power bus failure is
certainly possible, but how long should they continue after that?
If you choose too large a time frame you might over-write vital
information from the beginning of the incident, too short a time
and you still miss the last 'n' minutes of flight.

--
Bill Ranck                +1-540-231-3951                    ranck@vt.edu
   Virginia Polytechnic Institute & State University, Computing Center