Re: FMS capability (was: Re: AA965 proves Airbus bashers deadly wrong)

From: (FMCDave)
Organization: America Online, Inc. (1-800-827-6364)
Date:         27 Feb 96 23:00:30 
References:   1
Followups:    1
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  or MIME structure (Ed Hahn) wrote:

>Yes, assuming the computer had the information about terrain
>available.  However, because of the (correctly) stringent nature of
>certification rules, the FMS computers on board most equipped aircraft
>have an upper limit of 1Mword of memory or less (i.e. 2MB), due to the
>need to show that the electronics industry has experience with the
>components to have REAL, demonstrated MTBF figures.
The constraint on the Navigation Data Base memory size is not due to
certification.  It is due to the technology of the times.  The new 757/767
FMCS upgrade will have 16 MBytes for NDBs...simply because now it is
affordable to do that.  The 777 has a greater than 1MByte limit, but the
number escapes me now.

Certification is the issue though it is not due to size.  It is due to the
criticality of the function.  A complete safety analysis would need to be
done on this function, but if a terrain display based on an NDB update was
to be used as the primary means for avoidance of Controlled Flight Into
Terrain, then the criticiality of the function may be higher than most FMC
computers are certified to.  In this configuration, I would presume to
leave the GPWS in as the "belt and  suspenders" function due to the
consequences of the failure.  The criticality of the function would affect
the hardware (reliability), software (integrity via DO178B), and the DATA.
 The data would be the tough part.  Currently, there is a 28 day cycle for
update of navigation data.  This might not be considered adequate.  So,
one might say, data link the stuff up.  Well, current data link systems
are also not certified for the reliability or integrity of a function this

So, after saying all of this negative things; let me close by saying that
CFIT is a serious problem.  Terrain display coupled with GPWS will help
flight crews avoid these accidents by providing them information.
However, there are no easy solutions, implementation of this function will
be a significant systems engineering task.  If any company tells you
otherwise, hold on to your wallet.
David Allen
Project Manager, CNS/ATM
Opinions are mine and not my employer's