Re: B757/B767/B777 Stearing system?

Date:         11 Feb 98 04:24:04 
From:         Pete Mellor <>
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jf mezei <"[non-spam]jfmezei"> asked:

>Karl Swartz wrote:
>> The first three A340s were all retained by Airbus, though it appears
>> that only the first was a prototype.  I have no idea why they wouldn't
>> have sold the other two.
> Could this have to do with a need to test multiple versions of software
> and how they react to various situations ?

I doubt it. The FCS on the A340 is designed on very similar
lines to that of the A320: 5 interconnected boxes, each of which
is dual channel. The outputs of the monitoring channel and command
channel are compared on every cycle, and if a mismatch is detected,
the box shuts itself down and the others take over as much of its
function as they can. The software in the two channels is diverse
(different languages, means of compilation, etc.).

Having two working versions of the software in each box is therefore
essential before an A3XX can even get off the ground (not to mention
the problem of getting an airworthiness certificate).

This is something with which Airbus have a lot of experience. (The
A320 went into service in 1987 (88?). The software is thrashed on
the bench and then on the "iron bird" simulator before being allowed
anywhere near a moving aircraft.

This does not mean that they always get it totally right first time,
and the software does get updated during the working life of the
model, but I don't think that the reason for retaining two production
aircraft had anything to do with the need for a flying testbed.

Peter Mellor, Centre for Software Reliability, City University, Northampton
Square, London EC1V 0HB, UK. Tel: +44 (171) 477-8422, Fax: +44 (171) 477-8585
E-mail:, Web: