Re: Boeing 777 - Totally Irresponsible?

Date:         04 Jan 97 03:55:52 
From:         jfmezei <>
Organization: SPC
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A few more comments on the "unreliability" of a "million lines of code"

A single program with a million lines of code will be more prone to
errors than 100 programs with a thousand lines of codes each.

If your software runs on a proven operating system, or on different
computers, modules are not likely to pollute other modules with memory
leaks etc. So you are looking at certifying separate smaller modules
instead of one huge module. The ods of errors in smaller modules is much
smaller than in a larger module.

Furthermore, in a "client-server" environment, you can test how the
server will respond to requests (whether valid or invalid) and how the
client will respond to responses (valid or invalid). This is not as
easy to do whith one large program with multiple subroutines calling
each other.

Whether the software is bug free or not is not really the issue. What is
the issue is how the software reacts to unanticipated inputs (eg: plane
in the air but landing gear system tells the computer it is on the
ground or vice-versa).

If such conditions are known, they would be documented in technical
manuals for the plane so pilots know to to handle such situations, so
they can also be included in the programming so that the computers know
how to handle this situation.

If such conditions are unknown, then the pilots will have to respond to
erroneous warning lights on a conventional planes, or erroneous
behaviour or warnings on a FBW plane.