From: (B. Douglas Reeves)
Organization: Tanasborne Graduate Institute
Date:         15 Jun 95 14:25:27 
References:   1 2 3
Followups:    1
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In article <airliners.1995.755@ohare.Chicago.COM>, (Jean-Francois Bosc) wrote:

 |I think the main point is that systems are careful 60 seconds
 |per minute and 24 hours a day. And software reliability is
 |increasing, and will keep increasing.

This is a common misconception.  Software, as it becomes more complex,
spawns a huge number of "bugs", most of which are "1-in-a-million"
errors which are likely not to show up in labratory software testing or
flight testing.  For an excellent discussion of these problems, see
"Software's Chronic Crisis", Scientific American, Sept. 1994 and "The
Risks of Software", Scientific American, Nov. 1992.

While both Boeing and Airbus are producing fly-by-wire aircraft, the
fact that the computer has ultimate control in the Airbus systems
versus the Boeing system where the software plays more of an advisory
role makes the Airbus system more vulnerable to software errors.

An excellent example of this is the Airbus A330 crash.  My understanding
of the reports is that the pilots were making an engine out takeoff and
that the combination of (1) the use of TOGA vs Flex49 power setting and
(2) the nose-down input from the co-pilot made the autopilot engage
(1) late and (2) in altitude acquisition mode.  The autopilot then
kept increasing the pitch until the pilot realized what was going on
and three seconds later took control and pitched the aircraft down.
By then the speed had decayed to ~100kt.  The aircraft was unrecoverable.
Apparently the software killed them.  Good thing it wasn't a revenue
flight.  I guess Airbus will be fixing that piece of the software, but
how many other "bugs" like this are there?

I must commend Airbus for (1) making Boeing rethink its practices and
(2) advancing aircraft technology, but their problems lie in human
interface design (side stick w/o feedback) and reliance on software to
protect the aircraft.


D. Reeves

|B. Douglas Reeves     Dept. Env. Sci.& Eng.    Oregon Graduate Institute|
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