Computers have played second fiddle to analogue techniques in safety-critical systems and though airlines are switching to fly-by-wire technology, an unknown safety record is their major worry
THE use of computer technology has
grown immensely and software is being
used to develop varied products ranging
from trains to toasters. But as the power
of digital hardware has grown, the size
and complexity of the software needed
to control it has als6 increased.
Traditional software development is
labour-intensive and error-prone, and
the software industry urgently needs
cheaper and more reliable ways to
develop programmes. If a Pc crashes,
only data is lost, but if a computer controlling an aircraft or a chemical reactor
misbehaves, the consequences can be
catastrophic.
Computers have traditionally played
a subservient role to more conventional
analogue technologies in safety-critical
systems. Analogue technologies, leave
the pilot or operator in fall control and
are designed to well-known fail-safe
principles. No such principles exist
for softwares, so designers of digital
systems have to test their programmes
extensively.
Extensive use of computers allow
the Boeing-777, which went into service
in June 1995,-,to be flown in by two
crews instead of the traditional three -
a flight engineer is not required. The 777
contains more than two metre lines of
software code, four times that of its predecessors, and is Boieng's first commercial
aircraft to use digital fly-by-wire technology for flight control system. Built by
GEC Marconi, it took five years and 200
people to develop the system.
Digital technology was used because
manufacturing costs are lower than
those for the analogue system. Boeing's
main competitor, the European consortium, Airbus Industry, was the first
to use digital fly-by-wire technology to
reduce costs.
Airlines are wary of new technologies because of expensive service an dfear of unreliability. For digital fly-by-
wire flight control systems, there is
the additional worry of IA unknown
safety record.
To calm such fears, Boeing exhaustively tested the software for one year inthe laboratory and another one in the
air, twice the test period for earlier techniques. Cost is not the only factor in
building safety-critical software. Testing
complex software requires thousands -of
test cases and combinations of operating
parameters to be generated and fed into
the software to see how it responds.
Traditionally, this is done manually,
which is both costly and tedious for the
people involved and prone to error.
Rational software, a us-based firm, is
working with Boeing on a set of tools to
automate test-case generation, thereby
cutting testing costs. Boeing engineers
have developed an algorithm to reduce
the time needed to generate test cases
from several hours to a few minutes.
Software testing detects mistakes
made by the programmers who
wrote the programme code. But fundamental design flaws will not be revealed and even extensive testing uncovers
only about 70 per cent of the errors in
the programme. The rest, serious or
otherwise, are passed
Y-wire system on to the customer.
Phantom withdrawals
from bank teller
machines, telephone
exchanges that do
not communicate, and
space shuttles that
refuse to take off are all
manifestations of software bugs overlooked
during testing.
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