|University of Bielefeld - Faculty of technology|
Networks and distributed Systems
Research group of Prof. Peter B. Ladkin, Ph.D.
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|This page was copied from: http://www.open.gov.uk/aaib/gobme/a28.htm|
At about the time of the accident to G-OBME,
reports were starting to be received by the manufacturer that
certain control modules were exhibiting unusual behaviour when
subjected to interruptions in the 28V DC power supply. It appeared
that the module became 'dormant' for various lengths of time following
the interruption but with no fault indications apparent to the
crew unless they performed the cockpit self-test, in which case
the affected audio and visual warnings of overheat and fire failed
to illuminate. Detailed investigation showed that the problem
lay in a microcircuit from a particular vendor and units liable
to latch-up could be thus identified. While the module manufacturer
devised a modification to their equipment, Boeing issued an Operations
Manual Bulletin No 89-2 dated 6 March 1989 to all operators.
This essentially called for flight crews to perform a test of
the Fire/Overheat detection system after initial power-up or after
a power loss or transfer to No 2 generator bus. The time interval
between power loss, or transfer, and the test was later revised
to one minute in recognition of the fact that some units might
not latch-up immediately. Should the system fail the test in
flight, crews were advised to land at the nearest suitable airport.
It has been noted that the behaviour of individual modules exhibiting
latch-up tended to vary both with respect to the time taken for
the condition to occur and its duration.
|This page was copied from:||http://www.open.gov.uk/aaib/gobme/a28.htm|
by Michael Blume