Appendix 2.8

LATCH-UP CONDITION

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.

Last modification on 1999-06-15 by Michael Blume