Executive summary
On Wednesday 12 March 1997, at 0032 hours, Boeing 747-419 ZK-NBU was enroute from Auckland to Los Angeles as Flight NZ18. The Captain's electronic primary flight display no longer displayed flight reference information and appeared blank. An internal failure within the brake system control unit, due to water ingress, in turn caused a fault with the inertial reference unit data bus supplying information to the Captain's display. An alternate inertial reference unit was selected and it failed to supply the required information. The selection of a third inertial reference unit resolved the problem and the Captain regained his display. The two affected systems were turned off and the aircraft returned to Auckland without further incident. Safety issues discussed include the adequacy of the Quick Reference Handbook and the vulnerability of an inertial reference unit to an external fault. Safety recommendations were made on both these safety issues.
Related Recommendations
Carry out an investigation, and take any necessary action, to ensure the design requirement for an IRU not to be faulted by an external failure, such as a shorted or open circuited data bus, is met.
Review the Quick Reference Handbook for the B747-400 series regarding the procedure for Inertial Reference Unit (IRU) selection by the crew, after an EICAS IRS fault message is displayed, to determine if any change is necessary to ensure all three IRUs are not switched off unnecessarily.