Recommendation Date
Recipient Name
Boeing
Text
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.
Reply Text
In regards to the Quick Reference Handbook (QRH) , we have concluded that a revision to the QRH as requested in the subject report does not conform to the 747-400 flight deck procedures philosophy.
The basic philosophy is that EICAS messages are the primary cue for accomplishing a non-normal procedure. If a message is displayed the flight crew refers to the associated non-normal procedure. Non-normal procedures generally address single failures. It is not possible to develop procedures for all situations particularly those involving multiple failures. Therefore, the IRS procedure has been written assuming a single failure. If a second failure occurs when selecting an operable IRU, the first crew must use appropriate judgement to determine the best course of action. It is not possible to write the non-normal procedure which correctly addresses all possible multiple failure scenarios.
The basic philosophy is that EICAS messages are the primary cue for accomplishing a non-normal procedure. If a message is displayed the flight crew refers to the associated non-normal procedure. Non-normal procedures generally address single failures. It is not possible to develop procedures for all situations particularly those involving multiple failures. Therefore, the IRS procedure has been written assuming a single failure. If a second failure occurs when selecting an operable IRU, the first crew must use appropriate judgement to determine the best course of action. It is not possible to write the non-normal procedure which correctly addresses all possible multiple failure scenarios.
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