Just a quick question to all you techheads in the know
My understanding of the B777 system is that the 3 inputs to the ADIRU and the same 3 inputs to the SAARU are independently compared internally and provided 2 or more of them are the same the data is valid and the Capt and FOs PFD/ND receive the ADIRU data.
The FCOM states that "single channel operation occurs when the ADIRU
AND SAARU air data are invalid" (my emphasis on the AND)
Two questions
1) Do you only get the NAV AIR DATA SYS when both ADIRU and SAARU fail or just when the ADIRU fails, and if so how would you know if the SAARU had failed?
The standby instrument receives independent data from the centre Pitot and Static ports via independent ADM. The altitude and airspeed are independent of the ADIRU and SAARU and in most types the attitude is also independent (in some variations SAARU provides attitude)
2) Is the stby instrument receiving independent data at all times or only when the AIR DATA/ATT switch is in ALTN?