B777 ADIRU SAARU & Stby instruments
Thread Starter
Join Date: Mar 2007
Location: Up North….
Posts: 502
Likes: 0
Received 0 Likes
on
0 Posts
B777 ADIRU SAARU & Stby instruments
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?
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?
Last edited by felixthecat; 20th Jul 2011 at 09:00.
Felix,
You should not assume that “invalid” and “failed” are the same. To me, “invalid” in this context is when a computer cannot determine with enough certainty whether a value is correct or not.
You get NAV AIR DATA SYS when both ADIRU and SAARU have no validated output. They are not failed however. The output is no longer the result of a blend of sensors. The ADIRU shows air data from the left pitot/static on the captain’s side and the SAARU shows air data from the right pitot/static on the FO’s side. The task of choosing which side, if any, is correct and taking appropriate corrective action is left to the crew.
The STBY instruments always receive data from the center pitot/static system via independent ADMs. The ADIRU and SAARU also use data from the center pitot/static sensors via their own ADMs so there is no complete independence here. The AIR DATA/ATT switch only serves to force the captain or FO PFD and ND to display SAARU or ADIRU single channel air data respectively and SAARU attitude instead of normal ADIRU data.
Hope this helps.
DJ.
You should not assume that “invalid” and “failed” are the same. To me, “invalid” in this context is when a computer cannot determine with enough certainty whether a value is correct or not.
You get NAV AIR DATA SYS when both ADIRU and SAARU have no validated output. They are not failed however. The output is no longer the result of a blend of sensors. The ADIRU shows air data from the left pitot/static on the captain’s side and the SAARU shows air data from the right pitot/static on the FO’s side. The task of choosing which side, if any, is correct and taking appropriate corrective action is left to the crew.
The STBY instruments always receive data from the center pitot/static system via independent ADMs. The ADIRU and SAARU also use data from the center pitot/static sensors via their own ADMs so there is no complete independence here. The AIR DATA/ATT switch only serves to force the captain or FO PFD and ND to display SAARU or ADIRU single channel air data respectively and SAARU attitude instead of normal ADIRU data.
Hope this helps.
DJ.
Join Date: Jul 2011
Location: Here
Posts: 194
Likes: 0
Received 0 Likes
on
0 Posts
Absolutely correct ... one of the big mistake a pilot can do reading the Boeing manual is to interpret what is written ... if you can try the NAV AIR DATA SYS on the sim and you'll see different instrument readings.
