Originally Posted by Marty-Party
Can't speak for the Airbus, but Boeing VNAV usually gets its QNH from the Captains altimeter sub-scale setting. A cross-check is made to compare the sub-scale setting with that of the F/Os altimeter. Also the VNAV database knows the approximate transition level and flags a caution is still on STD.
Just for interest, the kit in the B717 (which is the same kit found in MD-11 and B777 I believe) has two FMS Nav systems, (Versatile Integrated Avionics systems, VIAs) left and right, that work in a dual mode where both work out their own nav solutions and monitor the other. Each generates their own Flight Director displays, and one operates in slave and one as master, but that has nothing to do with PF/PNF or CAPT/FO, only to do with which unit powered up first at the beginning of the day's flying. Each VIA takes the baro subscale setting from its respective side. The QNH cross check is automatic -- an amber "ALT" warning is displayed on both PFD's if the CAPT and FO QNH are not identical. The autoflight system is normally engaged as AP1 taking flight management information from the Captains VIA and AP2 fed by FO FMS. Should the QNH mismatch be allowed to persist the AP will fly to and capture altitudes set on the 'side' that is controlling the flight path, or the altitude selected in the ALT SEL window at the baro setting on the side that is engaged.
I enjoy running the 717 FMS trainer for my 737-800, 747-400 and 767 friends -- it makes them jealous!
