Back Up Speed Scale (BUSS)
The following is an extract from Airbus safety First Magazine Issue No 5.
"Back Up Speed Scale (BUSS). In order to decrease the crew workload in case of unreliable speed, Airbus has developed the Back-Up Speed Scale (BUSS) that replaces the pitch and thrust tables. The BUSS is optional on A320/A330/A340. It is basic on A380, being part of the ADR Monitoring functions.This indication is based on angle of attack (AOA)sensor information, and is therefore not affectedby erroneous pressure measurements.The BUSS comes with a new ADIRU standard (among other new system standards), where the AOA information is provided through the IRs and not through the ADRs. This enables selecting all ADRs off without loosing the Stall Warning Protection. The AOA information provides a guidance area inplace of the speed scale. When the crew selectsall ADRs OFF, then:
•The Back-Up Speed Scale replaces the PFD speed scale on both PFDs,
•GPS Altitude replaces the Altitude Scale on both PFDs.
The Back-Up Speed Scale then enables to fly ata safe speed, i.e. above stall speed and below maximum structural speeds, by adjusting thrust and pitch.
The BUSS will be displayed once all ADRs are switched OFF. Therefore, on aircraft that have the BUSS, when the flight crew cannot identify the faulty ADR(s) when performing the troubleshooting, or when all ADRs are affected, the flight crew will switch OFF all ADRs, and will fly the green area of the BUSS. However, if the safe conduct of the flight is affected,the memory items must still be applied beforetroubleshooting.As the BUSS is associated to the ADR monitoring functions, some unreliable speed situations can be automatically detected (e.g. new ECAM warning “NAV ADR 1+2+3 FAULT”), and some ECAM procedures will lead to the BUSS activation by requesting to switch OFF all ADRs."
(source: PPruNe)
Jeff