AOA / Speed lwr limits.
In the digital world they need to set a upper and lower limit.
You have only limited bits in a binary word and have to deal with accuracy, resolution, standarisation and......odds.
E.g. the odds to get into ALTERNATE LAW = 10^-5 per Flighthour (10^-3 under MMEL) as advertised.
The Speed indicating limits of ISIS (directly connected to Pt/Ps) are 5 - 600Kts, so backup is available.
AIB did develop the Back-Up Speed Scale. (Article dated Dec. 2007 - sorry can't
get direct link working.)
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 table. 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 atack (AOA) sensor information, and is therefore not affected by erroneous pressure measumements.
The BUSS comes with a new ADIUR standard (among other new system standards), where the AOA information is provided through the IRs and nor through the ADRs. This enables selecting all ADRs off without loosing the STALL WARNING PROTECTION.
The AOA information provides a guidance area in place of the speed scale. When the crew selects all 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 at a safe speed, i. e. above stall 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 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 before troubleshooting.
As the BUSS is associated to the ADR monitoring funcitions, 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.
****************************************************
But all of that wouldn't make any difference to this SWISS CHEESE case.
BTW ADIRS are also common in modern B brand airliners.