However the irreversibility part was quoted by some one...
Hi vilas,
You are correct.
According to FCOM DSC-31-40: Backup Speed/Altitude Scale:
"The BackUp Speed Scale (BUSS) enables to fly the aircraft when airspeed indications are unreliable.
When the BUSS is activated:
The BUSS replaces the normal speed,
The GPS altitude replaces the barometric altitude scales.
The BUSS is displayed on both PFDs when the flight crew turn off all ADRs.
The activation of this BUSS is not reversible.
The BUSS information is based on the angle of attack (AOA), and depends on the slat/flap configuration."
Thanks for the info as this behaviour is different to our sims.
there is no way of knowing that UAS has disappeared.
FCTM:
"At high altitude, typically above FL 250, the cases of unreliable speed situation are mostly a temporary phenomenon: They are usually due to contamination of the pitots, by water or ice, in particular meteorological conditions.
In-service experience shows that such a contamination typically disappears after few minutes, allowing to recover normal speed indications."