Zaphod are you sure your QRH does not tell you what to do below FL250 on BUSS a/c?
Extract from current A321 QRH Unreliable Airspeed:
AFFECTED ADR IDENTIFICATION
PROBE/WINDOW HEAT........................................................ ..............................ON
ALL SPEED INDICATIONS................................................. ......................X CHECK
ADR3 and STBY speeds use the data of the same probe.
If at least one ADR is confirmed reliable:
RELIABLE AIR DATA........................................................ ........................... USE
UNRELIABLE ADR P/B(s)........................................................ .................... OFF
If affected ADR(s) cannot be identified or all ADRs are affected:
When above FL 250:
ONE ADR......................................................... ...............................KEEP ON
TWO ADR P/Bs.......................................................... ............................. OFF
This prevents the flight control laws from using two coherent but unreliable ADR data.
For flight continuation, Refer to Pitch/Thrust Tables.
When below FL 250, if speed still unreliable:
ALL ADR P/Bs.......................................................... ............................... OFF
SPD......................................................... ........................... FLY THE GREEN
NAV ADR 1+2+3 FAULT Procedure................................................... APPLY
Extract from current A320/321 FCTM:
IN-SERVICE EXPERIENCE OF HIGH ALTITUDE PITOT OBSTRUCTIONS
Analysis of the in-service events shows that:
‐ The majority of unreliable speed events at low altitude are permanent situations, due to the
obstruction of pitot probes by rain, severe icing, or foreign objects (refer to the table above).
‐ 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 specific meteorological conditions. In-service experience shows that such a contamination
typically disappears after a few minutes, allowing to recover normal speed indications.