BUSS is only available when all ADR's are off and it is irreversible.
I found it in the FCTM, the FL250 has to do only with the inflight experience that icing usually disappears during the descent to that level.
But although not documented, BUSS should be useable at all flight levels. (ADR 1+2+3 Fault ECAM)
Last edited by Lantirn; 28th Oct 2016 at 09:52.
Reason: Correct ecam