There are 3 values for ground check: 20 ft , 25ft and 75 ft
The 75 ft is an old rule of RVSM regulation, that any altimeter should agree within 75 ft of airport elevation.
Is airbus more stringent and restrict it to 25 ft tolerance between any PFD and airport elevation ? In which case FCOM Special operation RVSM is wrong... Just reproduced an old generic rule not applicable anymore Stating " Check that the PFD altitude indication (QNH reference) of ADR1, and also of ADR2, does not differ from the airport elevation by more than 75 ft"
I am sure that the guy who wrote "FCOm special operation RVSM " is not the same author of "Supplementary NAV , altimeters tolerance"
I understand it like this : PFD 1 or 2 with airport elevation 25 ft
PFD 1 and 2 , should agree within 20 ft
And the 75 ft does not apply anymore
Or PFD 1 with airport elevation 75 ft
PFD 1 and 2 --> 25 ft
And I missed something about the 20 ft tolerance