Altimeters tolerance

[Citation2]

FCOM Supplementary techniques:
ALTITUDE TOLERANCES PFD 1 or 2 at ground check : plus or minus 25 ft (8 m)

MAXIMUM DIFFERENCES BETWEEN ALTITUDE INDICATIONS ALTITUDE (ft) COMPARISON BETWEEN FL/SPEED ADR 1 and ADR 2 (on PFD) GND CHECK 20 ft(6 m)

So how do you interpret this ?What is the altimeter tolerance on A320 at ground check? 20 ft or 25 ft?

The wording is different , with OR & AND. initially they say PFD 1 OR PFD 2 -->25 ft
Then ADR 1 AND ADR 2 on PFD ---> 20ft

Can anyone shed light on this?

[ricky-godf]

Hello..

my understanding is that the two altimeters (ADR 1 & ADR 2) should not indicate more than 20ft difference in reading between them, at ground level. (this tolerance increases with your flight level).

Further, any of the two altimeters should not indicate more than 25ft discrepancy between the indicated altitude and your aerodrome altitude (ground check).

Ricky

[Citation2]

The tolerance with airport elevation is 75 ft:

FCOM special operations : RVSM

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.

So I dont think that it refers to airport elevation

[Citation2]

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

[Microburst2002]

There is, apparently, a discrepancy, but I think it is not exactly a discrepancy:

if for instance PFD1 altimeter on the runway threshold differs 40 ft with respect to the elevation in the plate, you would be OK for RVSM, but anyway you would not be OK for flight, RVSM or not RVSM...

[Citation2]

In that case should we assume that the 25 ft rule overrides the 75 ft rule?
And keep the 20 ft rule between pfd 1 and 2?

[Microburst2002]

Yep, that's what I do every day.

But now thinking, the usual question: and what will we do if one day I see PFDs with a more than 20 ft discrepancy (with same QNH of course). And What if altimeters read more than 25 ft different than elevation (if accurately known and QNH is accurate, too, which is a lot of assuming).

Enter the MEL?