Thanks for your PM Guiones. Much appreciated.
Appreciate the feedback on the FAR's too. As an EU-OPS operator, I would guess FAR 25 still applies. i.e. Transport Category Aircraft
FAR Part 25 Sec. 25.841 effective as of 07/05/1996
(1) If certification for operation above 25,000 feet is requested, the airplane must be designed so that occupants will not be exposed to cabin pressure altitudes in excess of 15,000 feet after any probable failure condition in the pressurization system.
I would suggest the MEL 'ethos' that Airbus may be using is the probability of SINGLE pack failure.
....ignoring OEB 149 of course.....The probability of two pack failure events when your have two servicable packs (i.e. one fails followed by the other) is much smaller and hence the MEL limitation applies on single pack ops with SINGLE failure case. A DUAL PACK FAILURE is double failure ( Probability of Pack One Failure x Probability of Pack Two Failure) and thus is not a probable failure condition.
As far as the rates of descent go. The Airbus maximum acceptable leakage on flight test is 850 fpm but I can only guess that they use some form of modelling to define their emergency descent profile.
Thanks for the discussion!
We'll get there in the end!