@Meikleour
I see a subtle difference between a "Variable ZFW schedule" and a straight choice of MZFW from a list of alternatives. The former gives a lot more operational flexibility if written "You can use a ZFW up to 172T provided you keep the TOW below 209T, or you can use a TOW up to 215T if you keep the ZFW below 167T, with suitable variations in between". The latter is a simple once and for all decision yielding MZFW. I am assuming you were offered the former.
After all this time it is difficult to recover the then existing numbers, but it seems to me that AI were offering operational flexibility at constant fatigue life.
Calculating fatigue life (actually damage tolerance is a more accurate statement) is a tortuous process involving calculation of manoeuvre and gust loads at various points in typical missions and them summing them, appropriately weighted for exposure time, to get the total damage. Since most of the time is spent in cruise, this condition will dominate the calculations. The usual calculation is a midcruise 1g load with a typical (say 10fps) gust.
Obviously the exact numbers will depend on the airline's route structure, but it is a fair assumption that if an airline opts for higher than standard ZFWs it is because it expects to carry higher than average payloads more often than average. One can however get a feel for the anticipated average situation by looking at the worst case.
In this particular instance they were offered 209T TOW with 172T ZFW which would give a mid cruise (half fuel) weight of 190.5T or 215T TOW with 167T ZFW giving a mid cruise weight of 191T. In other words they were offered flexibility of operation and additional carrying capacity when needed without sacrificing fatigue life.
Hope this helps!
Last edited by Owain Glyndwr; 6th Jan 2017 at 15:26.