Hi
lion-g,
I must admit to deliberately waiting a day or two to respond to you, in the hopes that some Airbus knowledgable person might reply, I'm strictly a BCAR, FAR 25, and CAO 20.7.1B nerd. The JAR I respect greatly, but not done any work with any of their products (apart from plagiarising a few good ideas)
).
All of that is a round-about way of saying that I do not know much about how Airbus do things.
To go to your last sentence "
Is this legal ?", yes it is, but quaint, a bit 1950'ish.
Having already admitted to not knowing Airbus' mode d'emploi, my reaction is that they've over-simplified things at the expense of optimisation of Takeoff Weights. As you've said, for a given weight, the speeds are constant irrespective of the other variable conditions. What the Airbus engineering people have now done is to make the MTOW(perf) as well as the STOP MARGIN 'fit' the speeds. That's safe, but off-optimum. On the other side of the Atlantic they would have taken the variable considerations (including MEL) and optimised a Weight / Speed schedule for the condition.
Boeing are 'guilty' of doing the same thing for some MEL conditions (to keep things simple), but for more routine semi-unusual cases such as Wet Runway, optimised a whole new set of data. An example of this would be dispatch with a Hydraulic pump U/S, which directly affects gear retraction time and distance, thus imposing a 1st segment penalty. The technique used would be to use the normal V-Speeds, but apply a weight penalty to the MTOW.
A good idea to wait for a response from an Airboos expert, these remarks are just my thinking out aloud
Regards,
Old Smokey