Thanks spinwing for the helpful comment!
As far as I am aware once an aircraft is accepted as being Category A (even if only with a dispensaton) and is able to demonstrate Class 1 ability, the captain trims his load to suit the aircraft performance. It might well be that the F1 or F2 can only manage 2 pax in the back, but it can still operate according to Class 1 performance criteria.
Hence the interest in having the STC and the reason I need to update the figures!
There must be somebody who could give me a pointer please!
