but can anyone elaborate on why a minimum ZFW?

I cannot elaborate because I do not know the reasons. As I said earlier it was not a limitation that would normally be approached in service, even when we took aircraft down to St Athan for major servicing and they had had most of the role equipment removed. I do not even recall it being discussed on the OCU course and they did tend to go into the dark and dusty corners of the books.

Rather than speculate on the reasons I shall enquire of a couple of acquaintances who are much better qualified than I am. They have done the Difficult Sums Course at ETPS.

exmpa