'swh' yr quite right there most are a contractual requirement but not at all bases.
I recall a long time ago now b4 the EN AAV based contract came into effect (2000) that it stipulated twin Eng A/C (not new though) & certain hrs for it's drivers.
They did drop the hrs some years into the contract if I recall as there was a time where the RFDS had trouble getting pilots mid way thru the contract as the Airlines where playing vacuum cleaner at the time.
As a side note here I heard some time ago now that the Twin requirement in the contract was born about due but not limited to the paramedics union being strongly apposed having it's members going in SE machines due the associated dangers of the obvious, they had been used to twins anyway (C404's) just prior to the new contract.
Wmk2