According to a report in the Sunday Times last weekend the Conservatives, Labour and Liberal Democrats will offer Scotland greater tax powers, including setting air passenger duty and vehicle excise duty in the event of a No vote.
If that is the case air passenger duty will devolve to Scotland, and almost certainly significantly reduce, whether the electorate votes Yes or No to Independence.
If Scotland does remain in the UK it's very likely that the Wales government would then also be offered the same power re air passenger duty.
The result would be another slant on the West Lothian Question with Westminster, consisting of MPS from the entire UK, responsible for English airports' air passenger duty with possibly a chancellor representing a Welsh or Scottish constituency. There are recent precedents with Brown and Darling.