 alex Salmond yesterday called for air passenger duty (aPD) to be cut in an effort to increase the number of international air routes from Scotland.  The First Minister pledged to campaign for Scotland to win control of the tax in May's Holyrood election, to put pressure on Westminster to devolve the issue.  Mr Salmond said: "If you doubled the number of flights, you could halve the tax."  He was speaking as he officially opened Ryanair's enlarged maintenance base at Prestwick airport, with the no-frills airline claiming aPD was restricting Scottish route growth.  The tax has also been widely criticised by other airlines and airports. Passengers at UK airports are charg 