From the Scottish Governments "Programme for Government 2015-16" published the other day - first time I've seen commitment to dates
Programme for Government 2015-16
The Government’s Programme for Scotland 2015-16. The document sets out the legislation for the coming year, as well as summarising the Scottish Government's key achievements and main goals for the future - both legislative and non-legislative
AIR PASSENGER DUTY
We are committed to replacing Air Passenger Duty (APD) with a more competitive regime that better supports our goal of boosting international connectivity and business growth.
We have established a forum, with membership from the airline industry, Scotland's airports, environmental groups, business organisations and tax professionals. This forum will support the development of our policy proposals, including helping us to shape a policy consultation on a Scottish APD which we plan to launch this October. The Scottish Parliament will be able to legislate for a replacement Scottish tax only once the new Scotland Bill has been passed by the UK Parliament.
We will reduce the burden of APD by 50% with the reduction beginning when we introduce a Scottish APD in 2018 with a view to abolish it completely when resources allow. We are currently consulting with stakeholders on the best way to apply that reduction across new and existing routes.