If it's like other apprenticeship selections, it will usually be a mini set of gcse's (i.e maths and physics). Then there might be a pyschometric test (rotation in a set of gears for example). Also a technical test (just a simple one, they want to know your basic knowledge of aeroplanes).
I have also heard of selection essays, i.e how a jet or piston engine works, theory of flight etc.
Hope this helps,
Good luck and let us know how it goes
