It's specialist & calibrated tools like a pitot/static tester and weighing equipment that are sometimes needed. This is workshop kit that is regularly checked and calibrated, and you have to be well trusted to be allowed to use it.
The French have given some competent owners resticted part 66 licences so that under EASA they can continue to maintain their own aircraft. When I tried to get the CAA to look at this idea the guy I contacted was genuinely appalled. Even though my qualifications and experience are more than required, the UK licence fees makes a UK part 66 licence (for a competent DIY owner) a non cost effective option. In any case "it's just not done" for an amateur owner to have a professional licence to service his own aeroplane.
