MGD - I think you will find that NCA is a trade or specialisation and that you serve on tours. Officers serve in a Branch, NCA dont, or has it changed that much in 13 years since I left it for a Branch Appointment
NCA is an aircrew branch, containing specific trades/specialisations such as Flight Eng/Crewman etc. Like everyone else, NCA serve on tours, but this is an admin related issue, as 99% of tours are flying tours, with postings often to the hangar next door.