Back in the seventies, at the height of the PIRA activity, I remember an order went out asking airmen not to salute any officers in civvy dress they met in a public place. They were only to give a nod of acknowledgement. I believe it started in NI, but spread to some bases on the mainland. It was funny at times as you would see an airman you recognised approaching and his right arm would give an instinctive twitch before settling back down by his side.