Many years back when the Flight Checking Force re equipped with the mighty Argosy there were still a number of Varsities around and Navs had to be dual capable, so very often we had to calibrate landing aids using a balloon theodolite which put the aircraft upside down and the receiver aerial was in the wingtip. Anyhow there was one Varsity which went to Berlin...a lot. Navving this beast down the corridor on a decca flight map was challenging with occasional interceptions from the GDR Migs. I wasn't happy one day and announced that I would go aft and take a drift reading. Don't step behind that curtain I was advised if I still had prospects for a family. It was all worth it for a night on the Kdamm and the Munchener Hofbrauhaus.