The Chipmunk was a basic training plane whose UK specification was modified to meet RAF requirements, which included the downward identification light, to train pilots to use a standard procedure. The same light was fitted to other aircraft particularly fighters and bombers in WW II. Ground to Air Radio communications were very primitive at the time and in some cases, such as anti- aircraft guns there was no means of challenging an unidentfied aircraft by radio. It was done by morse lights. The theory at least was that the aeroplane. when challenged, responded with the "letter of the day". If he failed to do that correctly he could be shot at. I'm not sur how practical this was and I'm not sure how visible the light is from the ground, but there are mentions of this procedure in some of the books written about there experiences by WW II RAF pilots. It seems difficult to imagine that one was supposed to calmly tap out the letter of the day on a dark turbulent night, while trying to keep the plane on course, at the risk of being shot at if you get it wrong. The procedure was obviously overtaken by the introduction of IFF or what we now call a transponder that replies to a radar challenge, but I suspect they continued to teach the old procedure for many years.
Nigel
