Probably at a wedding reception when I was one of the groomsmen, with one of the bridesmaids in a sort of side-room down a hall off the main reception room. I later discovered that she had a boyfriend who was one of the wedding guests, which made me feel rather bad. On the other hand, she had been eyeing me up and flirting with me at the wedding rehearsal and all through the wedding day itself, and she was obviously gagging for it.
Also in the back garden of a friend's house during a party, in the absolutely pouring rain. We were both 17, and I remember that she had trouble explaining to her parents how she had got so wet.
In a sleeper on a train (don't suppose that's crazy, really) but never in a plane, sadly.
But a few years a couple on a train in Kent (in England) were fined, not for having sex on the train but for smoking afterwards. Other passengers just sat and watched (or more likely pretended to read their books and felt embarrassed) while the couple fucked but, as soon as they lit up, three people went and complained to the guard on the train.