Not honking
When I was rear crew instructing on the Mk1 Nimrod there was a young Sgt siggie who was sick on every trip as he was going through the course. I had some theories about airsickness and I made it my mission to get him through ONE trip without hurling. Like others above have mentioned I decided to keep him so occupied on his radio op duties that he wouldn't have time to think about being sick , and apart from one or two rolling eyes moments I/we succeeded. As we rolled out on the landing run he was looking green "get into the port beam lookout and stay sharp as we taxy in " I yelled at him; off he went, and now stable and slow and with a horizon to look at I was sure he'd be OK. He was - until the student pilot trod rather heavily on the brakes as we came to a final halt and we all had a lurch forward. As I went round the end of the nav crate to say "Well done son, you see you CAN do it" He projectile vomited into the goldfish bowl window....... Not a pretty sight! I was absolutely knackered, my voice had gone and he looked so crestfallen! He did get a bit better as time passed but I think it always bothered him in his subsequent career.
My main theory was that it's caused by emotions of stress. Stress of a pressured learning situation like an OCU, just about jump out of an a/c, setting off to do a dinghy drill in roughish WX in the firth or as it used to be in Plymouth sound, for some just being in an aircraft and having no control of your environment, stage fright...... lots of situations. I actually was present in the desensitisation suite at Farnborough when one of my ex-studes was there for day one of the process. The lovely lady who oversaw the process briefed him carefully and zipped up the cover on the turntable and as we walked back to the console to star there was the amplified sound of a projectile vomit over the speakers - and NOTHINg HAD MOVED YET! Sheer stress and nerves.
The Ancient Mariner