Hi,
This has happened to me 4 or 5 times... you may have "vasovagal syncope" (situational fainting) and it's likely related to the low oxygen on the airplane... I usually get it after I've fallen asleep for a few minutes, on the way up (I've not gotten it on descent)-- you breathe more shallow when you sleep, so get even less oxygen (and it's low on airplanes to begin with).
I wake up feeling sick, get up, and pass out near the bathroom (never usually actually sick).
The last time, my husband was with me, and while I didn't stand up, he said I was out cold, with my eyes open.
Not sure what to do about it, but now that it's happened a few times for me, I think I'm going to pursue this with my doctor-- I've read about the monitor, and also tilt-table testing... I fly a lot, and I can't keep letting this happen (though if I don't sleep, it doesn't happen).
Next time you fly, ask your doctor if you can wear a holter monitor. If you do faint, he/she can check the recording to see if there is a correlation between your HR and the fainting episode.