Old knife - 1980s, was not very secure. My UAS instructor's knife + sheath came out & slipped down the side of the seat unseen (See Pingdit's pic - the press stud came undone). On the subsequent solo aeros, I got knifed in the head during a reverse stall turn. Fortunately it was still sheathed & hit me on the side of the bonedome. Even more fortunately, I'd been very well taught & remembered to recover to S&L before sorting out the problem.
I recommend the Canadian Army Knife, by Grohmann, partly because I have one & am very happy with it, and partly 'cos they're made locally,
with love 