L.A.B. said:
http://www.boltscience.com/pages/helicalspringwashers.htm
Helical spring lock washers have been in use for well over 100 years. They are still used on many applications in the belief that they will will "lock" the nut/bolt to the joint and prevent loosening. The body of evidence, based upon both experience and experimental results, is that they do not prevent loosening and can be shown to actually speed up the rate of loosening in many cases. Junker originally showed in his work published in 1969 that these washers are ineffective in preventing loosening.
Although I've read that and the FAA says in AC 43.13-1B that they're not to be used in primary structures or where frequent removal is necessary, I find when removing very large nuts with split ring lock washers from equipment that often they tear quite a bit of metal from both the nut and base material. I can't help but think they are actually locking the fastener in that situation. The tearing/gouging happens long after the initial loosening has begun.
Having said that I agree that they have no place on an engine or just about anywhere else where a metal to metal joint is made and sufficient torque can be applied to stretch the fastener.
Having worked on aircraft for years I am fond of self locking nuts, castle nuts on anything subject to rotation (rotation is the key, cotter key, that is), and safety wire for anything that can't be secured in other fashion and shouldn't be allowed to come loose.
