dkatwa Please excuse the smart a** remarks which you can always expect on this site. If you were to stop the main rotors from turning, the helicopter would drop like a rock. All certificated helicopters are designed to autorotate in the event of an engine failure. The same applies in a tail rotor failure when the pilot initiates an autorotation. Rather than try and explain it myself, you can get a pretty good explanation here: http://en.wikipedia.org/wiki/Autorotation_(helicopter)
Thanks Rotornut.....actually, I did not post it as an April Fools joke...coincidence, I assure you!
I was thinking along the lines of a failure just before landing, say 5 feet...the following on youtube will give you an idea of what I mean....dropping 5 feet on the skids would be better (I imagine) than the problems caused by the violent rotation and flip over/ flying blades etc....
Not as stupid an idea as it sounds (to some people)...
Of note, someone had a good idea to add something like this into the Lynx once; a crash detector that would shut down most of the aircraft's vital systems.
The system is disabled because the risk of it going off giving you an immediate double engine failure with added total electrical failure when it's not supposed to outweighs that of it doing any good in a crash.
I can't be sure, but from the OP's reference to a sort of "deadman switch" like he has on his lawnmower, I don't think he meant to suggest that we actually stop the rotors immediately. I think he just meant a kill switch to kill the engine.
And so to dkatwa I say that many helicopters do have a "kill switch" that is in the pilot's hand all the time. We call it the "throttle" (even though a turbine engine doesn't have the same type of throttle as a piston engine lawnmower). In a tail rotor failure scenario, many pilots can cut the power "fairly" instantaneously, thereby cutting torque to the main rotor, which is the problem that causes the spin when there's no device to stop it (e.g. that tail rotor). But momentum being what it is, the fuselage does not stop spinning immediately. And of course, when you cut the power to the rotor it slows down pretty quickly, so if you're still in the air you might hit the ground pretty hard.
However! Larger (and some smaller) helicopters sometimes put the fuel control levers ("throttles") on a separate quadrant either on an overhead console or on the floor. What this means is that the pilot must move his hand from the controls to move the throttle(s). And yes, this can be a problem, as you can imagine.
Perhaps it would be a good idea to put an electrical button or trigger or something on the controls that would shut the engines down. But unlike a lawnmower it cannot be something that has to be held in all the time - our flights can be pretty long sometimes...and!...we pilots need to take our hands off the controls from time to time (to stuff a sandwich into our gobs). In any event, helicopter designers have not seen fit to install such a device, and we pilots have not called for it. What can I tell you...
Luckily, tail rotor failures are not all that common.
For what it's worth you could get a rotor brake as an option on Bell 206 IIIs and perhaps other helicopters. However, it is not to be used in flight and it's only purpose is to stop the the main rotor at low rpm on the ground.