Suicide to fly w/o opto kill switch. Power failure = plane going wild.

This one is programmable and the electronics are 100% sealed. http://www.electrodynam.com/rc/EDR-107/index.shtml

And for the people using an A123 (1100mah good for 100+ minutes on a single cylinder engine), put a 1N400x (1 A) or 1N500x (3 A) between the ignition switch and the kill switch for an average .7 volts voltage drop, where x is any number from 1 to 9.