I've used these on all of my 1/5th scale rc cars and trucks and they saved me more than once.
http://www.davesmotors.com/s.nl/c.88...t.A/id.3569/.f

These are great because if the Rx voltage is too low it wont even let you start the engine, or if it gets low the engine dies, also if connection is lost it will kill the engine, and i can use a channel on my Tx as an on/off switch. I'm sure the airplane variants are just as good/usefull.