How could I add a kill switch to my trainer, (petrol) so if I lose signal the engine cuts out, I was thinking of using one of the spare channels on my controller to have a second servo on the throttle that somehow pulls it back when signal is lost but am unsure if this is possible.

kalaharidw,
With a regular radio it's difficult, if not impossible to set up what you are trying to. When signal is lost there is nothing to tell the servo to "kill you motor". In some of the "higher end" radio they have the ability to set actions for the plane in the event of signal loss. Hopefully some of the more radio smart members will fill in where I am leaving off on this.

you would need a failsafe (futaba has one coming out soon and there are a few others) or as mentioned with some of the newer PCM radios (my 9cap does it) you can program a servo to go to a certain set point when signal is lost.... ie: throttle to idle etc etc

First of all, you should be able to maintain a signal as long as you can see the airplane. If you loose the signal, shutting down the motor is really the last of your worries. You need to have control of the airplane and without the signal, you have none.

As for the pre-set servo functions, the higher end transmitters do have a thing called "failsafe" mode. This "IS" when the receiver looses the signal from the transmitter, the servos go into a pre-set position. Most people, when experiencing want to have the engine go to idle. As for the settings of the surface controls, some want them at neutral while others want a little up elevator and a little bit of aileron to make somewhat a turn should you loose the signal. As far as I'm concerned, I like to have the throttle at an idle and all the surfaces at the last know position before the signal loss. I'd rather not try to figure out a new starting point once the signal is regained.

Now, that all depends at what attitude and altitude you're at when you loose that signal. Anyway, a novel could probably be written about failsafe and it's advantages/disadvantages, but to answer your question, you'd be looking for a radio system with failsafe to provide you with a change in throttle setting when the signal is lost..

there are aftermarket devices called failsafes that plug inline between the servo and the receiver. these set the servo to preprogrammed positions if signal is lost. I have used them on my model subs to shut down engines and blow ballast when signal is lost.

look for an rc car failsafe, they are about $20 and will do what you want. But, as someone else mentioned, if you lose signal shutting the engine down won't matter that much.

I'm curious. You said the failsafe is something built into the transmitter, but that when the receiver loses the signal it puts the servo into a pre-set position. I'm curious how the transmitter can effect such a change in the receiver if the receiver has lost the signal. Isn't the failsafe function actually built into the receiver?

Chuck... You're simply correct. However, you pre-set the failsafe utilizing your transmitter, right? Meaning, it's just a function of the higher end radio "systems".

Yes, it does no good if one buys a "high end" transmitter and then uses a "low end" receiver.