Kill switches are essential for the larger gas engine aircraft. The purpose of the safety kill switches are to allow you to kill the motor regardless of failures in the wiring or throttle linkage. 

Lets say you took off, and are flying circuits. All of a sudden, you notice that the throttle is stuck at about 1/4 throttle, and you have not control over the engine speed. You know you have a full tank of gas, and at the expected fuel burn, you won't experience a dead stick for about another 45 minutes or so. You can try to outlast your batteries, attempt a hot landing and possible crash, or use your kill switch. With most giant scale aircraft, setup properly with the various ways to kill the engine, this would be a routine situation. You would use the servo driven kill switch.

So, lets talk about these kill switches, and how to implement them.

First, is the throttle cut off. Most smaller glow engine planes use these, and it will be used here as well. Basically, the throttle servo is used to close the throttle, cutting off the fuel and air supply to the engine, thus killing it. Usually, this is nothing more than using the throttle trim to perform this magic. However, you can also program a transmitter switch to do this task on many of the newer transmitters or computer radios. We will assume you are familiar with this method, and move on. If not, ask other pilots around the flying site on how do set this up.

Second, is the manual kill switch. Typically, this is  the ignition on/off switch in between the battery and ignition module. This will turn on the power to the ignition system, thus allowing the engine to run. By turning it off, you cut the power to the ignition system, and thus kills the engine. Or, it is a switch on the black lead coming out of the magneto on a magneto system. This grounds the magneto, turning off the current required to fire the spark plug, which in turn kills the engine.

Usually, this is all that is needed in most circumstances. However, lets assume a failure of the throttle linkage in flight, and was the case noted in the beginning of this article. Since the plane is flying around at 1/4 throttle, it will last quite a while in the air at that throttle settings. There is a possibility that the gas will exceed the battery supply. Second, since the throttle linkage is broken or jammed, you can not use the throttle cutoff or throttle trim to close the throttle. Further, you can not reach the manual switch mounted on the fuselage!

To solve this problem, a servo driven kill switch can be installed. This servo is controlled by a switch or lever on the transmitter. It is independent of the throttle linkage, and independent of the manual kill switch. Should either fail, the servo driven kill switch will still work. To be truly independent of the manual kill switch, you would use a separate switch, as well as separate and redundant wiring. Any failure in the manual kill switch will not effect the servo driven kill switch. The diagram shows the independent wiring of the ignition switch or manual kill switch, and the servo driven kill switch.