The now out-of-production Rotax 503 engine has 2 cylinders, with a total displacement of 500CC, producing 50hp at 6800RPM. Starting the engine is fine, but killing the engine with one single press of a button seemed to be a challenge.
The thumb kill switch
This seemingly trivial issue wasn’t that trivial. The kill-button needed to be small, and a small pushbutton such as what was needed did not support a double pole. Without a double pole, it is impossible to short the magneto supply to ground, as we could not, and cannot tie the two wires together. We found some fat push button, with a double pole to do the job, but that wasn’t going to fit in the slender hand-grip. We could have provided two separate toggle switches to kill the engine, but when flying a powered paraglider with both hands, you’d need a thumb switch. A simple matter had become a bit complicated.
We tried some solutions with diodes, but then realised that the supply to the magnetos were AC, and the half cycle-conducting diodes could not kill the engine. The idea was to prevent the out-of-phase supply to one magneto from reaching the other magneto, but when we shorted the two magneto supplies to ground via the diodes, the RPM fell, but the engine didn’t die.
The only solution was the use of a double-pole electromechanical relay with a single control.
So, from the on-board 120W 12VAC peak-to-peak generator, we pulled the two phase wires, and passed them through a bridge rectifier. This gave us pulsating DC, which we smoothened via a 2200uF, 25V electrolytic capacitor. That was also to store charge to drive the relay for a sufficient amount of time. The capacitor rating was chosen based on the current drawn to drive an electromagnetic relay, and the duration for which it must be driven, which we set to 5 seconds.
This is important as the relay is in the normally-open condition. Energizing the relay from the smoothed generator supply would contact the magneto connections to the ground, killing both spark plugs. This would make the engine die, but a dying engine would produce a lesser generator voltage, which can, without a sufficient capacitor, de-energise the relay, allowing the engine to re-ignite. This would have led to an oscillation of kill-run-kill-run, which is very dangerous should you need the engine dead for whatever reason.
The hand wired circuit was coated with hot-melt adhesive, to sufficiently water-proof it. It was then placed in a small plastic box found in a local store, and then fastened to the frame of the powered paraglider. After the necessary wiring, the circuit was tested several times, and it performed flawlessly. Although exhaustive testing for uncertified airplanes isn’t required, we wanted to be really sure that this circuit will work.
And it does very well, on two powered para gliders.