Dual Cylinder Rotax Electronic Engine Kill Switch

The Rotax 503 is a two cylinder engine that is no longer made. It has a total displacement of 500 cc and produces 50 hp at 6,800 rpm. Starting it is easy. The challenge was stopping it with a single press of a button. This project covered two builds of a thumb kill switch.

Why it was not simple

The kill button had to be small, because it sat in a slim hand grip. A small pushbutton of that size does not have a double pole. Without a double pole, you cannot short both magneto supplies to ground, and the two wires cannot just be tied together. We found a fat pushbutton with a double pole, but it would not fit in the grip. Two separate toggle switches would work, but a pilot flying a powered paraglider with both hands needs a thumb switch. A simple matter had become complicated.

What did not work

We tried diodes first. The idea was to stop the out of phase supply to one magneto from reaching the other. But the supply to the magnetos is alternating current, and diodes conduct only on half the cycle. When we shorted both magneto supplies to ground through diodes, the rpm dropped but the engine did not stop.

The working design

The only answer was a double pole electromechanical relay with a single control. The on board generator is rated 120 W at 12 V AC peak to peak. We took the two phase wires from it and passed them through a bridge rectifier, which gave pulsating DC. A 2200 uF, 25 V electrolytic capacitor smoothed it. The capacitor also stored enough charge to drive the relay for long enough. We chose its rating from the current the relay draws and the time it must stay on, which we set to 5 seconds.

That timing matters because the relay is normally open. Energising it connects the magnetos to ground and kills both spark plugs. But a dying engine makes less generator voltage. With too small a capacitor, the relay could drop out and let the engine fire again. That would cause a dangerous cycle of kill, run, kill, run when you need the engine off.

Build and testing

The hand wired circuit was coated in hot melt adhesive to keep water out. We put it in a small plastic box from a local shop and fastened it to the paraglider frame. After wiring, we tested it many times and it worked every time. Uncertified aircraft do not require long testing, but we wanted to be sure. The circuit works well on two powered paragliders.

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