Rotax 503

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The Rotax 503 is a 50 hp (37 kW) two-cylinder, two-stroke aircraft engine from Austria, made for ultralights. It was very popular for years, but production stopped in 2011. A Russian company later released a close replica called the RMZ 500, and Rotax’s other two-stroke option for aircraft is the partially water-cooled 582.

The 503 is piston-ported and air-cooled, with cooling from a built-in fan and cowling or by free air. It can run on pre-mixed fuel/oil or use oil injected from an external tank. It has two independent ignition systems and can use one or two carburetors. Fuel pressure comes from a manifold-driven pneumatic pump. An optional High Altitude Compensation kit is available. Propeller drive uses Rotax gearboxes of types B, C, or E. A standard muffler is included, with an optional after-muffler. Start options include a recoil starter, with an electric starter available as an option. An optional 12V, 170 W alternator with external regulator is available, along with an intake air filter and optional intake silencer.

Maintenance guidance includes teardown and inspection every 150 hours and overhaul every 300 hours. The engine is not certificated and is intended for experimental aircraft, where sudden engine stoppage can occur. It should be used only in daylight, visual flight rules conditions to allow a safe no-power landing if needed.