Rotax 503

503
Rotax 503 trade show display
Type	Piston aero-engine
National origin	Austria
Manufacturer	Rotax
Major applications	Quad City Challenger Blue Yonder EZ Flyer
Produced	until 2011

The Rotax 503 is a 37 kW (50 hp), inline 2-cylinder, two-stroke aircraft engine, built by BRP-Rotax GmbH & Co. KG of Austria for use in ultralight aircraft.^[1][2]

For decades the engine was one of the most popular and reputedly reliable aircraft engines in its class (two-stroke, under 60 horsepower),^[3] and it remains widely used and supported.^[4]

As of 2011 the Rotax 503 is no longer in production.^[3][5] However, a Russian manufacturer has developed an approximate reproduction, the RMZ 500.^[3] Rotax subsequently offered only one other two-stroke engine for aircraft, the partially water-cooled Rotax 582.^[4]

The Rotax 503 is piston ported with air-cooled cylinders and heads, utilizing either an engine driven fan and cowl, or free air cooling. Lubrication is either by use of pre-mixed fuel and oil or oil injection from an externally mounted oil tank. The 503 has dual independent breakerless, magneto capacitor-discharge ignition (CDI) systems. It can be equipped with either one or two piston-type carburetors. It uses a manifold-driven pneumatic fuel pump to provide fuel pressure. An optional High Altitude Compensation kit is available.^[1][2][4]

The engine's propeller drive is via a Rotax type B, C, or E style gearbox. The standard engine includes a muffler exhaust system with an extra after-muffler as optional. The standard starter is a recoil start type, with an electric starter optional. An integral alternating current generator producing 170 watts at 12 volts with external rectifier-regulator is optional. The engine includes an intake air filter and can be fitted with an intake silencer system.^[1][2][4]

Rotax recommends teardown and inspection every 150 hours, and overhaul every 300 hours.^[4]

The manufacturer acknowledges the design limitations of this engine, warning pilots:^[1]

"This engine, by its design, is subject to sudden stoppage. Engine stoppage can result in crash landings, forced landings or no power landings. Such crash landings can lead to serious bodily injury or death...This is not a certificated aircraft engine. It has not received any safety or durability testing, and conforms to no aircraft standards. It is for use in experimental, uncertificated aircraft and vehicles only in which an engine failure will not compromise safety. User assumes all risk of use, and acknowledges by his use that he knows this engine is subject to sudden stoppage...Never fly the aircraft equipped with this engine at locations, airspeeds, altitudes, or other circumstances from which a successful no-power landing cannot be made, after sudden engine stoppage. Aircraft equipped with this engine must only fly in DAYLIGHT VFR conditions."^[1]

Data from OPERATORS MANUAL FOR ENGINE TYPES 447, 503 & 582^[1]

• Type: two-stroke air-cooled aeroengine
• Bore: 72 mm (2.84 in)
• Stroke: 61 mm (2.4 in)
• Displacement: 496.7 cc (30.31 cu in)
• Dry weight: 31.4 kg (69 lb) (dry, no exhaust system)

• Valvetrain: piston ports
• Fuel system: pneumatic pump pressurized
• Fuel type: regular autofuel
• Oil system: premixed in the fuel at 50:1 or oil injection
• Cooling system: fan or free air
• Reduction gear: Rotax 'B' gearbox: 2.00, 2.24 or 2.58 ratios; Rotax 'C' or 'D' gearbox: 2.62, 3.00, 3.47 or 4.00 ratios

• Power output: 37 kW (50 hp) at 6800 rpm
• Compression ratio: 10.8 (theoretical)
• Fuel consumption: 15 L/hr (4 US gph)
• Power-to-weight ratio: 1.18 kW/kg (0.72 hp/lb)

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