Rotax 503



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.

The Rotax 503 features piston ported, air-cooled cylinder heads and cylinders, utilizing either a fan or free air for 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 and 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.

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.


Rotax 503 DCDI


The manufacturer acknowledges the design limitations of this engine, warning pilots:
"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."

As of 2011 the Rotax 503 is no longer in production.


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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)


503 UL 2V DCDI
Cycle: 2
No cylinders: 2
Bore: 72 mm
Stroke: 61 mm
Compression: 6.3
Displacement: 496.7 cc
Cooling: Air
Ignition: Electronic Ducati
Reduction: Mechanical Gear ‘B’ ‘C’ or ‘E’
Reduction ratio: 2-4:1
Weight: 42.8 kg
Max pwr: 49.5 hp at 6500 rpm
Max torque: 55 at 6000 rpm
Fuel consumption: 360 G/hp/hr
Price 1998: 19 693,98 Fttc


The 503 B-Box with Pull Start, DCDI, (dual carb, dual ignition) of 52 hp weighs 323 lb.
Rotax 503 DCDI, B-Box w/Electric Start & Mag End: 343 lbs
Rotax 503 DCDI, C-Box (dampener) w/ Pull Start: 331 lbs
Rotax 503 DCDI, E-Box w/ Electric Start: 347 lbs