The Rotax 912 S3 is a 100 HP four-stroke, 4-cylinder aircraft piston engine from the Rotax 912 series, widely used in light aircraft and sport aviation. It is very similar in core architecture to the 912 ULS but configured as a certified variant (S series) with installation differences depending on aircraft type certificate.

It uses an integrated propeller speed reduction gearbox (PSRU) and in the S3 configuration includes the Type 3 gearbox setup, which is typically associated with:

constant-speed (CS) propeller compatibility shafting
modified gearbox output shaft design
overload clutch protection system
Key features:

100 HP output at ~5800 RPM
4-cylinder horizontally opposed engine
Liquid-cooled cylinder heads + air-cooled cylinders
Dry-sump lubrication system with separate oil tank
Dual carburetors
Dual independent electronic ignition
Electric starter
Integrated gearbox (i = 2.43) with overload clutch
Type 3 gearbox variant (CS prop-ready shafting configuration)
Designed for certified aircraft installations (FAR 33 / EASA TCDS variants)