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Rotax Aircraft Engine

Rotax Aircraft Engines, III

Rotax four stroke aircraft engines have proven themselves as reliable pieces of machinery. They can be bought as experimental (UL) or certified (A and F) engine and there are four main models (injected too) to date. The original engine is the 80 hp 912, from which the 100 hp 912S was developed and Rotax also has a 115 hp turbocharged 914 engine. In 2015 Rotax announced the 135 hp fuel injected turbocharged 915iS.

Recognizing the different models is easy: the 912 has black cylinder covers, the 912S green/blue (blue for the french market), the 912iS is green but without the carburetors and the 914 has red covers and a turbo. These are the most obvious differences.

Flying with these small engines, compared to the ordinary Lycoming or Continental takes some time to get used to. They are also equipped with liquid cooling and have an electronic ignition, this is new to some pilots.

Maintaining these little engines takes some time getting used too as things are just a bit different compared to standard aero engines. Below some tips and tricks.


Oil ViscosityOil Viscosity
Rotax Oil Viscosity Chart

As the gearbox is also lubricated by the engine oil it therefore needs a type of oil suitable for gearboxes. Motorbike oil is fine for this purpose. Normal car or aircraft engine oil is not suitable and it will ruin the gearbox eventually. Use API SG or higher with high pressure additives for gearboxes. As for viscosity we recommend 10W40 or 15W50, as this will do fine in most moderate climates, see the chart.
Service Instructions SI-18-1997-R5 and SI-912-016-914-019 from Rotax explains it in more detail.
We also recommend Liqui-Moly engine oil for Rotax Engines, for more info see the website of Liqui-Moly Aero.

This engine can run on AVgas and Mogas. Mogas is preferred and Rotax recommends that when using AVgas for more than 30% of the time, oil change interval must be shortened from 100 hours to 50 hours. This is due to the high lead content of AVgas. Semi-synthetic oil must also be used when running on AVgas, as full synthetic oil has difficulties cleaning the lead deposits inside the engine.


At the end of 2011 it became known that some crankshafts had developed problems during recent manufacturing, from Rotax: "During a production process review, a deviation (double side straightening) in the manufacturing process of certain Part Number (P/N) 888164 crankshafts has been detected, which may have resulted in cracks on the surface of the crankshaft.".

Make sure the engine complies with Rotax Service Bulletin SB-912-059-914-042 on this important issue.

Carburetor flanges

The Bing carburetors are mounted to the intake by means of a large rubber flange. This flange needs to be checked every 50 hours for cracks. These cracks occur on the inside and you will need to remove the carbs to see them, if any.

If these cracks are allowed to progress an engine failure is possible. Usually if one fails the engine will start running rough on low RPM and doesn't start as easily anymore. Read Service Bulletin SB-912-030-914-019-R1 about this.

Waterless Coolant

Rotax Cylinder Head TemperatureRotax Cylinder Head Temperature
Rotax CHT Temperature Chart

The engine is equipped with water cooled cylinder heads with a pressurized system containing ethylene glycol or NPG+™. Early models used a 0.9 bar pressure cap but the latest models use a 1.2 bar cap. See Service Bulletin SB-912-043-914-029 below.

Optimum temperature can be read of the chart left. More information on the new pressure cap and required coolant is in this Rotax Service Bulletin SB-912-043-914-029.

Rotax GmbH recommends a waterless coolant for its engines, more can be read in this Evans article from Phillip Lockwood and these Evans installation instructions.

Operating fluids

This engine needs proper coolant fluid and an oil suitable for gears to operate within limits set forth by Bombardier Rotax GmbH, Service Instruction SI-912-016-914-019 has all the details.

Electronic Ignition

The engine is equipped with a standardized electronic ignition unit, see Service Instruction SI-912-013-914-016. Personal experience has led to the following recommendation: when flying near high powered radio transmitter stations do keep a safe distance from those antenna towers. Normally you need to be 1000 ft higher than any obstacle within 600 meters, in this case stay at least 2 miles away.

Certain radio stations can emit high powered radio signals and flying close to them has the effect that these radio signals sometimes can be heard in your intercom. If this is the case, the electronic ignition can probably 'hear' them too.
The result of these unwanted signals in the electronics will be that the ignition is going to run irregular/erratic and could even fail at some point and the engine will start running rough, missing power strokes or even stop working altogether resulting in an off airport landing.

Ground contact

Both ignition units and the coils need a good ground contact with the engine and to the battery. If this is not the case then expect either no drop at the ignition test or even a stoppage at runup. Intermittent RPM drops while flying are also possible. The wires in the loom are of such quality that they will break eventually, with the above mentioned results. Update: newer engines have a much higher quality teflon-like wiring wich does not chafe and brake its insulation so easy anymore.

Written by EAI.

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