Rotax Aircraft Engines, III
Rotax four stroke aircraft engines have proven themselves as reliable engines. They can be bought as experimental (UL) or certified (A and F) engine and there are three main models to date. The original engine is the 80 Hp 912, from which the 100 Hp 912S was developed and they also have a 115 Hp turbocharged 914 engine.
Recognizing the different models is easy: the 912 has black cylinder covers, the 912S green/blue (blue for the french market) 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 an electronic ignition, this is different for most pilots.
Engine Maintenance
Rotax Oil Viscosity Chart
As the gearbox of the engine 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 will ruin the gears eventually. Use API SG or higher with additives for gearboxes. As for viscosity we recommend 10W40, as this will do fine in most climates, see the chart.
Service Instructions SI-18-1997-R5 and SI-912-016-914-019 from Rotax explains it in more detail.
This engine can run on AVgas and Mogas. Mogas is preferred. Rotax recommends that when the engine uses 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, full synthetic oil has difficulties cleaning the lead deposits in the engine.
Crankshaft
At the end of 2011 it became known that the crankshaft had developed problems during 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 the engine complies with Rotax Service Bulletin SB-912-059-914-042 on this important issue.
Carburettor flanges
The Bing carburettors are attached to the engine by means of a rubber flange. This flange needs to be checked every 50 hours for cracks. These cracks are mainly 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 CHT Temperature Chart
The engine has 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 a proper coolant and engine 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 the antenna's. 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 an engine 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...
