Rotax four stroke aircraft engines have proven themselves as reliable pieces of machinery. They can be bought as experimental (UL), sport (S) or certified (A and F) engine and there are four main models (fuel injected too: i models) 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 BRP-Rotax announced the fuel injected 141 hp 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. The 915 is a turbocharged version of the 912iS with blue covers. These are the most obvious differences.
Flying with these small engines, compared to the ordinary Lycoming or Continental takes some time to get used too. They are also equipped with liquid cooling, some models have fuel injection and an electronic ignition (ECU), this is also new to some pilots.
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 sooner or later. Use API specification 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 for that.
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 4-stroke engines, for more info see our pages on Liqui-Moly Aero.
Oct 2020: BRP Utility and Diagnostics Software (BUDS) and relevant converter hardware is available for all ROTAX aircraft engine 912 i Series and ROTAX 915 i A Series. This converter connects the computer via an USB port to the serial interfaces Lane A and Lane B of the engine control unit (ECU), fuel injected engines only.
This software enables the aircraft technician to upgrade firmware, read logs and other important engine parameters. For more info see the next link to Rotax Tool Tip BUDS.
July 2019: SB-912-073/ SB-914-055 Replacement of Carburetor Circlip is a mandatory (within 25 hours) service bulletin by Rotax for certain engines serial numbers and spare parts. Be sure to comply with this. Dec 2020: In rare circumstances Rotax noticed fracture of these circlips resulting in a rough running engine. Watch the next FlyRotax video about Circlip replacement and the steps to perform it.
Mar 2019: An Alert Service Bulletin (ASB) was released stating that the dual fuel pump assembly MUST be checked for fuel leaks inside of the fuel pump housing on both type of engines. See ASB-912 i-010 / ASB-915 i A-006 - Inspection and/or Replacement of Fuel Pump Assembly for ROTAX Engine Types 912 i and 915 i A (Series) for more details.
Dec 2018: exhaust valves on the types 915 i A, 915 i B and 914 may have had a friction weld issue possible resulting in a fracture of the exhaust valves on these engines. Other engines which have valves with part number 854113(4) must also be exchanged. Engines with serial numbers listed in Alert Service Bulletin 915i-003 and 914-054 are affected. Parts kit nr: 481375 is available for repair.
Dec 2018: Turbochargers on some Rotax 915 engines suffer from a lack of lubrication. Be sure to comply with SB-915 i A-005, SB-915 i B-005 inspection of turbocharger assembly before the next flight!
Oct 2017: Due to deviations in the manufacturing process of the valve push-rod assembly, partial wear on the rocker arm ball socket may occur. This wear might lead to a rocker arm cracking / fracture which in consequence may lead to a malfunction of the valve train. Possible effects are rough engine running or an unusual engine operating behaviour. You must follow Rotax Service Bulletin SB-912-070, I-008 or SB-914-052 for more information.
Nov 2017: there was a manufacturing deviation with the gasket on some oil filters (part number 825016 with date code between: 10/17 and 38/17, these also have a green sealing gasket. Inspected good oil filters have green dot) from Rotax. Make sure to check SB-912 i-009 / SB-912-071 or SB-914-053 for the exact details on this issue. Detailed information in this FlyRotax TechTips #4 video.
May 2017: In the course of continuous development and standardization, new floats (pair) ROTAX® part number 861188 have been introduced. These new floats (pair) are manufactured with a higher compaction rate and contain a recessed brass guide. This Service Instruction also refers to two other issues with the carburetors on certain engine numbers, namely a separation of the outer skin and sinking floats. Read more in this Service Instruction SI-912-914-029 from BRP-Rotax.
Dec 2020: Service bulletins SB-912-074 / SB-914-056 need to be complied with within one year after publishing of this SB. Check to make sure that your engine and carburetor serial is on the list. More info in the next video from FlyRotax.com: Exchange of floats. Part no. 861189 (marked with a R) is the correct float for your carburetors. The symptoms are rough running at low RPMs on the carburetor engines.
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.
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.
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. Field experience has shown that these caps fail sometimes. In my case the system started loosing coolant and did not reach operating temperatures properly.
Optimum temperature can be read from the chart left. More information on the new pressure cap and required coolant is in this Rotax Service Bulletin SB-912-043-914-029.
This engine needs proper coolant fluid and an oil suitable for gears to operate within limits set forth by BRP-Rotax, Service Instruction SI-912-016-914-019 has all the details.Written by EAI.