Rotax Aircraft Engines, II
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 they also have 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.
Operating a Rotax
Before starting it is recommended to open the external oil tank, leave the cap off and rotate the propeller by hand (ignition off, of course) until you hear a murmling sound coming from the sump. This way you know for sure all oil is in the tank and the engine is loose (especially helpful in wintertime). This procedure also pre-oils the engine.
Oil level check
After completing the previous procedure its time to check the oil level, see Service Instruction SI-27-1997 for more details. You can do that after the flight too, but be very careful then. Hot oil can burn very nasty. Be also careful NOT to overfill the tank. Read Service Bulletin SB-912-040 / SB-914-026 to see if you have the correct dipstick as Rotax changed that item too some time ago.
Cold starting a Rotax is not that difficult: use the fuel pump (if any) for about 5 seconds (check pressure, max 0.4 bar), activate the choke fully, place the throttle in idle position (or else the choke will not work on these Bing carburetors) and activate the starter. A ten seconds maximum will save it from overheating, after period this you will need to wait two minutes to cool the starter before you take another attempt to start.
When it starts, close the choke slowly and take over with the throttle to keep RPM constant. If it does not start it is probably flooded: close the choke, open the throttle and start again. Close the throttle when it fires. If it still does not start you might have used not enough choke.
When the engine starts, keep it running at 2000 RPM as this keeps the gearbox loaded and avoids damage. Do not gas it like a car. Warm for the first two minutes up at 2000 RPM, this way the oil slowly warms up and is distributed through the engine. After two minutes increase RPM to 2500 and continue warm up. The engine is ready for taxi and run up checks when the oil reaches a minimum of 50 °C. Normal oil operating temperature is between 75 °C and 110 °C. Make sure that the oil reaches this temperature to boil off any condensation that might have accumulated, at least once every flying day. This will keep oxidation out of the engine.
The ignition test is done at 4000 RPM with a maximum drop of 300 RPM and no more than 120 RPM difference between both ignition circuits. If you have a hydraulic constant speed propeller then this would be a good time to cycle the propeller lever three times to circulate the cold oil in the propeller back to the engine. For electric propellers it will verify proper operation of the electronics.
Keep the RPM at 4000 and check if the voltage from the regulator is at least 13.8 - 14 V so that it supplies power to the electrical system; check amps if equipped with an ammeter (low positive charge to the battery: < 10 A and reducing). If your aircraft is fitted with a vacuum pump for the gyro's: verify that the suction gauge is in the green.
All of this is done while keeping an eye on oil temperatures and pressures. If all checks are done reduce RPM to check stable idle (at least 1400 RPM) then return to 2500 - 2700 RPM.
The Rotax engines capable of producing sufficient power at altitude are the turbocharged 914 and the 915iS, keeping their continuous power of 100/125 hp up to 15000/23000 ft (the 115/135 hp is only available at 5800 RPM for a maximum of five minutes at a time). All other types experience a loss in power with altitude. It's a sort of automatic throttle reduction and it reduces at about 1" manifold pressure every 1000 ft of altitude gain.
This brings me to the next point: I have seen pilots reducing RPM just after takeoff between 300' and 500' AGL. After asking them why, they said it was to reduce engine stress during take off. They probably did not realize that by reducing power it will take longer to get to altitude and the engine needs more total RPM to get to a certain altitude. Plus that the engine may run 5500 RPM forever and 5800 RPM for five minutes and this will normally get you higher than 300' AGL in those five minutes.
The point I am making is this: leave the power at 5500 and get to a safe altitude first (>500' AGL) before you make any changes. It will do NO harm to the engine at all. Just follow the instructions of the manufacturer and you will be fine.
And, changing power at such a low altitude (when things are running smoothly) is usually not a good idea. It is wise to get some altitude first before any changes are introduced. This will give you options in case of trouble.
Rotax CHT Temperature Chart
During flight oil temperature should be between 75 °C and 110 °C, cylinder head temperature should also be around 80-90 °C and must not exceed 135 °C depending on the type of coolant used, for details: move your mouse over the image to the right. Oil pressure is in the range of 2 - 5 bar (29 - 73 PSI) and on average around 4 bar (59 PSI).
After landing and taxi back make sure to cool the engine for a couple of minutes before shutting down. Else the risk of boiling cylinder heads will be high. Cooling a 914 is even more important, reason: the turbo. The turbo can be red hot while operating and runs at a maximum of 160000 RPM !! It needs a minimum of five minutes cooling time before it can be shutdown. Turbo chargers are expensive machinery so make sure to cool it properly and that the engine is within operating temperatures (min. 50 °C) before commencing take-off.