Are we talking about the Rotax 912/914 series specifically, or all Rotax engines?
I have only made a handful of flights behind a Rotax 912/914, and in all cases as a passenger. I have never flown behind (or in front) of a Rotax two-stroke. But for years I've toyed with the idea of building my own aircraft, so I've read a lot on this subject over the years.
What I've seen is that the Rotax 912/914 series have a record of utter reliability. No issues whatsoever. Provided that they are maintained and operated properly. Most of the items have already been said: Turning the prop to evacuate the oil from the dry sump to the separate oil tank, balancing the carbs, Mogas in favor of 100LL and so forth. Both the pilots and the engineers need to understand that this engine is NOT like your average Lycosaurus, and needs a different kind of engine management/maintenance.
On the other hand, the two-stroke Rotaxes have a less than stellar record. Most problems, AFAIK, can be traced back to the fact that idle power means no or virtually no lubrication in a two-stroke. So if you're in the habit of making steep glide approaches with zero engine power, you're slowly thrashing the engine. With predictable results.
Furthermore, I can well imagine that if you have a fleet of Lycosaurus-powered aircraft and one Rotax-powered aircraft, that "the odd man out" effect may well be an issue. Particularly if you have renters that only do a few hours a year, and maybe only one or two hours a year on the Rotax-powered aircraft. We've had that same situation where we had one Diamond DA40 TDI on a fleet of otherwise Lycosaurus-powered aircraft. Trying to get all pilots up to speed on the differences required a lot of training.
I have been flying behind a 912 since 2007 - it had 800 hours then. Now it has got 1650 hours - past its TBO of 1500 hours, as well as it is past its 12-year calendar TBO. Still silk smooth, starts first time every time regardless of outside temperature and humidity (and it can get very cold here in Lithuania), oil consumption is so low that I do not have to add any between the oil changes that come every 100 hours, which means I do not even fully use a 4 liter canister of oil per 100 hours. In 6 years of ownership the only non-scheduled expense was changing one of 4 springs that hold exhaust manifold in place. I also added aftermarket oil thermostat because the engine had a tendency to run cool.
I am very happy with 912. Its performance, reliability, low fuel consumption and and maintenance costs answer my brief completely. I have no experience with other aero engines, but looking after the Rotax is easy. I do not find it troublesome to check the coolant and oil level - it takes less than 5 minutes on my aircraft. There is nothing else to look after, really. No valve gap adjustment. No mag timing. Does not require conservation procedures if you do not use it for more than 30 days. As with the oil changes I do the carburetor synchronizing myself. Takes up to 10 minutes.
It is really hard for me to imagine how people end up having problems with this engine. You really should try hard like running it on Avgas while using synthetic oil or flying without coolant.
When we were operating the Tecnams (only stopped due to political reasons but I still co-own and co-built a Rotax powered kitplane) they were run on 100LL.
No issues at all. Oil was AeroShell Sport Plus 4 changed every 50 hours. As per Rotax instructions, >30% 100LL results in more frequent oil, oil filter and spark plug changes. Gearbox inspection at 600 hours.
No operational issues whatsoever with 100LL. We now have access to Total UL91 so run our Rotax on that.
Education is what is required but steely-eyed sky gods (pilots and engineers) can't be told that they need to learn slightly new procedures when operating Rotax's.
Handy hint - gurgle to get the oil back in the tank after flight - couple of blades and it's done. Much quicker than with a cold engine.
It is amusing that the unlearned pass on all these unfounded rumours on reliability. Would they blame an aircraft type as being dangerous because it kept crashing whereas no one had actually taught the pilot to fly....?
The only reason I do not use fully synthetic oil is that time after time I cannot avoid fueling with 100LL, which is totally fine with the 912, apart from, as mentioned by many others, more frequent oil/spark-plug changes.
Other aspects that add to the reliability of this engine is the absence of possibility to mismanage the air/fuel mixture and thermal stability owing to its liquid-cooled heads. You only have to watch that oil temperature has reached 50 degrees Celsius before seriously revving it. Never had a problem of reaching the upper limits for oil/head/coolant temperatures even on very hot days.
Jan Olieslagers mentioned "acceptable" power to weight ratio. If I am not mistaken 912S power/weight ratio is leading among the aero engines of comparable power.
Oh, and it is so much quieter when compared to Lycomings and Continentals.
How many of the Rotax 912s in current operation do people think will be running in 25-50 years? Its a serious question, not meant as a dig, because I intend to fly my current engines for perhaps 25 more years before selling them. They are currently 42 and 67 years old, one never major overhauled and running just fine, the other overhauled twice so far and likewise a good engine. I think their lives are indefinite with key components replaced or reworked every now and again. The engine accessories (carb, mags, alternator) can be similarly overhauled.
My cost to overhaul a four cylinder aircraft engine is conservatively on the order of $15K or 11K Euro or 9K GBP, assuming my own free labor and work performed under supervision. Actually one of them costs a fraction of that to overhaul but its only a 65 HP engine while the second is a 150 HP O-320. New PMA parts including complete new cylinders are available for both engines from multiple suppliers at reasonable cost, and I cannot imagine I'll need much from the original engine manufacturer within my lifetime. I might have to do one overhaul on each engine, and expect that in 25 years the aircraft will sell for current value plus inflation with them attached.
What would the plan be, and how much would I pay if I currently owned a mid-time Rotax 912 instead, and had the same 25 year (plus 100% margin) time horizon to maintain my solely owned aircraft before resale? The Rotax needs to be airworthy the whole period, and be serviceable for resale at the end of the period.
I know a guy who flies a 912 that is approaching 4000 hours and is yet to be overhauled. This particular engine is an early example from the beginning of nineties, when the TBO for 912 was 600 hours.
I received an estimate from Rotax representative a month ago. Without seeing the engine they estimate that if there are no surprises lurking inside the factory overhaul for my engine should be around 8,500 - 9,000 euro. New engine is 14,000 Euro. I really do not know if the overhaul is worth the trouble. The new engine would give me another trouble free 2000 hours/15 years, while the overhauled one would still be 1500 hours 12 years.
Indefinite lifetime for an engine? Sounds interesting. However, there must be a reason why Lycoming will only take an exchange core which had no more than definite amount of overhauls on it.
However, there must be a reason why Lycoming will only take an exchange core which had no more than definite amount of overhauls on it.
The main reason is that the crankcases often need skimming on their mating surfaces, and after 1 or 2 skims they can't be used anymore because they are outside the limits. Every time you skim them (machine a bit of metal off the mating surfaces) you have to re-bore the central hole for the crankshaft bearings (etc) and eventually you can't do that anymore.
The crankshaft and conrods and AFAIK the other steel parts have no life limit, provided they are within limits wear-wise.
The problem with exchange engines is that everybody plays the same game: you overhaul your own engine over and over, and you give it away for exchange only when it is totally shagged and beyond economic repair. Also a lot of people send in engines which have been shock loaded, without declaring it. That's why Lyco etc need to set some limits on what they take back, because a lot of it is junk they can't use. I looked at an exchange engine when I was doing the SB569 crank swap and most of the ones I saw were around 5000-10000 hours TT. Apparently this is normal.
Silvaire, are these costs for certified or non-certified engines? I have been told the pricetag on new certified 912 engines would be far north of 25k€.
Yes, there's quite a difference between certified and non-certified. Apart the technical aspects, that is. A certified one would not meet Silvaire's requirement anyway, having a max. certified age of 12 years.
Back to the opening question and its mention of "professional" use: the manufacturer has the name "B_ Recreational Products", that should say something I think. These engines were never conceived for professional use. That their inherent good quality does allow it may have been a surprise to the makers, too...
@Sadowsky: about the power/weight ratio, I was comparing to car conversions and to the Jabiru 4-cylinder, one of the few realistic alternatives. I think it is lighter than the 912, with no reduction gear and no coolant liquid; but it must make up for that with a larger displacement. Checking the specs, both claim 60 kg "dry" weight, but the Jabiru is, by its nature, "drier" than the 912...
They are currently 42 and 67 years old, one never major overhauled and running just fine, the other overhauled twice so far and likewise a good engine.
Is this true for the majority of cases though? Most Lyco/Continental engines of any age I've known about are really a bit like Trigger's broom - "50 years old, had five new brushes and four new handles" as it were. (Perhaps at most they still have the same crank case).
