“Using cheap hoses just because they look the same as the original hoses is a bit like fitting a chocolate pin to a hand grenade. There is a very good reason that Rotax & the airframe manufacturer puts a life on hoses.”
Even for a Rotax 912 in a ULM this is stressed in the POH, or at least it is on the Super Guepard.
I am a life long sceptic on these sort of statistics without being able to study the methodology behind them.
Here there is a largish market these days whereby owners of certified Rotax powered aircraft will, when the engine reaches TBO or when a 2nd TBO becomes due, sell the engine into the ULM or experimental. I wonder how these figures have been treated in these statistics.
Without the methodology its database we could define our own statistics. As an example of this I would invite you to look at engine failures over the last few years on this very forum.
You might well get the impression that Cirrus engines are the most unreliable. Are they? Or is it because the reports are more publicised. As in the media are fascinated by the fact that a aircraft can safely return to earth under a parachute, whereas the Bonanza or PA28 which a pilot manages to land safely back on the airfield or even some other field, gets no attention at all.
Experience with Rotax engines installed in Sportcruiser aircraft has illustrated that the MAJOR reliability issue is the quality of the flexible rubber hoses.
Using cheap hoses just because they look the same as the original hoses is a bit like fitting a chocolate pin to a hand grenade. There is a very good reason that Rotax & the airframe manufacturer puts a life on hoses.
Silvaire wrote:
Air cooling is a good solution for light aircraft, especially if you want an engine that is ‘industrial’ is its ability to operate without much attention, and accordingly doesn’t have much fussy ancillary hardware. I am happier with that kind of engine for my service in my aircraft.
Most Rotax engines are non certified. For a certified installation (of any engine), part of the certification is to make sure the engine parameters are within specs under normal operation. This is obviously not required for a non certified installation. Some manufacturers still do a good job, others don’t. This translates to a certified installation (of any engine) can be operated without much attention. If special attention is needed, this is duly communicated in the POH. For a non certified installation, the default state so to speak, is to always pay attention to what is going on with the engine. It takes longer to know the aircraft.
if you want all the “hot” bits to remain more or less the same size, you have to provide water cooling.
That’s not actually correct. I have for example several air cooled motorcycle engines that run under 0.0015
inch (0.03 mm) diametric piston to cylinder clearance when cold and that small clearance is maintained when the pistons are heated and expand. The reason they can do that is that the cylinders are made of the same material as the pistons, and are coated with a very thin layer of hard nickel/silicon to make them last. Purely air cooled aircraft engines aren’t made that way, except for some experimental engines. Production air cooled aircraft engine cylinder technology predates the possibility to reliably hard coat aluminum cylinders.
In principle, the reason for liquid cooling is to increase power density and operating speed, as I mentioned above, particularly in applications where introducing a gearbox on the output is not an issue. Also, for e.g. cars and motorcycles it provides a secondary benefit in noise reduction and ability to idle for long periods in traffic that is irrelevant to aircraft, and it makes implementing digital fuel injection easier because one coolant temperature probe tells the story for all the cylinders.
Regardless, air cooling is a technically valid solution for light aircraft, especially if you want an engine that is ‘industrial’ is its ability to operate without much attention, and accordingly doesn’t have much fussy ancillary hardware. I am happier with that kind of engine for my service in my aircraft.
Well, yes, if you want all the “hot” bits to remain more or less the same size, you have to provide water cooling 
And that introduces a major failure mode. In terms of R&D and “engineering for reliability”, GA is roughly in the 1970s
and back then, when you drove along any major road, the roadside would be lined with these, with steam coming out of the engine compartment Especially the uphill parts.
In cars, this has been solved, cheaply, with lots of engineering, and in GA it has been addressed with varying degrees of success, and done better on say a DA42 than on a Eurofox (better quality pipes and fittings).
LeSving wrote:
No. All this stuff is there to: Increase reliability (by optimizing the core engine running parameters)
Are you saying that having more than ten separate fluid hose sections connected with engine, radiators etc. with clamps – where failure of even one hose or clamp makes the coolant or oil go away – means that they wanted to increase reliability?
I think – they had to do it – otherwise the engine would be either much heavier or produce half of the current power in order not to overheat.
Since I maintain my own homebuilt aircraft, I spend a lot of time reading about engines, and as much as I can about what happened when an engine fails.
Many engine failures, as we all know, are the result of bad or inadequate MX. What’s interesting is that none of this is new, and I believe we have not done a good job of sharing information about the frequent causes of engine stoppage with those that need it, like A&Ps and pilots.
Catastrophe engine failures do happen, and are thankfully rare. A more common event is a bolt or fitting coming loose or breaking. Some installations are just bad, and something comes loose, or rubs against something and fails. If it’s a fuel line or oil line, you’ll be on the ground quickly, and sometimes on fire. Neglected MX, like not cleaning the lyco oil suction screen, has killed people.
Every mechanical device has its limitations, and it’s important to understand them. Engines vibrate and move – account for that fact. Some systems need regular maintenance, like mags. Wires harden and break. Some hoses have limited lifetimes and will collapse or degrade and block passage of the fluid. Filters need changing or cleaning.
If you are paying someone to maintain your engine, be sure to get someone good, who likes to learn. There’s a huge difference between the good mechanics and the bad ones.
IO390 wrote:
Because Rotaxes are used almost entirely in homebuilts,
They are? I thought most UL’s and LSA’s use Rotax engines? (In Europe at least.)
Water cooled heads are used on the Rotax 912 etc to support higher power density, in other words to get the heat out of a small cylinder head that’s making more power and waste heat than it otherwise would because it’s running twice as fast and firing twice as often. That reduces weight at the expense of complexity, which does indeed reduce reliability somewhat. The higher rpm and higher power density also requires the reduction gearbox, which likewise increases complexity but has less potential than the cooling system to be screwed up by the airframe installation design or hamfisted mechanics, and twin Bing motorcycle carbs which have lots of reliability issues unless watched carefully. Fuel injection should be the best thing ever for four-stroke Rotax reliability… if it were done right.
The upside of the 912 other than power density is that the same relatively small Nikasil lined aluminum cylinders running at high rpm can tolerate poor quality fuel, and they don’t burn oil. It’s an engine that is well designed in terms of thermal and mechanical stresses (I’ll overlook the somewhat nutty built up two stroke style crankshaft because it seems to work OK here) but it requires somebody to be watching over all the external ‘stuff’ in order to be reliable over calendar and operational time. In that regard it’s not particularly fault tolerant and requires maintenance to preserve reliability.
So in my view a 150 HP Lycoming like my O-320 while perfectly adequate in terms of thermal and mechanical stresses probably has less margin in those respects than the less powerful Rotax, meaning it isn’t as likely to last 4000 hours if you were to completely ignore TBO. And the Lycomings large steel cylinders will always use a little oil, and power density will always be lower. On the other hand by comparison it’s pretty much a set and forget engine, it doesn’t need much attention over its lifetime to be reliable and its also very fault tolerant. Those Ill maintained Cherokees that @IO390 mentions don’t fall out of the sky often. My O-320 has gone 51 years since manufacture and has never been apart. The mags and spark plugs are really the only areas that need periodic maintenance, the plugs mainly because we can’t get the 80/87 fuel for which the engine was designed.
Raven wrote:
That’s why Rotax will always be much less safe than Lyco/Conti.
No. All this stuff is there to:
As with everything else, it’s the weakest link that breaks the chain. The thing is, some aircraft manufacturers are good at this, some aren’t. Some owners do not maintain these items as they should.
