How is that related to engine make or even piston vs turbine?
Paul‘s video is great, however only a part of it is about piston engine reliability, the overall theme is about why engines quit, which is a different topic.
The Federal Aviation Administration (FAA) was quoted as stating turbine engines have a failure rate of one per 375,000 flight hours, compared to of one every 3,200 flight hours for aircraft piston engines.
Rotax delivers the engine. The installation is done by the aircraft manufacturer. This includes such things as radiators, cables, hoses etc + eventual thermostats and eventual vanes and so on. It’s a whole bunch of stuff. Not all aircraft manufacturers are good at doing that.
Also, all the hoses etc have to be replaced every 5 years. On non certified aircraft, high quality hoses can typically be replaced by cheap and poor quality stuff.
A whole lot more can go wrong, and it has nothing to do with the engine as such. A well maintained good installation will run for ages. A poorly maintained bad installation can, with a little bit of luck, become only as good as a Lycoming 
Rotax delivers the engine. The installation is done by the aircraft manufacturer.
That system was cleverly used by Diamond to avoid the company going bust, when Thielert engines started failing. Diamond walked away from liability, saying they are not responsible for the engine
And Thielert blamed Diamond for the failures, saying the coolant system was badly designed by Diamond I remember talking to one of their reps at some exhibition.
I don’t think anyone here knows the details of the contracts between Diamond and Thielert. Thielert may very well have been responsible for the installation in a certified aircraft. After all that is the normal procedure when installing more complex stuff, and clear lines are drawn where the liability of one part ends, and the other part starts.
Most Rotax engines are non certified, and sold off the shelves. The installation is done according to the Rotax Installation Manual.
LeSving wrote:
This includes such things as radiators, cables, hoses etc + eventual thermostats and eventual vanes and so on. It’s a whole bunch of stuff.
That’s why Rotax will always be much less safe than Lyco/Conti.
You have NIL cables/hoses/radiators/car style ignition harness on O-200/O-235.
As well as stupidly placed carborators on a rubber seal just above red hot exhaust pipe.
That rotax hose stuff is to do with the installation. You can absolutely install a Rotax with the same hoses from a TSIO550 in a new SR22 and they will last the same amount of time.
I looked into this in great detail when helping restore a Rotax powered plane recently. All the coolant hoses were replaced with silicone hose that will never need replacement. Fuel hose I got the highest quality R9 spec hose I could find. Same for oil hose, then firesleeved it + added heat shield where necessary.
Because Rotaxes are used almost entirely in homebuilts, the installations are highly variable and not everyone knows how to look after the engines properly. I agree the carbs and mounts are a weak spot, but it’s a manageable issue.
Also once you look at the sheer state of the lyco/conti installs on a typical club aircraft, especially some owned by local UK schools (who carry out their own, notoriously crap mx) you will become suspicious of everything you fly behind.
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.
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.
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.)
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.
