Significant problems with Rotax engines?

since the problems always seem to occur on climbout, with the aux fuel pump ON, our shop’s suspicion is more that the problem has to do with having TOO HIGH a fuel pressure. To me, that sounds more plausible.

Yes, it is at least a plausible theory. The Bing carbs were designed for gravity-fed supply and you can overcome the floats with too much fuel pressure, more so if the floats are damaged. You would then flood the engine and should see evidence of fuel coming out of the overflow, stains etc particularly with 100LL given the amount of blue dye it has.

Too much pressure from an electric fuel pump ? So then the mechanic was an idiot. You can have common Pierburg pumps that do max. 0.5 bar and are used on lots of cars with floats, no problems ever and Bings no different with their floats. *BUT*Bold** these Bings were known for a while for failing floats when they selected unsuitable plastics obviously. Owners were advised to do weight checks and any float exceding the specified weight was exchanged – at own costs. As they say, most carb problems are electric, but I would not think so in the takeoff phase when most electronics are not yet hot. I wonder how long this takes for reading about facts in these stories, lots of rumours only . . . Vic

A mechanic does not specify or adjust the fuel pressure on an aircraft as a normal practice. If the wrong pump or pressure regulator were installed by a mechanic, that might cause a problem as might a faulty component but in general the fuel supply pressure is a function of the pump and fuel system design specified by the airframe manufacturer.

If it were a problem with the floats that would be easily solved because they can be replaced in a few minutes on Bings, it’s a particularly easy job on those carbs.

My experience after 570 hrs on the 912 B23 is indeed that there are instances of overpressure. But only on final appch after switching on the aux pump. I’ve reached the point where I don’t use the pump any more on final, first checking normal fuel pressure of course, because I don’t care for warnings at that point in time.

I have my floats checked every 100 hrs, even though this famous quality problem on them is now years ago, just to be sure.

My little 912 runs and has always run like a sewing machine and, although I look forward to see what comes out of the investigation, will continue to happily fly, with 95 car gas, 5% ethanol and supposedly all kinds of other nasty stuff

The Swiss Transportation Safety Investigation Board (STSB) has published the final report of an incident that happened on the 13th of January 2022, link (German only).

The Aquila AT01, equipped with a Rotax 912 S3-01, was returning from a training session, and following a T&G the engine developed severe vibrations. The FI took over the controls, and manage an elegant return to the departure field, with no damage to the aircraft nor its occupants.

The investigation showed a cracked #2 exhaust valve. Borescope view:

After cylinder removal:

The German text for the next picture (Deepl):
The deposits could not or only partially be removed for further microfractographic examination, which made analysis difficult. The deposits consisted mainly of combustion residues and lead or lead compounds. The defective zone in the valve disk showed clear signs of
of melting. A metalographic section was made through this defect zone.

Report Conclusions (Deepl):
The serious incident, which resulted in severe vibrations and a loss of engine power, is due to the fact that an exhaust valve on one of the cylinders was defective.
This defect was caused by deposits with a high lead content in the combustion area of the cylinder head and explicitly on the valves, which were caused by operating the engine with a fuel mixture with an increased lead content.

Report Recommendations (Deepl):
It should be remembered that, according to the documentation of the engine manufacturer Rotax, the engines should be operated with unleaded fuel such as MOGAS if possible.
If leaded fuel such as AVGAS 100LL is used for certain periods of time, the engine manufacturer’s recommendations regarding shortening the interval for a compression test and an oil and oil filter change must be observed and, in case of doubt, interpreted conservatively.

So 100LL is the cause Probably lots of different causes by the looks of it. But I don’t fully understand the above case. How would this be possible without the engine running rough before it happened? Or did the deposit come from somewhere else in the combustion chamber and accidentally positioned itself on the seat when the valve closed? It’s more a one in a million in that case.

IMO mogas 98 (ethanol free) is s known thing. The only concern is vapor lock, which is easily mitigated.

It’s more a one in a million in that case.

I’m inclined to think that issuing government reports and manufacturer service bulletins as a ‘solution’ has seemingly replaced the idea of finding the real cause for these engineering problems. It says a lot that the ‘cause’ in all these reports is the fuel, any fuel grade selection is wrong it seems, thereby shifting responsibility to the operator. Ass covering versus engineering as an inappropriately unconstrained first impulse.

IMO mogas 98 (ethanol free) is a known thing. The only concern is vapor lock, which is easily mitigated.

Except for its long term total unavailability in many or most of the places where Rotax engines fly, and the ongoing trend towards that being the case worldwide.

Silvaire wrote:

I’m inclined to think that issuing government reports and manufacturer service bulletins as a ‘solution’ has seemingly replaced the idea of finding the real cause for these engineering problems. It says a lot that the ‘cause’ in all these reports is the fuel, any fuel grade selection is wrong it seems, thereby shifting responsibility to the operator. Ass covering versus engineering as an inappropriately unconstrained first impulse.

Indeed. This is a disease, and it’s everywhere. Perhaps just an inevitable result of years of focus on “safety and risk” ? It would be interesting if someone could go through SBs and incident/accident reports over the years in an orderly and scientific manner and show how this has developed over the years.

Thinking about it, regarding fuel related problems with Rotax powered aircraft that I personally have been heavily involved with, I can thing of three cases. This also happens to be all the Rotax powered aircraft I actually have been heavily involved with The common denominator is the Bing carburetor.

1. WT-9 Dynamic used for towing gliders. It had a standard 912 ULS. Now, this aircraft obviously has a very hard life. It’s hard to think of a way to abuse the engine more than towing gliders Nevertheless, the main problem was the carburetors. Problems were rough running for no apparent reason and difficult to start.
Solution: After trying everything with the Bings, the entire fuel system was replaced with FI from Edgeperformance. Solved everything. No fuel related problems ever since, 8-9 years. (Later we also installed big bore kit from Edgeperformance, but that’s a separate thing).

2. Pipistrel Alphatrainer with 912 UL. Started to observe random power/rpm surges. Hard to pinpoint the issue really. To me it was 100% consistent with vapor lock, but then again not really, because the engine didn’t run all that rough. But, mean who has 100s h of experience with vapor lock? It’s a bit like pinpointing a problem blindfolded. The engine looked OK, ran OK when testing it. It was in the spring, the weather getting warmer, and it all really pointed to running winter blend mogas in warm weather. Winter blend mogas is even more susceptible to vapor lock than summer blend. Then it was OK for a while, a month perhaps, and it for sure looked like vapor lock. There was one nagging issue though, the engine was sometimes hard to start, similar to the WT-9. Then all of a sudden it happened again. Dismantled the carburetors, and for sure, the floats were not floating properly (I don’t know how they actually should float, but they were all sitting in different positions in the fuel, which for sure could not be right). Sent the carbs to Edgeperformance for complete service, and the engine has been running OK ever since, and easy to start. A better solution would be FI, but it’s a big job.

3. ICP Savannah. This for sure can be attested at least in part to sloppiness/finger trouble, but the core problem is the Bing. I have never heard about such a problem with a C-172 for instance. This is a high wing aircraft, and it has a main fuel ON/OFF handle that cuts fuel to the engine. Forgetting to turn off the fuel and the engine will eventually get filled up with fuel if left for a week or so. This is dependent on how the filter/heat box assembly is set up. We just mounted a small drain line on the heat box assembly, so if it happened again, the fuel would drain out instead of into the engine. I’m still not completely sure how this is actually supposed to be though. The engine also have to run on idle, and it will need fuel to do it Poor idle tuning? Not sure yet, but it’s nothing that FI wouldn’t fix.

IMO, the Bing carburetors is a problematic issue for sure. They are also extremely fiddly using “bicycle gauge” wires and hardware that binds and bends if over torqued ever so slightly (slightly more than finger tight). Why Rotax keep on using these carburetors is a mystery. There probably are no better alternatives all things considered. Their long time solution is FI anyway.

Silvaire wrote:

Except for its long term total unavailability in many or most of the places where Rotax engines fly, and the ongoing trend towards that being the case worldwide.

I don’t understand this. Norway follows EU regulations regarding this, and there is absolutely nothing that prevents fuel resellers from selling ethanol free 98. In fact, the regulations are made so that it’s beneficial for them to do so, as long as they sell a certain total quantity of biofuel anyway, which they do simply by selling 95. Ethanol is not the only biofuel they can use. Obviously diesel will not work with ethanol for instance, it has to be blended with biodiesel. Shell has since long had their own biofuel, which is a 100% gasoline drop in replacement made from wood. It makes no sense whatsoever for a reseller to blend 98 with ethanol, when 90+ % of the gasoline they sell is 95. But, perhaps in Norway they are open about it and say so (and guarantee it. It’s often the main reason to use 98), while this is not PC on the continent?

Besides what kind of fuel are all the people with classic cars using in, say Germany? Or all the boaters everywhere? It for sure cannot be 95 mogas with 10% ethanol?

This whole fuel issue is so much politics and ignorance and superstition, it’s hard to discuss it in a productive manner.

As to “vapour locks” I´d like to know the positions of fuel tanks, below carbs or above ?? I bet there was never vapour löck on Cessna 172 types as fuel is gravity fed into carbs. On Rotax aircraft tanks are mainly behind passengers I guess so in take-off situations I ´d like to have an electric fuel feed pump on . I think they had Pierburg pumps on Rotaxes in earlier times but no longer ?? With the type in my link below there is definitely never a problem with drowning the engine by too high fuel pressure and nobody has to “adjust” pressures with these. The setting is made at the factory for less than 0.5 bar and the pumps are stuck onto old carburetted cars with their floats , never been problems from that, fit and forget – and these pumps are ON ALL THE TIME , for thousands of hours. On a 300 hp plus engine you may want two of them on the aircraft, one way valves in strategic places certainly. So vapour locks can be excluded definitely when you suspect this to be a problem. Plastic floats in these Bings were problematic for years and checking them is easy and never wrong. But then, even on Lyco carbs they had numbers of failed floats some time ago. The Bings got rubber membranes actuating the slides – you change them in periods, say 4-5 years, buying from mc dealers or Ebay. Otherwise they do their business on motorbikes since half a century already. Starting the engine is a matter of engaging the primer/choke system as was designed, so this may be the root of some owners problems. As to the burned ex valve in photo above, my thinking is unsufficient valve clearance. Lead deposits were never a problem in cars while they had adjuster screws on rockers , so valves hit their seat rings with some ticking easily overcoming/cleaning any lead or dirt on the seat rings. I am very suspicious about automatic hydraulic tappets which may work or may not work as designed. So then one valve may not seat properly and burn with obvious defects. Nothing to do with lead or alcohol , just valve clearance lost. That engine was not old so I do not think there was a seat sinking into the alu head from softness by years of use. More like some flaw in the valve train . Vic

Pierburg fuel pump

In the Stag my wife uses UL98 and an additive which can be bought at most supermarket car accessory places. It’s a very tiny amount of additive per fill up.