Rotax 912 valve spring retainers failing

[Jerrytex]

https://data.ntsb.gov/.../GenerateNewestReport/102076/pdf (https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/102076/pdf?fbclid=IwAR3G8VO7k6Ipj1T2Su0p7nwRToNWNojsSV94P 6-eui5JMD0JGjZb74KEbyg)

Ran across this report and was wondering if anyone has any additional info of this?

There have been 18 failures of valve spring retainers. 7 to 2000 hours. This recent one brought the plane down, thankfully no one was hurt but this is concerning since I can't seem to find any info on service bulletin info, other than a proper oil purge, which according the this NTSB report may or may not be the issue.

[Shadowrider]

Interesting! If its improper oil purging, its crazy that they build oil pressure and are running but have air in the line somewhere? I cant understand how that is possible?

[Eric Page]

The most surprising thing that I found in that document is the 40 degree maximum bank angle limit for an aircraft that's equipped with a Rotax 9-series engine. It would seem to me that this limitation makes any Rotax-equipped airplane unsuitable for flight instruction. How do you practice steep turns, turns around a point, stalls, spins, chandelles, etc. without exceeding 40 degrees of bank?

Frankly the limitation doesn't make much sense. As long as you maintain positive G, the engine should be oblivious to bank angle, right? What am I missing?

[Shadowrider]

Yes good point Eric. If that where the case I think I would of flamed out 200 times by now.

I do remember watching a video from Lockwood about the 912 and if you don’t purge your oil system correct at about 80 hours you will have issues if I recall which is very surprising it takes that long to show up? I will see if I can find it.

[alexM]

The most surprising thing that I found in that document is the 40 degree maximum bank angle limit for an aircraft that's equipped with a Rotax 9-series engine. It would seem to me that this limitation makes any Rotax-equipped airplane unsuitable for flight instruction. How do you practice steep turns, turns around a point, stalls, spins, chandelles, etc. without exceeding 40 degrees of bank?


Interesting observation. From that link there is this:

Review of published guidance materials from some of these manufacturers revealed however that the
Rotax engine bank angle and G limitations were not published in the flight manuals or pilot’s
operating handbooks, and in many cases, the maximum published bank angle limitation for the
aircraft was 60°, which exceeded the Rotax published limitation.

That could impact flight training significantly.


Frankly the limitation doesn't make much sense. As long as you maintain positive G, the engine should be oblivious to bank angle, right? What am I missing?
For that answer we need our friend (or enemy), trigonometry.
A 60 degree bank (while holding altitude) with 2G will have a 1.73G inward pull but it will always have a 1G downward component drawing oil to the lower cylinder head and, more importantly in this case, away from the upper cylinder head.
The 40 degree limitation is really not well thought out. It should say something like "don't exceed 40 degrees continuous bank angle, and do not exceed 60 degrees for more than ** seconds".
Note that these failures didn't occur during flight maneuvers, they occurred in straight and level flight at a different time.

[Shadowrider]

What I don’t get is how the heck do you know if you have air in the lifters? You purge the oil system and prime the pump and build oil pressure but still can have air in the system that will cause your spring retainer to fail hours later? Maybe someone can explain that to me?

[avidflyer]

Alex, I would think that if the aircraft is in coordinated flight, the g force will be straight down in the aircraft, not in relation to the ground. I know there are videos of Bob Hoover pouring water in a glass during a loop. How is a coordinated bank to the right or left any different. Think to of all the small certified planes that can do a loop or barrel roll with gravity feed fuel systems. Maybe I'm all wet, but that's how I see it.

[alexM]

Alex, I would think that if the aircraft is in coordinated flight, the g force will be straight down in the aircraft, not in relation to the ground. I know there are videos of Bob Hoover pouring water in a glass during a loop. How is a coordinated bank to the right or left any different. Think to of all the small certified planes that can do a loop or barrel roll with gravity feed fuel systems. Maybe I'm all wet, but that's how I see it.

That makes more sense than what I said, but then it seems the 40 degree limitation makes even less sense.

[Dave S]

Rotax documentation, on occasion, falls prey to language translation. Regarding the stated 40 degree bank - my suspicion is that might well be intended to be what we describe as "pitch" in english, which would make some sense with the design of the oil tank and the location of the crankcase drains. Coordinated 60
degree bank.....naaaaaaa:rolleyes:

If a person does the purge explicitly per the instructions, the method to test the lifters for air is included - I also suspect that not everyone does that part as it is a bit if a pain and costs some rather high priced O-rings.

I don't understand why Rotax seemingly is not concerned about a total purge after an oil/filter change since removal of the oil filter certainly introduces air into the pressure side of the oil system; and, cleaning out the oil tank also admits air into the suction side. The orientation of the oil filter out the side of the crankcase also makes a place for air to remain after filter change as the oil leaves the filter through the center hole - but since that can't be seen, maybe nobody thinks about that.

My suspiciuon is that the engine can digest and purge some air, but not a great quantity such as what you have when an engine is new or overhauled.

Although it doesn't happen all the time, I am not convinced the air thing is the total picture on this issue. Conveniently, the air issue can more likely be blamed on operator/mechanic error than on engineering if people don't explicitly follow the directions.

I can also see the possibility of periodic inspection requirements of the retainers some day. The documented 18 cases by Rotax over 4 years gives about 4 incidents per year; and, likely more than that considering the entire population of 900 series engines ever built and and possible unreported cases that have been repaired.

But - I like the engine and will continue to fly behind it:)

[Benbell4140]

This is exactly what failed on a rotax powered Rans S7 that I previously owned. Luckily it was on final and he made a flawless landing.


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[Jerrytex]

I don't understand why Rotax has not addressed this? I guess I am going to do the lifter air check that Dave mentioned and hope that Rotax comes out with a SB soon.

[Dusty]

I wasn't aware of the bank / pitch limitation, but air in the 912 is a problem.

I only have experience with one retainer failure but have seen plenty of valve damage, usually related to air leaks in the suction side of the oil system or improper oil change procedure such as starter cranking to get pressure after opening the system.

Hand prime only, preferably with the filter slightly loose to effectively bleed any air before it gets into the pressure systym.

My theory, for what is worth , is that air will gravitate upwards toward the crank journals and only oil, free of air will flow down hill to the cam and lifters.

Starter cranking just feeds foamy oil everywhere due to the speed.
Run the engine up to temp with the cooler covered if necessary then do the rocker test (the "o" rings unless very old will go again)

[Slyfox]

ok I give, what is the rocker test. I'm thinking take the cover off and push on the rocker for air in the lifter. dah I don't know. help me out. When I change my oil I never take any hose off, I remove the tank cover and suck out the fluid. I have the new filter sitting on the floor and fill with new oil, sometimes takes a couple times, oil soaks into the filter. I remove the old filter and immediately put on the new filter. takes very little to purge the system. that's my oil change procedure.

[Shadowrider]

Can a rotax not run on 3 cylinders. If you lose a cylinder it will bring the engine down? Seems like if you lost a valve on on cylinder it should keep running?

[Jerrytex]

The Rocker test:

https://www.youtube.com/watch?v=UwARvpIP9-k&t=266s

About the 9:00 mark he talks about checking for trapped air.

The crazy part is.....if there is air, the fix to "untrap" thee air is to run the engine.....

[Dave S]

Jerry.

One of the comments in the report you posted from the NTSB was that a test was performed to see how long the engine had to run to bleed the air - something like a bit over 6 minutes at idle for that experiment. This is empirical information we don't normally see.

One of the problems with bleeding lifters on a boxer engine like the rotax - the lifters are laying on their sides in the bores. On engines with the lifters oriented upwards the trapped air exits up through the metering valve feeding the pushrod pretty well. With a horizontally oriented lifter, some of the air is above the lifter metering valve and its only way out is between the lifter piston and lifter body which is machined to incredibly close tolerances. Warming up the engine/oil at idle speeds facilitates the bleed rate between the lifter piston and lifter body and is likely the route the remaining air has to go. Thick cold oil does not flow as well. Rotax fit and finish of the engine parts is extremely close compared to many other engines.

Dusty's comments about air up, oil down and easy does it is golden. I guess we are more like plumbers than a person might think!

There is probably quite a difference in hammering stress on the affected parts between idle speed and running at 5000+ rpm. How many of these instances occurred at cruise RPM vs idle - pretty much all and none as well as I can tell?

I think there is a lot more to find out about this - the NTSB report is a good start. I am still not convinced air in the lifters alone is the only factor.

[avidflyer]

Can a rotax not run on 3 cylinders. If you lose a cylinder it will bring the engine down? Seems like if you lost a valve on on cylinder it should keep running?


Running on three cylinders is not so bad, vibrations and such, but if the valve shatters the piston, and the connecting rod bends, ect, ect then the engine probably wont keep the plane in the air