Rotax 912 valve spring retainers failing

[Jerrytex]

https://data.ntsb.gov/.../GenerateNewestReport/102076/pdf

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

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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