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Do your purging properly


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Corey, I read the attached document.  Focusing on the main event, ie, the 912-equipped Tecnam that had an engine failure, I find it very hard to believe that the valve spring retainer would fail due to "air trapped in the hydraulic tappet".

As we know, air in the hydraulic tappet would result in reduced valve valve travel and reduced valve spring compression (ie, the tappet gets "soft").  Consequently, a soft tappet results in a reduced load on the valve spring retainer, not an increased load that might exceed its design limits (the engine might not run to full power given reduced valve travel but that is a different issue). So, air in the tappet is an unlikely cause of valve spring retainer failure.  

Further, the retainer has little or no movement relative to (i) the split cotter that secures the retainer to the stem of the valve (when the valve spring is in compression - as it always is on an assembled engine) or (ii) the valve springs themselves.  So, it is not likely to fail rapidly in the setting of alleged oil starvation since other parts in the engine are much more dependent on a steady flow of oil and would fail much sooner in such a circumstance than would the retainer.  

Overall, the report was unconvincing regarding the series of 912 series engine valve spring retainer failures on which it commented.  To suggest that they were due to the introduction of air into the oil system (during flight) appeared to me to be speculative and unsupported by actual evidence.

Thanks for sharing.


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It is my understanding from Rotax training that when there is air in the lifter that it creates slack in the valve train, The slack allows the valve to slam closed instead of closing smoothly on the ramp of the camshaft. The slamming of the valve creates the stress. At least that is what I recall.

I remember reading about a similar issue with aerobatic Lycoming engines and valve failures. A standard practice when you do a compression check, and there was leaking past the exhaust valve was to stake the valve. This involved tapping it with a hammer to let it close rapidly smashing any carbon build up that was keeping it from closing properly. They started having valve failures after this procedure had been performed.

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I admit I have always thought it strange thereasons we are given, but there is one I can believe:

Proper inflation and deflation of the tappets are important to deliver oil to the cylinder head. The oil is absorbed into the tappet, an while it is being compressed, the oil is pushed out through the crown and into the pushrod bore, and up to the head. From there, It enters the rocker arms and gets sprayed onto the valves and lubricates the rocker bearing. A lack of lubrication wild cause all sorts of havoc.

in addition, cylinder 1 exhaust is the last tappet to get oil. the oil goes from the front of the engine, past 2 and 4, around the back, past 3 and onto 1. So i can believe cylinder 1 being oil starved easily.

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Tom, I can see how a soft tappet - and resulting slack in the valve train - might batter the face of the valve against the valve seat in the way that you described (depending on the cam profile).  Still, if the valve spring retainer is fracturing from such slack then I have to wonder if  the retainer was designed with sufficient strength.  

Corey, hydraulic tappets, in steady state operation, have almost no oil flow into and out of the very small volume of the tappet.  It is hard to see that the oil in the tappet is the source of lubrication for the top end (pushrod to rocker, rocker to valve stem tip).  I have no doubt that a lack of lubrication to the valve train is undesirable.  I fail to understand how it results in failure of the valve spring retainer before other parts fail,  in particular.    

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I thought that too at some point but never followed up. You got me thinking about it again. One thing is true though: The pushrod tube itself is the oil return, the pushrods are the oil supply and are fed via the tappets. There is no other possible way for oil to get to the heads, and I know for a FACT that oil goes through the tappets, pushrod tubes, and rocker arms.

So I went down the rabbit hole. Unfortunately I know of no rotax documentation that has this, but I know continental does. I realized it doesn't have to compress to still allow oil through; and confirmed there are calibrated bleeds in the tappet that supply the oil through the pushrod tube. This seems to be typical of quite a few pushrod lubrication systems.

From the TCM Continental Engine Lubrication System document:


Oil is supplied to the hydraulic lifters through oil holes in the crankcase. These holes align with the
groove in the body of the tappet. From the tappet sockets the oil flows through the hollow pushrods
to the rocker arm passages to lubricate the rocker bushings. The valve stems are lubricated by an oil
spray (mist) from the rockers. The oil returns to the crankcase from the rocker boxes through the
pushrod tubes, then back to the sump through the open center of the oil sump mounting flange.



Hydraulic valve tappets transfer oil from the main oil galleries to the cylinder overhead. Oil flows
through the hollow push rods to a drilled oil passage in the rocker arms. Oil that flows through and
exits the rocker arms lubricates the valve stems, springs, rotocoils and retainers. The oil then falls
to the lower rocker cavity returning to the crankcase and sump through the push rod housings.


I don't have any such diagrams for rotax or internal lifter drawings, so you have to draw your own conclusions, and see that rotax parts have the same holes and configurations.

Here is a rocker arm, notice the bore inside of the bushing to lubricate it, and the bore where the pushrod is.


Here are the pushrods. Notice the borehole.



The tappet with the oil supply bore shown in the middle.




Anyways, I bring this up because if a rocker arm gets insufficient lubrication, I could see the rocker getting a little sticky, as well as the pushrods having trouble sliding around in the cups. Under the right conditions, I could see the exhaust valve having trouble closing until the cam has rolled significantly, then slamming shut and putting a lot of stress on the spring retainer when there is enough pressure to unstick the rocker arm.

As an additional point, flat tappets and pushrods are designed to rotate in operation. If you ever pull a flat tappet off a continental, lycoming, and if I recall, a rotax (but I might be wrong on that), and see the face only wearing in one spot instead of a circular pattern as the cam lobes are slightly offset to impart a torque on the tappets, something is very likely seriously wrong and requires investigation, and failure is impending. Same with the rods.

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  • 3 weeks later...

The referenced NTSB accident reports states  "Review of the engine manufacturer’s published guidance revealed that air could be introduced into the oil lubrication system through several means, including exceedance of the maximum bank angle limitation of 40º,...."

The Rotax SI on purging says nothing about bank angle.  The Rotax Installation Manual says nothing about bank angle.  The Rotax Operators Manual says:

"Bank angle - Deviation from bank angle:  Max 40° Note Up to this value the dry sump lubrication system warrants lubrication in every flight situation."

The Tecnam P92 (accident aircraft) Operators Manual approves bank angles not to exceed 60°.  Flight Design CTSW Operators Manual approves bank angles not to exceed 60°.

FAA Practical Flight Standards for Sport Pilot call for Steep Turns of 45° +-5°.

How is it that the aircraft manufacturer speaks of angles of 60° and Rotax says something about 40°?

What does Rotax mean when it says deviation from bank angle?  How does one deviate from a bank angle?  Is Rotax talking about uncoordinated flight?  If so, if one is in an FAA 45° turn, what does one do to "deviate" 40°?

Note that Rotax did not use the wording NTSB did.  Rotax talked about a deviation from bank angle but NTSB talked about a 40° bank angle which we assume based on everything we've ever been taught means referenced to the horizon.  Rotax did not say reference to the horizon - it said reference to the bank angle.  I'd like to find a German language Rotax Operators Manual to see how it reads.

My bottom line is I do not trust the NTSB observation on 40° as a factor in introducing air into the hydraulic lifter system.




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The "bank angle of 40 degrees" is an utterly pointless things anyways. In a CT, I could roll hard from left 40 to right 40 uncoordinated at high speed and have a high chance of ingesting air, but also roll coordinated and gently into a left 70 and not ingest anything.

In the end, Rotax's oil system design to me needs robustified.

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Attached is the oil tank drawing.

The long rod is the outlet pipe. Its designed to go down most of the way to the bottom of the tank.

Now, the CT has a roll rate of 160 degrees a second (CTLS AOI). That's nearly half a roll in a second. If I am not coordinated, then that oil is going to get slung around in the reservoir much like sloshing fuel in the wings.

If I am applying rudder correctly while entering a roll. I will counter the adverse yaw and while the oil will still move a little bit. It's not going to slosh side to side, and I've never been able to roll fast enough to lift myself out of a seat so I don't think the oil will either.

I don't claim it's a guarantee, jist potentially.

There's also another consideration. CTs are certificated to -1.2 Gs right? If air ingestion is an issue, then rotax technically would be zero. Plus if you go to a negative G, the engine coughs if it's a carb engine, and you start blowing oil out of the breather.


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40 degree Bank limit is absurd. If this were the case no one would use a 912 in most aircraft, you couldn't even get a pilot's license using it. There would be thousands of drive train failures, not just a few. There are certified 912's out there such as in the Tecnam P2006, a twin engine aircraft. It could never get FAA certification with a 40 degree bank limitation. There are a lot of variables in this issue, leaking case pressure, tappet dimensions,  tappet alloy change, vendor manufacturing issue, maintenance issue, oil level, valve sticking issues, is it limited to certain years, the list goes on. Why only one cylinder failure? Why not multiple failures or partial  failures on same engine. This is not something Rotax wants to get wrong, it would cost them heavily.  I'm sure they will look at this quietly before publishing anything. Right now it's easy to blame purging. 

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"Bank angle - Deviation from bank angle:  Max 40° Note Up to this value the dry sump lubrication system warrants lubrication in every flight situation."

This is the only place I can find as a source for NTSB statement.  But this does not say reference the horizon.  We're all reading reference the horizon into it because none of us can visualize what Rotax means when it says 40° deviation from bank angle. 

What is absurd is NTSB drawing a conclusion from this confusing statement.

The bottom line of all of this is Rotax gets off the hook by implying it was a human failure in purging.

I'm still looking for a German Rotax Operators Manual and can't find one.

Nota Bene:  I asked Brett Lawton of LEAF and at his suggestion I submitted a CSRI.  I'll post their response.  


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6 hours ago, Jim Meade said:

I'm going to have to read your two last posts a couple of times to see if I can make sense of them.

In the meantime, I'm still searching for a German language Rotax Operators Manual to see if the 40° comment was mistranslated.  

In summary, I am saying in really crazy abormal movement could slosh the oil around.

But basically I have to TRY to do it.

I could do a coordinated 80 degree bank and the oil will sit in the bottom of the reservoir just fine and the system should continue to operate.

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