Thursday, August 06, 2009

AD2009-16-03 SAP Cylinder Cracking

AD 2009-16-03 results from reports of cracks in the area of the exhaust valve and separation of cylinder heads from the barrels of SAP cylinder assemblies with certain part numbers.

So now 8,000 of you (8,000 engines effected) need to inspect your SAP cylinders for head cracks per AD2009-16-03. Two inspection methods are presented in the AD:
  1. Visual inspection for combustion staining, and
  2. Pressure test.
After spending 30 years running a cylinder overhaul shop I learned one thing is certain: cylinders crack. No surprise here. Cracks in aircraft cylinders are not limited to any one manufacturer - all have had problems with cracks and will continue too given the hard work we ask of them. So even if your airplane is not directly affected by this AD, checking cylinders for cracks is important and can be incorporated into your normal inspections without much additional time or expense.
Lets discuss both of these inspection methods and introduce a third method of my own making:
Both inspection methods presented in the AD only detect cracks that extend completely through the wall and large enough to pass gas. That is one big crack and on the verge of outright fly-apart fracture.
Lets look at a cylinder for what it is: a pressure chamber that gets pressurized and de-pressurized with very hot corrosive gasses 20 times a second. If a flight is 1 hour long you have one "thermal cycle" but 72,000 pressure cycles!
Pressure test:
There is nothing unusual in the requirement that if during a compression test, gas is leaking from the pressure chamber (and it's not going past the valves, and, its not going past the rings) then you need to start looking for cracks in the cylinder head. Well that's something you should do anytime you suspect your cylinder has a hole in the head and won't hold air. Only inconvenience is you have to do a compression check each 50 hours per the AD.
Staining Inspection ("black combustion leakage")
This inspection is much more interesting and anyone can do it with just a mirror and small flashlight.
Pressure Vessel Breach
The picture above shows staining from gas leakage out a crack in a Lycoming cylinder. At each 50 hour oil change you can look at the fins for staining. It just takes a few minutes of eye-ball time. This crack didn't get detected so lets see the end result:

So there you have it - spontaneous cylinder head separation. Notice the gas staining in the red circles. There was plenty of advanced warning. Inspection for exhaust staining on the outside fins of a cylinder should be a normal inspection item.

Gas Staining - pressure vessel breach

Lets take a more hidden example: See anything wrong with this O-200 cylinder below?

Well, you're not going to at this angle. You're not looking at the right angle nor in the right places. There's gas leakage and a crack right there between the yellow lines. Here is how you should look at the cylinder - look between the fins! That's where the crack penetrates - not at the end of a fin but at its base. Same cylinder but different view:

See the dark staining between the fins. Do you see the crack?

Back in the 1980's we did some experiments with gas or oil leakage, especially leakage between the barrel and head. We'd take cylinders that had oil and gas stains and pressurize them with 80 pounds of pressure to see if they leaked. Our experience was that they didn't leak in our test. Certainly if the crack is large enough and open it will leak but cracks like the one above would not leak air when pressurized. Our theory is that our test is at room temperature without the strains induced during the actual combustion cycle. Only cracks at their last stage - when they're big, large, and about to come apart, leak gas. So the compression test and soap and water is the final frontier - gas and oil staining come first.
There is another test that we used and that is the ping test. Here is how it originated: I got upset spending shop time cleaning cylinders and then checking them for cracks only to find out after 2 hours of labor that the cylinder was no good. Better to find the obvious crack before investing any shop time. Once a cylinder passes the ping test then we would further clean and inspect for less obvious cracks.
Take your finger nail or the plastic end of a pen and ping a cylinder fin. It should ring. If it goes thud then there could be a crack at the base of the fin. You can even take your pen and just stroke it down the cylinder head. The link below is a sound file so you can listen to the sound a crack makes! This doesn't work for fins that are in contact with baffling, they have to be free to ring like a bell.
Additional Information:
I have a E-Book available (windows only) at my web site that you can download called "Crack Detection Using the Unaided Eye"
Visit for more articles and software for mechanics

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