2nd opinion: Compression ok on a cracked cylinder?

[keepup]

Not trying to run this guy amuck at all cuz I have no experience with compression checks on cracked aircraft engine cylinders but is it really plausible that this guy's compression checks could have been normal?

http://flash.aopa.org/asf/pilotstori...WT.mc_sect=sap

[Andrew M]

First of all I couldn't pull up the link. But I can speak about looking for cracked cylinders.
Some one put out some iffy cylinders that had an AD against them, and although the compression test was part of the inspection, it was not the only thing that had to pass. In addition, the pistons was brought up from BDC, pressure in the cylininder, and soapy water applied outside looking for cracks. Also be aware that what a compression test determines is how well the running parts of the cylinder are working. It "might" find a crack. Internal pressures can be up arround 800psi (I think). between that and the temperatures, the joint between the steel cylinder barrel and the aluminum head are severe. The cracks that could develope may not leak much until it lets go; Bright light, vanity mirrors, look for exaust stains or fretting cracks. Nore often than not, it's from a spark plug hole.

[apkp777]

Thanks for sharing that. Yes, it is plausible that the compression check come up good. Most likely the crack began relatively recent to his incident. Most A&P's and IA's are going to focus attention at the cylinder barrel area for obvious indications of a crack. I suspect at the last annual it was fine.

I don't question his decision to turn back, I would have done the same. I am surprised that he made a right turn. He turned toward a park (trees). He ended up making a right 270 and then left 90 or 360 degrees of turning. Wow, in a Mooney to boot. If he'd have turned left he would have made a 180 degree turn and another 60 to line up with RW30 or about 270 degrees of turning.

A great lesson for us all.

[Experimental Chaos]

Years ago, while working as an A&P, I did a compression check (I think it was on a 200HP Piper Arrow) the compressions were all normal, good in fact. But I could see exhaust stains between the cooling fins. Bore scope and subsequent removal of the cylinder revealed a crack running from spark plug hole to spark plug hole. Sometime after this there was an AD issued calling for changing the mag timing from 25◦ to 20◦.

[rocketbob]

I had a crack in one of my cylinders last year (classic crack from the exhaust port seat to the lower spark plug hole.) Compression checked fine and I caught it during a borescope inspection.

[Canadian_JOY]

When I took my engine in for overhaul it was still making fantastic compression on all four cylinders and didn't have any signs of exhaust staining on the heads. All the jugs were pulled and the insides of the heads bead-blasted. Imagine my surprise when one of the cylinders showed a crack between the exhaust valve and spark plug! BTW, when the head has been bead blasted the metal is bright and the crack shows up as a very distinct black line, visible to the naked eye without difficulty.