Cylinder head cracks - how dangerous?

Compression checks might not find a crack that has a safety implication. We started blowing exhaust gaskets on one cylinder – at first I thought it was because the flange on the exhaust was warped (I’ve had that happen on other aircraft), but when we removed the exhaust we found that on the cylinder in question, one stud came out including the helicoil from a prior repair (no mention of that in the log books – it must have been done before I owned the plane). A dye penetrant inspection found a crack coming from the stud hole and it’s likely that given time the stud would have worked completely loose leading to quite a severe exhaust leak. None of this would be detected by a compression check or looking in the spark plug holes. Fortunately the symptoms were something that manifested itself as something that required us to take the exhaust off anyway so we found the real problem.

This recent report from French BEA is relevant: https://www.bea.aero/fileadmin/user_upload/BEA2018-0656.pdf . Sorry French only.

BEA2018_0656 local copy

The article Xtophe relates to not to a cylinder head crack, but a cylinder base barrel failure…
Nevertheless, though I’m fluent in French, I gave the translation of the conclusions (see, easy translation ) to the internet, here the results in English for you:
3 – CONCLUSIONS
The conclusions are only drawn from the information that the BEA became aware of during the investigation. They are not intended to determine faults or responsibilities.
Scenario
During a local flight off the south coast of Brittany, one of the engine’s four cylinders failed while cruising. This rupture is the consequence of a phenomenon of fatigue cracking of the steel barrel of the cylinder. The condition of the break seems to bear witness to an ancient process, although it cannot be dated. At the level of this break, the only singularity noted is the presence of corrosion. The pilot succeeded with the residual power to reach the coast. The engine then stopped and the pilot landed on a beach. During the landing roll, the aircraft rolled onto its back and came to rest in the water.
The lack of leak rate measurements recorded during the last two 100-hour maintenance visits did not allow the possible detection, during the visits, of an anomaly on the ruptured cylinder. In addition, the maintenance data available does not mention an annual search for corrosion for the F-GSBK due to its operation in a saline atmosphere.
The safety investigation could not determine the factors that contributed to the damage phenomenon which led to the complete rupture of the cylinder in flight

Thanks.
Leak rate measurement means compression tests.
The study suggest it could have been detected with compression tests. I would have favored oil analysis (my 0,2 cents).

This guy got something similar. I don’t know if @italianjon is still around.

I occasionally lurk ;)

Life’s got in the way of lots of forum posting.

My cylinder incident did on post incident inspection, seem to show an external influence. The way the vanes failed seemed to be consistent with a high speed object striking it, and the crack starting on the outside of the vanes.

I’ll see if I still have some photos.

Vanes meaning cylinder fins?

Thanks

Cylinder head cracks – how dangerous?

Define “dangerous”? Too many factors, the first one being how many cylinders are left to do the work…

Here’s a little bedtime story :
One of the first problems I’ve had with aircraft engines was one of those “dangerous” cylinder head cracks.
Once upon a time, my flying buddy Jack† and myself took our little French SIPA 903, Continental C-90 powered, to the PFA (yep, called the LAA today…) rally at Kemble. The year is 2004, the day a Friday.
Flew across France and headed across the British Channel/French Manche towards Bournemouth. Just having transited their airspace, a loud banging noise from the engine with heavy vibes… no good. Revs down at full power, not possible to hold altitude. Since I was PM (pilot monitoring) on this day, or assisting pilot, or whatever it was called in those days, I placed a PAN call with Bournemouth whilst diving into the NRST function of our portable Garmin GPS.
Popped into Popham (sic) some 6 minutes later for a safe landing on the welcoming green turf
Prior to the landing, ATC had us go to 121.5, and after landing I had to use the good service of a passing BA flight to relay our safe outcome.
Nevertheless, some 20 minutes later, whilst we were already assessing the damage with engine cowls opened, an armada of giant fire trucks, no sirens but plenty of blue/red lights, rushed on the field. Notwithstanding the trucks we discovered the crack in the cylinder and started inquiring with the locals as to whether the local Engineer might turn up tomorrow, on a Saturday. Finally it dawned on me and I stopped one of the fire trucks to ask them what it all was about: “we’re looking for a light aircraft that might have crashed in the surroundings”… Once I got to explain that we had been the target and we were alright, they left. I still chuckle at the situation
Saturday turned up to be a beautiful and lucky day, the Engineer showed up, and had, pure coincidence, 4 cylinders just removed from a C-150 engine a few days before. Picked the best one to lend it to us, and installed it on our mighty craft in a couple of hours. And we made it to the fly-in at Kemble!

Dangerous? Had it failed, either partially or worse with full separation, some 20 minutes prior to reaching the coast… well, I guess my guardian angels were just getting warmed up for future adventures 😎