Savvy Aviator Newsletter 2004-11

Last issue, we discussed the importance of avoiding high-time cylinder heads because of the risk of fatigue failure. This time, we have some spectacular photos of what can happen if you don't heed this advice.

In the last issue of this newsletter, I talked about the money-saving benefits of using reconditioned or continued-time cylinders rather than new ones when the need arises to replace cylinders between major overhauls, particularly on engines that are beyond mid-TBO. While I've done this numerous times on my own airplane and consider this a prudent and sensible way to save money, I stressed the importance of using only known first-run cylinders -- either by reconditioning your own cylinders or purchasing exchange cylinders from a trustworthy source. Or to put it more succinctly, make sure you don't buy junk.

More often than not, owners and mechanics who install reconditioned cylinders have no idea how much time is on the cylinder heads. All they know is that the jug came with a yellow tag or an FAA Form 8130-3 stating that the cylinder assembly is airworthy. That's not enough. As I pointed out last time, cylinder heads that have been in service more than two or three TBOs are far more likely to suffer fatigue failures, and one of those can ruin your whole day.

Since I wrote that, I've received some interesting emails and photos illustrating just what can happen if you don't heed this advice. Here's an interesting photo that an aircraft owner sent me. The camera is looking up inside a big-bore TCM cylinder that was removed by an overhaul shop from an engine that luckily had just been removed from service and sent in for major overhaul:

The white arrow in the lower-right part of the photo points to a fatigue crack that runs from the lower spark plug boss to the exhaust valve seat. It doesn't take a lot of imagination to see that this jug was a time bomb waiting to go off. Fortunately, the cylinder was retired just in the nick of time.

Another owner of a Lycoming-powered single based in Phoenix, Arizona, wasn't quite so lucky. Before we look at his photos, here's his story:

My wife and I were going to Page for a Breakfast Club event at the Wahweap Lodge. After a normal run-up, we took-off on Scottsdale’s runway 3. At about 500 feet, we turned towards the north. The take-off was normal, the climb-out was normal, at least for awhile.

At about 2800 feet MSL and about 3 miles north of the airport, I suddenly felt and heard a loud bang. My very first impression was that a tire had blown out. But after a very few brief seconds, I felt a slight vibration with an associated power loss. The elapsed time between the bang and my decision to do a 180 was maybe 8-10 seconds. I called SDL tower while in the turn and informed them that I was declaring an emergency, and told them that we heard a loud bang with a power loss.

The controller began scattering planes in all directions like kicking over an ant hill. It was not until I completed the turn-back that the windscreen began to take on a sickening opaque appearance. In less time than it took to think about it, all forward visibility ceased. I could still see out the side windows but nothing out the front.

As I reached the departure end of runway 3, we were still plenty high with a slight overshoot. A sharp bank to the right brought the white stripe that is along the side of the runway, into view. From there, it just a case of bleeding-off airspeed and trying to maintain our position over the runway as best we could.

Eventually we landed, and as we were rolling-out the engine finally wound down and stopped. I had enough momentum to make an exit on Alpha 4 and stopped just across the “hold short” line. The tower called to inquire if we needed fire equipment but I told them that it was a cylinder issue but ”I could sure use a tow”.

It wasn’t until my wife and I opened the cabin door that we began breathing again. As best that I can guess, the elapsed time between the big bang and clearing the runway was maybe 3 minutes ... but it seemed like 3-4 days.

From the position and altitude that we were in at the time of the incident, I believe that most everyone could have returned to the airport, but the thing that was the most difficult was not being able to see anything out the windshield. I don’t know how you can train for that eventuality but I truly believe that God had other plans for us and it was not yet "out time."

When the cowling came off and the owner surveyed the damage, here's the horrifying sight that he saw:

This is a classic head-to-barrel separation, caused by high-cycle fatigue failure of the cylinder head. Here's a slightly different view:

Yes, that's the top of the piston you're looking at. And here's yet another view looking forward from the aft side of the failed cylinder:

Come to think of it, perhaps this owner's timing wasn't so bad after all. Had this head-to-barrel separation occurred 10 or 20 minutes later, the outcome might have been a lot more serious.

Thanks to these owners for sharing their photos. A picture is truly worth 1,000 words, and these demonstrate that cylinder heads have a finite fatigue life better than any technical disseration I might offer.

Don't let this happen to you. Make it a point to know the history of the cylinders on your engine, and make sure that those cylinders are retired after they serve for two TBOs, three at most. Using reconditioned cylinders is fine so long as you know their history. But if you buy an exchange cylinder of unknown heritage, the head casting might just have been in service 5,000 or 10,000 hours, and you might have a fatigue failure waiting to happen.

So don't hesitate to send out your own cylinders for reconditioning, or to purchase an exchange cylinder from a reliable source that you trust to provide you with a first-run jug. That's a savvy way to save money, particularly when you need to change a jug on a high-time engine. But beware of overhauled cylinders that have an unknown pedigree. The fact that the jug has a yellow tag or a Form 8130-3 tells you only that the cylinder is dimensionally within service limits--it tells you nothing about how old the head casting is or how many hours it has been in service.