Undoubtedly, the most common maintenance event for piston engine airplanes is the oil change. It’s one of the items listed in FAR 43 Appendix A  as Preventive Maintenance, so an owner/operator with a pilot’s certificate can legally do it. It’s actually a pretty simple process, even though there are several ways to do it wrong.  But this isn’t about how to do an oil change, it’s about what one might do during such a mundane event that could prevent an in­ flight failure.

A Cessna 414A arrives at our airport as a new local tenant in need of an oil change. The oil change requires removing the cowlings, which exposes almost all the engine to easy viewing. It takes a while for the oil to drain, so other things are taken care of while waiting: an oil sample for analysis and completing that paperwork, opening the old filter for inspection, installing a new filter, and doing a little looking around for trouble.

Bob (the senior mechanic in my shop) is making good use of time while waiting for the oil to drain. It might appear that he’s just casually looking around, but in fact, years of experience tells him something’s not quite right with what he sees. He notices a minor blemish near one of the top spark plug holes and just can’t ignore it. Close visual inspection shows it to have all the earmarks of a crack.

Simple visual inspection to confirm a small crack in a cylinder head cast is a very unreliable test. Before going to the expense and, more importantly, the significant risk of pulling a cylinder, some method must be employed to confirm the crack is real.

The pen is pointing to the very end of the crack. You can see it moves across the cylinder to the right to the spark plug boss.

The pen is pointing to the very end of the crack. You can see it moves across the cylinder to the right to the spark plug boss.

The first, easiest, quickest and least expensive method is just a few rubs with some sandpaper. If a crack can be sanded away with a few strokes, then it isn’t a crack at all. A fine line of spark plug thread anti­sieze will ease out of its proper home and weave along the texture of the aluminum casting very convincingly mimicking a crack.  If the crack survives the sandpaper, then a next step is the dye penetrant test.

Dye penetrant is a simple nondestructive test that usually yields reliable results. A red dye is applied to the area and allowed to soak in, or dwell, for a few minutes. All the visible dye is cleaned away until no visible evidence of it remains. A developer is then sprayed on which draws any dye out of a crack and magnifies it to the point the crack becomes obvious.  In our case, this method easily confirms the crack is real and the cylinder is not serviceable. It’s time to contact the owner with the unexpected bad news.

It’s very important to remember the discovery of the crack did not occur during any FAA required inspection such as an annual or 100 hour.  The owner only contracted for an oil change, so the mechanic has no obligation to look for a crack and only a moral one to report it to the aircraft owner.  There is no authority given the mechanic to ground the airplane at this point. The oil change can be completed and appropriately signed off in the aircraft maintenance records in accordance with FAR 43.9 for only the work performed. 

Of course, the owner is notified and informed the cylinder is not serviceable as is, and its continued operation is not recommended. The airworthiness determination is now firmly placed in the hands of the person the FAA intended as directed in FAR 91.403, the owner. In this case, when presented with photographs of the dye penetrant test showing the crack, the owner wisely chooses to authorize some corrective action.

A little crack at the spark plug boss doesn’t seem too ominous until the other side of the crack is found. Once the cylinder is removed, a massive crack  is discovered that extends from the same spark plug hole all the way to the exhaust valve seat. We also see a severely burned exhaust valve with a small section already departed. Amazingly enough, the turbocharger blades aren’t damaged by the departing section of valve. Left in service, this cylinder would have been a catastrophe just about to happen.

The burned valve is just one of two issues in this picture. You should be able to see the crack from the left side of the burned exhaust valve seat over to the spark plug hole.

The burned valve is just one of two issues in this picture. You should be able to see the crack from the left side of the burned exhaust valve seat over to the spark plug hole.

It’s so important not to underestimate the value of a simple visual inspection and anyone can do it. Many owners perform preventive maintenance like oil changes and spark plug servicing. While the cowling is off, consider taking a look for anything out of the ordinary. One doesn’t have to be a seasoned mechanic to see when something doesn’t look the same as it did the last time the engine was exposed.

The most likely discrepancy one finds is some sort of leak. Leaks might be oil, avgas, exhaust or air. It’s also important to realize that leaks can go two ways, in or out.

Oil leaks are what we hear the most complaints about because they make the biggest mess. Few oil leaks are severe enough to ever show on the oil gauge dipstick. This is because it only takes about a cup of oil to coat the entire airplane.  The most common oil mess makers are valve cover gaskets and pushrod tube seals. Just a little torque on the mounting screws of the valve covers is usually all that’s needed to stop the oil slobbering.  Push rod seals are a bigger effort but rarely leak enough oil to be noticed anywhere except the inside of the bottom cowling.

Fuel leaks are easy to see and should always get one’s immediate attention. A blue stain on a cylinder head is a good first indicator of a cylinder head crack. Blue stains on a fuel hose or end fitting isn’t very costly to fix but not something to put off. You may see fuel stains on one of the induction pipes indicating an air leak as well as fuel leak. An induction leak may blow air/fuel out or suck air in depending on the pressure differential with ambient pressure.

There are many other possible items to find, but it’s not a list I can generate for you. The best advice I can give is to pay very close attention the next time the cowling is off and your mechanic tells you all he/she sees looks normal.  This is your baseline for future comparisons. There’s a lot to see, so take your time.  This is a perfect opportunity to make good use of that camera in your smartphone and not rely on your photographic memory.

If you notice any of these signs of leaks or other mechanical anomalies, it’s time to call your local maintenance professional.  We all hope that anything found is just something to watch until the next annual, but it’s best to know with some certainty if all is safe. The idea is always to find a problem and take appropriate action while you’re in control of the schedule. For some reason, airplanes like to ask for maintenance way away from home on those family vacation trips. It’s nice to be ahead of the game once in a while.

Preventive maintenance, when applied correctly, is one of the most cost effective operations one can do toward improving the safety and dispatch reliability of one’s Cessna.  To be clear, preventive maintenance should not be about summarily replacing parts just because they’ve reached some arbitrary age. It’s about finding proper evidence that something is amiss and taking only the required response. The real trick is knowing where to look and for what.

It doesn’t take years of experience to find a crack or other small anomaly during an oil change. However, it usually does take years of experiences or a single traumatic experience to understand the importance of proactively looking for signs of a problem.  This is just a heads up to save you the need to accumulate years of experience or wait for some traumatic experience. Start looking now.