You do not have a clue what it is? Neither did I.
We pilots, are never exposed to all maintenance details and worries. Yet the maintenance process of an aircraft represents much more than an annual inspection. We pilots are an important part of the loop when it comes to aircraft maintenance.
Often we can detect issues that the mechanic may not be able to reproduce. Further just « dumping » an aircraft in front of the hangar with a snag written as « some part US » will never help making friends in the shop other than making troubleshooting labour more expensive.
Sometimes, developing defects (snags) have strange symptoms and sometimes those defects are simply unbeknownst to the flying profession while being common knowledge in the hangar.
Aviation has an immense service department know as: « Just when you think you know it all ». I found over the years not to stray in there. Essentially there is always an issue that will surprise oneself as I have recently encountered a new one: engine crankcase fretting, Lycoming IO-320. The problem apparently occurs on all engines. The condition’s symptoms were apparent but not easily recognizable since the engine power output was quite exemplary. And by turning over a propeller one feels stiffness initially thinking that this engine really has good compression.
Thanks to fellow members of the « Airworthy Comanche Forum » and head grinding over at my AMO (Aviation R.Goulet) in CZBM, we were able to identify the issue.
Crankcase fretting is essentially a situation where improperly torqued crankcase bolts will allow micro erosion at the contact between the bottom and top of the crankcase. This has nothing to do with operating technique of the engine. On the other hand sometimes it is reported after prop strikes.
One way or another, one is looking at serious engine work ranging from crankcase replacement to repair by preci$ion machining.
How does a pilot recognize fretting? First when the starter has difficulty turning over the engine. The starter with difficulty will make it past the first compression turn. But indeed the « Bendix » does engage and the engine does turn.
By hand, a stiff to turn propeller is also a good clue. Not so much stiff at the compression but stiff all the way around, more difficult of course during compression. The issue of stiffness is greatly amplified when the engine is hot to the point that the prop is nearly impossible to turn. Such a situation will appear as a faulty Bendix not engaging but the voltage drop during starter engagement will be remarquable, excluding the issue. After the engine cools down, the stiffness disappears. This is very insidious.
How is all this occurring? Essentially, the main bearings within are being, as I understand, applied asymmetrical force from one half in relation to the other of the engine core. When the engine is hot the force increases due to thermal expansion but since oil is being applied, the lubrification removes all indication of friction. Hence in flight, all engine parameters are observed as normal. Once the engine is shutdown, lubrification stops and for about an hour and change your engine might be really difficult to turn or totally seized to the point of being unable to start it, forget hand proping, the force is too intense. This one would never observe as after shutdown we merely walk away for lunch overnight, etc…
in relation to the other (top) of the engine core. When the engine is hot the force increases due to thermal expansion but since oil is being applied, the lubrification removes all indication of friction. Hence in flight, all engine parameters are observed as normal. Once the engine is shutdown, lubrification stops and for about an hour and change your engine might be really difficult to turn or totally seized to the point of being unable to start it, forget hand proping, the force is too intense. This one would never observe as after shutdown we merely walk away for lunch overnight, etc…
This description covers what appears to be early symptoms. More serious consequences would evolve naturally up to a total loss of power, read here engine failure.
Other early symptoms can be observed: eventual metal chips picked up by the oil filter during the 50 hour oil change, loss of oil or even abnormal oil consumption.
Next time you fly out, something we never really accomplish, is to take time to check that prop to see if it turns freely when you have just shutdown, you may be surprised just prior entering that department mentioned earlier.