A good article on engine airworthiness

I think abnormal oil usage (too high or too low) does indicate something is not right

How would “too low” oil consumption be indicative of anything? There is no such thing as “too low” oil consumption. My Lycoming gets an oil change every 25h and I do not top off the oil in between, there is very little oil burn. However, anything above a certain level (which is below the recommended level) gets thrown overboard quite quickly and the oil gets dirty rather fast, too. This means that my pistons pressurize the crankcase and a fair amount of exhaust gases find their way into the crankcase. That’s not a problem though and rather normal for these ultra low tech 1940s engines.

Back when I still had a car with a combustion engine, I did an oil change after 3 years. The dipstick reading after 3 years was identical to when I originally put in the oil and the oil itself was so squeaky clean that I considered using it to make french fries. A 4-stroke engine should not have any oil consumption at all.

Cylinder compressions may not be indicative of anything, though if the leak is past a valve then you could be looking at a dodgy valve which could break or get stuck and smash the pushrod (this happened to somebody right here the other day – IO540 engine). It seems that Conti’s QA is much lower than Lyco’s when it comes to machining quality, hence their notorious low compressions and on those engines a low compression may well indicate next to nothing unless there is a sudden change.

It’s pretty straightforward to tell why a cylinder might be getting low compression – listen to it when you do the compression test. If you hear the air leaking out via the exhaust or intake the problem is a valve and it most probably needs to be fixed as soon as possible.

I don’t think the US for the most part is any better for forced landing sites than Europe. Much of south east Texas for instance is flat, but covered completely in piney woods. I’d rather ditch than do a forced landing into the tops of 150 foot tall trees. Out west it’s desert for miles and miles with little to no radio coverage in many spots and good luck with getting cell phone coverage, and the ground is covered in rocks. You better have a few gallons of water with you because you might not be found for days (even months, remember Steve Fossett?) and hitting one of those rocks on landing might get you injured. When I did a coast to coast trip back when I lived in the US out west I remember flying an hour without seeing evidence of another human being and with surprisingly few places you could put even a slow plane like a Cessna 140 down due to ravines, rocks and other obstacles.

The Rocky Mountains are also big and there’s not many suitable places for forced landings. I’ve flown over the mountains in the US various times and much of the terrain is extremely remote, extremely jagged, and you’re below the peaks in a normally aspirated aircraft or without oxygen (and the MEAs are often above the altitude where you want to be in a NA plane).

See the article. No oil going to the top ends e.g.

I see no reference to this claim.

This can cause sticking valves. Have you had that?

Valves stick because coked up oil gets between the valve stem and the valve guide. The base problem is that valves get too hot, possibly due to insufficient oil flow in Lycomings.

A quart (1 litre) every 10hrs on a 540/550-sized engine should be the minimum consumption if the oil is actually going where it should be going. I burn about one every 8hrs.

Sorry, I don’t buy that. The engine shouldn’t burn any oil at all and it’s not a quality if it does. It’s a closed system, both the pistons and the valves should provide for a tight seal. 1 quart oil burn every 8 hours is a lot. And the statement about being air cooled isn’t correct either, you can build air cooled engines with much better sealing nowadays.

The challenge of reducing oil consumption to zero is increased by large displacement, steel air cooled cylinders. The reason for large cylinders is to save weight by having fewer of them. A while back there was a thread on direct replacement aluminum cylinders which interested me a lot, because cylinder sealing could potentially be improved.

Aluminum bored air cooled motorcycle cylinders have notably better oil consumption that their steel lined predecessors, although the bigger they get the harder the challenge and they are all a lot smaller than aircraft cylinders. Some manufacturers are better than others for whatever reason: the Italians seem to have it figured out whereas BMW for whatever reason has had problems with air cooled cylinder sealing, break in and oil consumption for a long time.

One of things I’ve noticed in Europe is that by comparison to my flying in the US, there is more often a suitable forced landing field in Europe. In particular, GA airports in Europe are surrounded by fields, whereas in the US, GA airports are often situated in sprawling urban or suburban areas. In the mountains of both continents it’s equally a problem.

So you’re claiming oil should be burned under the rocker cover and be pushed into the cylinder chamber by the piston?

“BMW for whatever reason has had problems with cylinder sealing, break in and oil consumption for a long time "

When was this?

1993 to date, on air cooled cylinders greater than 500 cc displacement. Before that they had a good period from about 1981, when Nikasil lined cylinders were introduced and when cylinders were a little smaller.