Actually Rotax is all aluminium in addition to dry sump type and therefore much less prone to get rust if flown infrequently.
WhiskeyPapa wrote:
Ok, so is use once a month for two hours infrequent?
It is actually as clear as mud.
http://www.aviationpros.com/article/10387461/corrosion-how-does-it-affect-the-internal-engine
The article above makes good reading. My understanding of the issue of internal corrosion, was that as an owner, you operate your engine, in the manner that best suits your needs and requirements. I have owned planes since 1996, and as yet, have still to come across engine damage through either lack of, or over use. In my world, I must be doing something right.
It is actually as clear as mud.
That’s a good article.
But nobody will stick their neck out and make categorical statements on whether once a month for two hours is OK.
Obviously it depends on all the factors that affect corrosion itself.
But look at it another way: would you buy a plane which has been doing 24hrs a year for years? A 20 year old plane with 480hrs? There should be more alarm bells ringing that you would get on a date with a woman carrying a boiled rabbit. That is a really very low time, for the typical European parking situation and distance from the sea, etc. Very low for the engine (especially as the flying was almost certainly not regularly spaced out), very low for the pilot (which means all kinds of things e.g. a likely lack of funds and thus a poor attitude to fixing defects, low currency e.g. bad landings and this will be 5x worse if it is a syndicate of 5), and probably there will be multiple avionics issues caused by moisture having extra time to eat circuit boards. I wouldn’t touch it with a bargepole. Well, I would, discounted by the engine overhaul cost and then some more. On a prebuy, I would do a really good inspection and would look for 100% avionics functionality including all autopilot modes.
OTOH it might be perfect, but without a really good inspection you won’t know, and you can’t inspect the engine especially as any savvy seller which have changed the oil and the filters shortly before putting it up for sale. The airframe should be a lot easier to check, however and in the end that is what a lot of people buy if it is clean.
I have owned planes since 1996, and as yet, have still to come across engine damage through either lack of, or over use. In my world, I must be doing something right.
If you had opened them all up, that would be a good data point. But, it does happen. OTOH unless the engine has been opened and inspected, it isn’t a data point because even a badly corroded engine will run.
Peter wrote:
But look at it another way: would you buy a plane which has been doing 24hrs a year for years?
It’s odd that you picked that number because it’s precisely the number of hours my plane and engine did annually between 1971 and 2010 
It was owned most of that time by an obsessive compulsive guy who worked on it more than he flew it, and kept it hangared at his airport home. The last five years before my purchase were harder but partially in the desert. So it seems fine, at least for my use over the last five years.
I think there are as many different storage and corrosion situations with aircraft as there are aircraft. There are certainly averages but some of the best buys may be found outside of the norm.
If you had opened them all up, that would be a good data point. But, it does happen. OTOH unless the engine has been opened and inspected, it isn’t a data point because even a badly corroded engine will run.
Agreed. But if you open them up, that in itself creates even more problems. It therefore becomes a slightly mute point. You will only actually know otherwise, when it quits. However, I beleive that there are certain pre indicators to an engine, that may indicate the onset of its inability to continue. On a number of reports on recent incidents, related to an engine failure, it would appear that a number of pre indicators, low pressure, high cylinder temp, etc, were missed by the pilot/operator. They flew on into the engine disintegrating. Peter, you pointed above, to a lack of funds in certain instances, to maintain properly. I feel that this is the route cause. Consensus is that actually no one can prevent corrosion. Regardless of use. So, a thorough inspection programme, may be the only certain way to prevent issues. And even then…..
@BeechBaby
Helpful article! Thanks!
Peter wrote:
one reason put forward for car engines not corroding readily is that the sump breather is fed back into the inlet manifold (so the engine burns up any muck coming out of there) and is thus a lot less exposed to fresh air.
Piston rings are not absolutely tight in any engines, all engines get combustion gas blow-by from the combustion chamber into the crankcase rather than producing useful work and going out the exhaust pipe. Exhaust gases are CO2 and water in much greater proportion than outside air ever will even in a cloud. In operation the crankcase is full of an oil mist and exhaust gases going through, some sort of crankcase ventilation is necessary to preserve all the gaskets. In flight you get a permanent flow through a hot engine, water stays as vapor until it goes out (might materialize shortly as white vapor at the exhaust pipe if the air is cold, until it dissolves in the much drier outside air). But when you stop the engine or generally cool it down that vapor will condensate inside the crankcase and mix with the oil that will thoroughly spread it around the whole engine.
Blow-by gas takes with it some oil droplets flying around in the crankcase on the way out, closed crankcase ventilation (CCV) on road-going vehicles sends them back to the inlet because emission regulations are so low the blow-by cannot be released directly. Nowadays on trucks for example the oil mist is filtered out of the crankcase ventilation flow so only gases are fed back, because burning engine oil would exceed the emissions limit. The oil level not going down is a nice side-effect.
If people looked for corrosion marks inside a 12 year old car engine run every other week they would find some. The car would not be grounded for it though.
That and advances in materials used in said road-going vehicles.
I read that internal corrosion is only really a problem when the relative humidity is >50%…. which it nearly always is in most parts of Europe…. added to that is the moisture trapped in the sump as described by Arne… I believe a good strategy is to:
1) open the oil filler immediately on shut down and allow as much of the vapor as possible to escape, and
2) run a dehydrated air line into the oil filler venting at the breather….there are a few home-made devices or Aircraft Spruce also sells a unit…some advocate also blocking the exhaust pipe with a rag and perhaps a sack of silica gel…
3) use Camguard
AnthonyQ wrote:
2) run a dehydrated air line into the oil filler venting at the breather….there are a few home-made devices or Aircraft Spruce also sells a unit…some advocate also blocking the exhaust pipe with a rag and perhaps a sack of silica gel…
Be careful with home-made devices. We had one and a pipe came loose and fell into the oil sump. Luckily the pilot removing the device from the aircraft made the connection between a missing part and the possibility of a foreign object in the engine. The pipe was located using a boroscope and we fortunately managed to wiggle it up the oil filler tube without having to take the engine apart but it was a close shot!
Engine dehydrator threads here and here
Here is a corrosion case.
I started making a device which would push dry air up the exhaust, but never finished it. It contained 0.5kg of silica gel and the battery driving the fan would last a week.
Probably, pushing dry air up the breather pipe and letting it escape out of the oil filler would be better. I reckon that if you used non-dried air (i.e. ambient air) in this case, it would be best to stop the fan after say an hour or two.
