Why change oil in airplanes?

Cpt_Kirk

Cpt_Kirk

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Ted Striker
Bad title.

Piston engine in aircraft seem to burn oil at a faster rate than those in cars. By the time you need to change the oil (50hrs? Just a guess), wouldn't you have burned and replenished enough so that none of the original oil would exist?

I do not own an airplane.
 
If it is burning that much oil it probably needs an overhaul. If I burn 2 quarts in 25 hours I would be surprised.
 
I haven't used more than a qt over 35 hours since new. Have 1200 hrs still going strong.
 
I'm about to change my oil today. It's been around 40hrs and the oil just started getting dark. In that time I've only added 2qts. I don't consider that consuming oil. Also just because you're replenishing the oil doesn't mean it isn't collecting stuff that you dont want in your engine. All those byproducts of combustion are still in there regardless of how many quarts of new oil you pour in. Essentially every new quart is an old quart right after startup.
 
You don't change it because it burns away you change it because it degrades.
 
My suspicion is that it relates to abuse. Rental and training are probably much harder on the rings and on consumption in general. In my case I add maybe 2 qts per oil change at 50hrs.
 
Another hidden cost in the piston vs turbine debate. If you're like me, better at doing my job than pretending to be a mechanic, then I go to the shop to have it done. With the Aerostar that's a 3hr deal minimum with decowling etc. It ends up being $350-500 each time with oil filters, oil and labour.

If you do that every 25hrs as they recommend for turbo engines, on a 1800hr TBO, you're looking at $36K in oil changes over the life span of the engine! If you do it every 50hrs, it's still a considerable $18K. That's the same cost as your average hot section inspection, I would venture. Turbines go 800hrs between oil changes.
 
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stratobee said:
Another hidden cost in the piston vs turbine debate. If you're like me, better at doing my job than pretending to be a mechanic, then I go to the shop to have it done. With the Aerostar that's a 3hr deal minimum with decowling etc. It ends up being $350-500 each time with oil filters, oil and labour.

If you do that every 25hrs as they recommend for turbo engines, on a 1800hr TBO, you're looking at $36K in oil changes over the life span of the engine! If you do it every 50hrs, it's still a considerable $18K. That's the same cost as your average hot section inspection, I would venture. Turbines go 800hrs between oil changes.
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Recips with spin on filters do 200 between changes as well. That's what I normally do, plus with a fresh quart every 20 hrs or so my additive packages stay pretty fresh especially since I'm LOP within a couple minutes of wheels up. My oil analysis never come back with anything ugly in them.
 
stratobee said:
Another hidden cost in the piston vs turbine debate. If you're like me, better at doing my job than pretending to be a mechanic, then I go to the shop to have it done. With the Aerostar that's a 3hr deal minimum with decowling etc. It ends up being $350-500 each time with oil filters, oil and labour.

If you do that every 25hrs as they recommend for turbo engines, on a 1800hr TBO, you're looking at $36K in oil changes over the life span of the engine! If you do it every 50hrs, it's still a considerable $18K. That's the same cost as your average hot section inspection, I would venture. Turbines go 800hrs between oil changes.
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While a turbine(you didn't specify brand) may have a considerable amount of time between oil changes, it still will have maintenance on filter changes. And most engines have interval inspections, plus turbine washes.

And your number on a hot section cost is considerably low.
 
stratobee said:
Another hidden cost in the piston vs turbine debate. If you're like me, better at doing my job than pretending to be a mechanic, then I go to the shop to have it done. With the Aerostar that's a 3hr deal minimum with decowling etc. It ends up being $350-500 each time with oil filters, oil and labour.

If you do that every 25hrs as they recommend for turbo engines, on a 1800hr TBO, you're looking at $36K in oil changes over the life span of the engine! If you do it every 50hrs, it's still a considerable $18K. That's the same cost as your average hot section inspection, I would venture. Turbines go 800hrs between oil changes.
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Yes. You can do a hot section inspection on a PT6 for 18K.

But, if you find ANYTHING wrong, it is going to cost BIG bucks fast.

It is a very rare PT6 that gets opened up and then closed because everything is okay!
 
HSI are hard to put costs on, as we all agree. They can be anywhere from $10K to the cost of an overhaul. On average for a Garrett (which are the only numbers I have), it will be around $20-50K every 1800hrs. PT6's can be more expensive, I've heard.

But add to this - on a 2000hr turbo'd engine, how many have gone to TBO without changing at least four pairs of plugs ($4k at least)? How many have gone to TBO without doing some kind of cylinder work - valves, rings, cracked cylinder etc? That's another $5K at least if two of them have gone down. Turbines tend to get to HSI without much work.
 
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This is a big, giant, hairy subject. Too large to do justice in a few sentences. 1. Each combustion event forces a small amount of burnt and unburnt fuel into the crankcase where it mixes with the oil. Some of that product of combustion(CHON chains, hydrocarbons and water vapor, etc) is later evaporated off, and some emulsifies in the oil. 2. Mechanical shear works on the long-chain molecules that make up the additives for the multi-vis oils. As these molecules are sheared over time, the viscosity index, and modifiers no longer work as well as they started when new. 3. Particularly in aircraft, FeO2(iron oxide) is scuffed off the jugs or rings, or other metal components over time, along with other metals, which is why you often do a oil analysis. These metals are caught in the filter for the most part, but small enough molecules of carbons get through and cause wear.

There's a few other age related wear items that are not affected by refilling oil as it's consumed but these are the main ones. Again, big subject.
 
SixPapaCharlie said:
You don't change it because it burns away you change it because it degrades.
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My understanding (which may be wrong) is that it's not so much that it degrades---modern oils really hold up quite well---but more that it gets dirty, and builds up combustion by-products and other gunk that you don't want in constant contact with your engine, especially in ever-increasing concentrations.
 
Combustion remnants and humidity together form acids. The reason I change oil is that I hate corrosion and the older the oil the higher the acid content and the greater the corrosion potential. Fresh oil is cheap maintenance. And no engine or filter manufacturer I know of goes over 50 hours for a recommended oil and filter change. 25 hours for a screen. I personally use 50 for the filter and 25 for the oil.
 
Cpt_Kirk said:
Piston engine in aircraft seem to burn oil at a faster rate than those in cars. By the time you need to change the oil (50hrs? Just a guess), wouldn't you have burned and replenished enough so that none of the original oil would exist?
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Airplane engines are older technology compared to car engines. Air cooled and light weight means they have larger tolerances due to the thermal characteristics, and therefore use oil. It is just a characteristic, not a problem.
Oil holds contaminates in suspension, so that the oil filter can capture most of them, but not all. Replacing the oil burned adds some freshness, but after a while (50 hrs is a common timeframe), it makes sense to replace the oil, and filter to start totally fresh. Relatively cheap preventive maintenance. In reality, replacing the filter is probably more important than replacing the oil, but it is just as easy to do both. Long time ago oil filters weren't as prevalent as they are now in aircraft engines, so changing the oil was far more important (and that philosophy has stuck - not a bad thing overall).
Auto engines are designed (liquid cooled, tolerances, piston/ring design) to burn very little oil. Most customers see that as a sign of quality (no need to add oil between changes), and it also makes the EPA happier.
 
Stewartb said:
Combustion remnants and humidity together form acids. The reason I change oil is that I hate corrosion and the older the oil the higher the acid content and the greater the corrosion potential. Fresh oil is cheap maintenance. And no engine or filter manufacturer I know of goes over 50 hours for a recommended oil and filter change. 25 hours for a screen. I personally use 50 for the filter and 25 for the oil.
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That is why you want to add Cam Guard, it is specifically designed to control the exact byproducts developed by 100LL.

Another thing, this is also why you don't want a synthetic stock in your 100LL burning engine. 100LL is a completely different compound from MoGas, and produces a very different set of by products. The synthetic base stock in use, and all the Semi Synthetic Av oils use the same one, is formulated to deal with MoGas, and does a very poor job of dealing with the byproducts of 100LL, setting up a corrosive environment.

The key to oil life though is operating LOP and minimizing the amount of byproduct and contamination you create to begin with.
 
SixPapaCharlie said:
You don't change it because it burns away you change it because it degrades.
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Not degrades. Fouls.

Oil is pretty hard to degrade as long as it is kept within operating specs. It will, however, absorb combustion products from blow-by.

The black color is generally partially burned fuel and water.

Anyone who tells you their engine doesn't use oil is wrong. If the volume doesn't go down, it's being refilled with other stuff.

Additives can get used up as well.

To answer the original question, not all the impurities will make it through leaks, so you will accumulate them.

Another real big advantage (especially for airplanes) is early detection of mechanical problems through used oil analysis.
 
Yep, oil doesn't wear out, it gets contaminated, or if you get it above 420°F for petroleum oils you can sustain thermal damage, but if your oil is that hot, you have way serious problems.
 
Doesn't matter once the sediment of lead settles to the bottom between flights it will not rise off the bottom, and be removed by draining the oil on top of it.

Change oil early, before it can cake on the bottom of the oil sump, and do it after a long hop when the most of the lead is in suspension.
 
The "top-off" method is used in certain turbofan engines because oil consumption is such that you burn it off before it degrades. On most piston engines that's pretty rare to do. Although Henning may have done 200 hours between oil changes, most people don't. Since there's such a huge variety in oil consumptions on piston engines, that's hard to do.

Right now I've burned 0 quarts in 25 hours. Engines are tight and I run LOP with low CHTs.
 
Henning said:
Yep, oil doesn't wear out, it gets contaminated, or if you get it above 420°F for petroleum oils you can sustain thermal damage, but if your oil is that hot, you have way serious problems.
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Actually oil does wear out. It's ability to neutralize contaminants, particularly corrosive acids formed during combustion, is degraded over time simply because the neutralizing agents are expended.

Now do the hydrocarbon molecules get degraded? Not much. The molecules can be sheared by gear teeth which is why motorcycle oils have anti-shear additives.

Of course the engine oil is composed of both hydrocarbon molecules and the additive chemicals so one shouldn't ignore a component when describing the behavior of the oil in an engine.

There are systems, typically shipboard, which refresh the additives to engine oil and filter it clean enough for continued re-use but ya don't find those systems on aircraft.
 
stratobee said:
Turbines tend to get to HSI without much work.
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I realize you are an amateur when it comes to aircraft turbine engines, but you are getting some bogus information. :rolleyes:

If a turbine engine is maintained properly (maintenance program or factory manuals) you are going to spend some serious money. While they are more reliable than a piston counterpart, they are certainly not cheaper.
 
Clark1961 said:
Actually oil does wear out. It's ability to neutralize contaminants, particularly corrosive acids formed during combustion, is degraded over time simply because the neutralizing agents are expended.

Now do the hydrocarbon molecules get degraded? Not much. The molecules can be sheared by gear teeth which is why motorcycle oils have anti-shear additives.

Of course the engine oil is composed of both hydrocarbon molecules and the additive chemicals so one shouldn't ignore a component when describing the behavior of the oil in an engine.

There are systems, typically shipboard, which refresh the additives to engine oil and filter it clean enough for continued re-use but ya don't find those systems on aircraft.
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Right, it's the additive packages that deplete, and are refreshed with top up oil to what produces no degraded performance that I could find in analysis when adding as low as a quart every 20 hrs or so. Again, I predicate these results on maximized LOP operations to minimize the byproduct and contamination production.
 
For piston engines:
1) Change the oil according to what the manufacturer says at the very least. To do otherwise is illogical.
2) Piston engines love fresh, clean oil!
 
On a per mile basis, I change the oil in my car more often than my plane.

Eggman
 
Henning said:
That is why you want to add Cam Guard, it is specifically designed to control the exact byproducts developed by 100LL.

Another thing, this is also why you don't want a synthetic stock in your 100LL burning engine. 100LL is a completely different compound from MoGas, and produces a very different set of by products. The synthetic base stock in use, and all the Semi Synthetic Av oils use the same one, is formulated to deal with MoGas, and does a very poor job of dealing with the byproducts of 100LL, setting up a corrosive environment.

The key to oil life though is operating LOP and minimizing the amount of byproduct and contamination you create to begin with.
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I was considering adding cam guard at my next oil change. Now I haven't looked at it yet but is it a direct replace ante for a quart. So if I put in 7qts normally would it be 6 quarts of oil and a quart of cam guard?
 
simtech said:
I was considering adding cam guard at my next oil change. Now I haven't looked at it yet but is it a direct replace ante for a quart. So if I put in 7qts normally would it be 6 quarts of oil and a quart of cam guard?
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Use 1.6 ounces of camguard per quart of oil
 
Greg Bockelman said:
If it is burning that much oil it probably needs an overhaul. If I burn 2 quarts in 25 hours I would be surprised.
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Shopping for a 182.. I called a guy about his ad. Said his was burning about 2 quarts every 6-8 hours. 1000 hours, top end already done, low compression (low 60's), low hours flown per year. He wouldn't budge on his price. I was pricing his plane with a run out engine.

I said good luck.
 
SixPapaCharlie said:
You don't change it because it burns away you change it because it degrades.
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Err......not really. You need to change it because of the contamination not because the oil degrades. If we had 2micron filtering, it would last a very long time. If you could filter out all the contaminants, keeping it topped up would be all you need.
 
simtech said:
I was considering adding cam guard at my next oil change. Now I haven't looked at it yet but is it a direct replace ante for a quart. So if I put in 7qts normally would it be 6 quarts of oil and a quart of cam guard?
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That is correct, best way to do it is put in 2 quarts, add the can of Cam Guard, they up to your normal, minimal loss, quantity you top to with, my preference Phillips XC 20/50. IIRC you are replacing a pint per 6 quarts.
 
Unit74 said:
Err......not really. You need to change it because of the contamination not because the oil degrades. If we had 2micron filtering, it would last a very long time. If you could filter out all the contaminants, keeping it topped up would be all you need.
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It's amazing what centrifuge type oil filters accomplish, those were a cool invention indeed.
 
Henning said:
That is correct, best way to do it is put in 2 quarts, add the can of Cam Guard, they up to your normal, minimal loss, quantity you top to with, my preference Phillips XC 20/50. IIRC you are replacing a pint per 6 quarts.
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This sounds good for oil changes. What about when you topping up? Every 6 quarts, then add one of these?
 
Henning said:
Recips with spin on filters do 200 between changes as well. That's what I normally do, plus with a fresh quart every 20 hrs or so my additive packages stay pretty fresh especially since I'm LOP within a couple minutes of wheels up. My oil analysis never come back with anything ugly in them.
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Really, everyone is going to let this slide (well except for Ted's afterthought mention).

I've completely lost faith in you guys.
 
TazzyTazzy said:
This sounds good for oil changes. What about when you topping up? Every 6 quarts, then add one of these?
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I add paint of camguard at each oil change but not when topping up. But I typically only add two quarts 'twixt changes. I change oil and filter at 35 to 40 hours or 4 months whichever comes first. These days it's almost always 4 months unless I'm flying a lot of dogs.
 
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