Cold weather ops and oil viscosity

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lucius

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The FAA aeronautical knowledge handbook states on page 7-17:

Oil.jpg

The yellow highlight refers to cruise flight, when engine is at operating temperature, not engine start. I have lower oil temperature simply due to the colder outside temperature causing a more effective cooling effect of the oil cooler, irrespective of the oil viscosity. Therefore, I sometimes tape off a small part of the oil cooler.
Why is the yellow highlight correct?
Thank you!
 
If the oil is too viscous it won’t flow through/around the engine at low temps. An abnormally low oil temp (as opposed to normal lower temps in winter) could be caused by oil around the temperature probe that isn’t going to/coming from the hotter parts of the engine.
 
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It is likely a reference to using the wrong weight of oil in cold weather before multiviscosity oils were available. Modern 15w50 or 20w50 allow all year use and don't have to be swapped every spring and fall like in the old days.
 
William Pete Hodges said:
It is likely a reference to using the wrong weight of oil in cold weather before multiviscosity oils were available. Modern 15w50 or 20w50 allow all year use and don't have to be swapped every spring and fall like in the old days.
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Right, low viscosity oil or multiviscosity oil is important for startup during cold weather ops, but it is not of much importance after normal engine operating temperature has been reached. The excerpt from the FAA book above refers to cruise flight.
 
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MauleSkinner said:
If the oil is too viscous it won’t flow through/around the engine at low temps. An abnormally low oil temp (as opposed to normal lower temps in winter) could be caused by oil around the temperature probe that isn’t going to/coming from the hotter parts of the engine.
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I can see this happening in extreme cases, where the probe is mounted at the coolest point, i.e. outlet of the oil cooler. In my case, during cold weather ops, my oil temperature is always lower by about 20C-30C compared to summertime, using 20w50, but still in the green arc. I consider this normal, albeit not optimal and hence I am taping off the oil cooler partially during wintertime.
However, in the FAA commercial knowledge test, where this question is asked, the answer that a lower oil temperature in cold weather ops is normal in cruise is the wrong answer and the correct answer is that lower oil temperature is an indication of improper oil viscosity.
 
lucius said:
However, in the FAA commercial knowledge test, where this question is asked, the answer that a lower oil temperature in cold weather ops is normal in cruise is the wrong answer and the correct answer is that lower oil temperature is an indication of improper oil viscosity.
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not “lower” as in “lower,” but “low” as in “below limits.”
 
If the question would indeed state "below limits", then this is different. But that's not how the question is phrased in the commercial knowledge test. "low" is vague, since it can mean low but within limits.
 
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lucius said:
If the question would indeed state "below limits", then this is different. But "low" is vague, since it can mean low but within limits.
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In airplane parlance, “low” is “below normal” or “below limits.” Look in your POH for a “Low (something-maybe oil pressure)” checklist and see.
 
In the Emergency Section 3 of my POH the only reference to "oil" is in under "Turbocharger Failures" where it states: "Abnormally low oil pressure", not just "low oil pressure", which I would interpret as within the yellow or red arc.
 
lucius said:
If the question would indeed state "below limits", then this is different. But that's not how the question is phrased in the commercial knowledge test. "low" is vague, since it can mean low but within limits.
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An old history teacher of mine said that poorly worded test questions fostered discussion (and sometimes debate) which was more educational than the test itself.
 
lucius said:
In the Emergency Section 3 of my POH the only reference to "oil" is in under "Turbocharger Failures" where it states: "Abnormally low oil pressure", not just "low oil pressure", which I would interpret as within the yellow or red arc.
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So there is no “low oil pressure” reference for not abnormally low, either. “Low” means “not normal.”
 
A heavier (higher viscosity) oil will reduce friction between moving parts and thereby reduce temperature.

One cannot have everything with engines, even car engines. We have multigrades that get us closer to matching viscosity to ambient temperatures, but even they suffer thinning at higher temps, just less thinning, and they don't thicken as much at low temps. Many cars use a 5W20 oil year-round, and on the Canadian prairies there are folks that run 0W20 or 30 year-round. Makes starting in the cold so much easier and safer, since that thinner stuff is more easily sucked into the pump.
 
bentmettle said:
We have people in our club that squawk "oil pressure in bottom part of green". OK, so it's in the normal range. What do you want done about it? :rolleyes:
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Should be in middle of green. If the gauge is correct, it is on the edge of unacceptable pressure. It implies a trend of lower pressures, and if the trend continues it will soon be below acceptable. If the pressure is indeed correct, then the gauge which should show it in middle of green is becoming defective.

If you're flying and see the needle move from middle of green to lowest part of green, it would be a concern. Same idea with seeing it at bottom of green to start with.
 
I guess If I don't see it moving, I don't assume there is a trend. I see it within the range given as acceptable. If there was a single acceptable reading, I would have a green dot instead of a green arc, wouldn't I?
 
bentmettle said:
I guess If I don't see it moving, I don't assume there is a trend. I see it within the range given as acceptable. If there was a single acceptable reading, I would have a green dot instead of a green arc, wouldn't I?
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Just an expectation that the scale on the gauge is used like all other gauges. "Normal" is in the middle of the green. Leads one to expect that it was there at one time, and now it's not.

Technically, you're correct. Green is green. But being on the edge still tells me there is no margin, not a good place to be.

We have a plane in our club that is like that. Drives people nuts; probably the gauge going bad. Needs to be investigated. Is the pressure really on low side of acceptable, or is it the gauge? If low side of acceptable, is there something wearing out that is making is lower?

This is why we have gauges vs idiot lights in planes.
 
Back to the OP.....

Flew a club plane last Friday to get some approaches in with a buddy as a SP. Checked the oil of course in pre flight. Was about 45 at the time, but the night before got down to 25. (Welcome to Atlanta!)

The oil on the dipstick was like maple syrup! Our chief maintenance guy uses Shell 100 single weight all year long. More expensive than Phillips 20-50 BTW. I do wonder if we should switch to save some money on oil as well as help the engine on cold mornings. When the engine started I was counting the seconds until the pressure got to green. Seemed like an eternity.
 
JackL-3J said:
An old history teacher of mine said that poorly worded test questions fostered discussion (and sometimes debate) which was more educational than the test itself.
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If only the FAA allowed meaningful discussion of its written test questions - maybe could lead to more meaningful tests? (Not holding my breath for that one :rolleyes: )
 
WDD said:
The oil on the dipstick was like maple syrup! Our chief maintenance guy uses Shell 100 single weight all year long. More expensive than Phillips 20-50 BTW.
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Not smart at all. That engine is suffering wear on every startup.
 
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MauleSkinner said:
In airplane parlance, “low” is “below normal” or “below limits.” Look in your POH for a “Low (something-maybe oil pressure)” checklist and see.
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@lucius - this is indeed common aviation usage. "Low" means "below limits". Just like "high" means above limits.

For example, in the King Air emergency procedures and abnormal checklists are entries for the following:
- Oil Pressure Low
- Low Oil Pressure Indication (see explanation below)
- Fuel Pressure Low
- Fuel Quantity Low
- Hydraulic Fluid Low

All of which in the text of the checklists show that these warnings/cautions are for exceeding the limitations of each system. Not simply that the hydraulic fluid is lower than normal.

For example, the oil pressure limitations are 90-135 psi during flight. If the oil pressure gets below 90, you run the Low Oil Pressure Indication checklist, which advises you to run at a reduced power setting and gives that setting. If the oil pressure gets below 60 psi, the red warning light comes on and the checklist has you shut the engine down, or land at the nearest suitable airport using minimum power required. So, they are very much limitations where you take action based on the pressure falling below limits. If the oil pressure is simply a little below the normal amount, I will watch it, but will not take any actions since it's not a "Low Oil Pressure" indication.

Similarly, in the Challenger 605 checklists, we have the following entries:
- ATS Low Speed (and Overspeed)
- Engine Oil Pressure Low (there is actually a separate checklist for "Fluctuating Oil Pressure")
- Low Fuel Warning
- Low Idling
- Fuel Low Pressure
- Fuel Low Temperature
- Engine Bottle Low
- Hydraulic 1/2/3 Low Pressure
- Oxygen Low Pressure

And once again, in all these cases "Low" refers to being below the associated lower limit, not simply "lower than normal but still in the green".
 
Dan Thomas said:
A heavier (higher viscosity) oil will reduce friction between moving parts and thereby reduce temperature.
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I'm not sure how much effect that would have. The thicker oil would also have more viscous drag, which also ends up as heat. But I suspect combustion heat is by far the largest contributor to heat in the oil as the oil is heated by contact with the underside of the pistons and the cylinder walls.

In every airplane I've owned, the oil temperature tends to run a constant offset from the ambient air temperature. This is not surprising as the engine produces a certain amount of waste heat at any given power setting, and the heat that can be transferred to the air is directly proportional to the temperature difference between the hot surface(s) and the cooling air. On the small Continentals it seems to be around 100°F, at least that's what I observed on my T-Craft with its A-65 and also on my Parrakeet with its C-85. On a 50° day the oil temp would stabilize at 150°; on a 90° day it would stabilize at 190°. On engines with active temperature control like the thermostat on a car (or the "thermostatic bypass valve" or Vernatherm on a Lycoming) the temps will be more constant, at least until it's hot enough to close (around 180°) and send all the oil through the cooler, at which point it will track the ambient temperature by some offset, i.e. it may go over 180 but shouldn't go lower once it's fully warmed up and stabilized (unless the OAT is cold enough that it never gets to 180).

But, too, the thicker oil may resist flowing through the partially closed valve thus putting more oil through the cooler, which means more cooling.
 
Dana said:
I'm not sure how much effect that would have. The thicker oil would also have more viscous drag, which also ends up as heat. But I suspect combustion heat is by far the largest contributor to heat in the oil as the oil is heated by contact with the underside of the pistons and the cylinder walls.

In every airplane I've owned, the oil temperature tends to run a constant offset from the ambient air temperature. This is not surprising as the engine produces a certain amount of waste heat at any given power setting, and the heat that can be transferred to the air is directly proportional to the temperature difference between the hot surface(s) and the cooling air. On the small Continentals it seems to be around 100°F, at least that's what I observed on my T-Craft with its A-65 and also on my Parrakeet with its C-85. On a 50° day the oil temp would stabilize at 150°; on a 90° day it would stabilize at 190°. On engines with active temperature control like the thermostat on a car (or the "thermostatic bypass valve" or Vernatherm on a Lycoming) the temps will be more constant, at least until it's hot enough to close (around 180°) and send all the oil through the cooler, at which point it will track the ambient temperature by some offset, i.e. it may go over 180 but shouldn't go lower once it's fully warmed up and stabilized (unless the OAT is cold enough that it never gets to 180).

But, too, the thicker oil may resist flowing through the partially closed valve thus putting more oil through the cooler, which means more cooling.
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We know that thin oil does not lubricate well at elevated temperatures, and poor lube means more friction, and more friction means more heat. It's why we can't just run SAE 5 or 10 oil year-round in any engine. On the other hand, oil that is too thick exerts more drag on the moving parts, costing efficiency and making starting more difficult. Like everything else in aviation and engines, compromises must be made, and if we won't use multigrade oils, we end up with even more compromises.

In my A-65 I used Exxon W80 for several years, then switched to Aeroshell 15W50. My oil temperatures dropped by around 30 degrees. Some of that was the hot viscosity of the oil, which means it acts like an SAE 50 when hot. The Exxon W80 is an SAE 40 oil. The rest of it was due to the anti-scuffing additive added to the 15W50; it lowered friction some. That oil is good stuff, but there are some engines that do not like it, specifically the small Continentals that use the geared starter with the one-way friction Bendix clutch that lets the driven gear, which is always engaged to the crankshaft gear, spin up with engine RPM while not spinning the starter. The little friction cams in that clutch fly outwards so they're not contacting the drive drum when the engine is running, but during start they grab that drum, and also wear flat spots on themselves that, with the slippery Aeroshell, can no longer grab the drum and spin the engine. They will usually work OK once the Aeroshell is drained and something else put it.
 
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