I have had a few interesting questions about oil lately, including one that concerned oil temperatures in a Cessna aircraft in which the gauge green band had a reading of 245°F as the high limit.
The pilot was wondering if it was OK to cruise at or near that temperature.
I like this question because I can give a definite answer: No.
There are several reasons for not flying at that high of an oil temperature.
The only time that 245° oil temp is OK is on takeoff and initial climb-out.
Once you are at altitude, the temperature should settle down to around 180° to 200°.
The concern is that on most aircraft that are not equipped with a CHT or EGT gauge, the oil temperature gauge is the only indicator of what is going on in your engine. If you are cruising at 245° oil temp, this is usually an indication of a cooling problem or engine problem.
The 50° Rule
Another concern is that the oil will not hold up for too long at that elevated temperature.
A good general rule is that as oil goes through an engine, it will usually see some instantaneous temperatures that are 50° higher than bulk oil temperatures.
Therefore, at a 245° bulk oil temperature, the oil may get to around 295°-300° at some point in the engine. This is too high if it stays at that point for very long.
The 50° rule is also important on the low end of the range.
A little simple math will show us that an oil temperature of 160° or less means that at no time in the engine will the oil temperature be above that magic 212° needed to boil the water out of the oil.
That is why it is so important to get your oil temperature up to at least 180° during cruise conditions.
So, shoot for a cruise oil temperature in that 180°-200° range for best engine life.
Good oil resource article. Perhaps I missed something, but I read all about the what…but missed out on the how part. Meaning….how does one more manage oil temperatures at altitude. “Alert: There is no knob for that.” It’s either one of the two….normal or hot….as long as the engine is still running. While at altitude if it is hot….most likely it will stay hot, but yeah….one can climb to higher altitudes where air is cooler…..try that, but if it is an internal engine problem…..well…best be searching for some place to land.
I had an aircraft that routinely showed elevated oil temperatures. I suspected it was the gage which was a bulb and capillary type. After testing, it turns out it had a plus
25 degree error (>10% FS). I ordered another PMA replacement (same manufacturer) and tested it before installation. It had the same error. My point is that while erroring on the side of caution is always prudent, it is better to verify before going down a diagnostic rabbit hole.
Yeah…….repeated rabbit hole visits most generally result in finding a snake at some point. So yeah….well….getting the snakes out is what it’s about really……haha…no snakes on airplanes ok…..
Its amasing they still havent come up with a syniteic oil i remember the very early days of amsoil had a avaition oil for small plane the certifition cost more than the product .they stopped. Its funny there small engine oil temp has a much higher range than general avaition does.
In the 50’s I learned to lean a C-90 above 3000 MSL by pulling the mixture until the engine ran rough and then increasing the mixture until it ran smoothly. Today I fly a Champ float plane in Alaska with a C-90. Is there a better way to lean the engine?
Ben,
Is there harm in using up a quart of Type M oil when adding oil between oil changes?
TSIO 550K with 100 hours flight time.