FlyinFoSheezy

Need a dispute settled...

Pilot A thinks that a cold engine's dipstick will indicate a slightly higher oil level than a warm (or hot) engine. The reason is due to the cold oil "creep" up the dipstick.

Pilot B thinks the reverse is true (hot engine dipstick indicates higher oil level).

What is your answer?
And, is the rule the same for turbine and reciprocating engines?

Riverside

Check the oil when the engine is warm. Oil expands when it's hot and contracts when it's cold. Different temperatures will give you different readings

Kilgore Trout

Not to confuse the issue too much, but you should also check the engine oil level before starting a cold engine, to make sure you have a sufficient quantity to avoid damage at start-up.
Especially after maintenance.

jungle

Creep isn't going to matter, expansion from heat isn't going to be significant.
Follow the operating manual. Some turbine engines require quantity to be checked within a few minutes after shutdown, mainly due to dry sump leakdown.
Most piston engines use a wet sump and the reading hot or cold will be nearly the same.
Checking the oil before start is always a good idea.

rickair7777

All of my experience with aircooled engines (airplanes and VW's) indicates that the dipstick will read noticeably higher when hot.

For consistency, I always check car engines hot, since I usually do it while getting gas.

With an airplane, you need to check it before start, hot or cold, for obvious safety reasons. Like others have said, the difference is not large maybe 1/4 - 1/2 quart depending on the engine. Most engines are supposed to be checked hot, and their dipsticks are calibrated accordingly. The main point of this is that if you overfill a cold engine, you waste the extra oil because the engine will consume it rapidly.

jungle

How Much Oil Is Needed?

The engines on my Cessna 310 have 12-quart sumps -- 13 quarts if you include the quart in the spin-on oil filter. When I first acquired the airplane, my mechanic would fill the sump to its maximum capacity at each oil change. It didn't take me long to discover that the engines didn't like that, and promptly tossed several quarts out the engine breathers.

My POH states that the "minimum for flight" oil level is 9 quarts. So I asked my mechanic to service the sump to 10 quarts (instead of 12), and I'd add a quart of make-up oil when the level got down to 9 quarts. That worked better, but I was still seeing a fair amount of oil on the underside of the engine nacelles and the outer gear doors.

After I became a mechanic myself and learned about such things, I checked the Type Certificate Data Sheet (TCDS) for my TCM TSIO-520-BB engines, and found that an oil level of 6 quarts was sufficient to make good oil pressure in all flight attitudes from 23° nose-up to 17° nose-down. Armed with this information, I decided to experiment with lower oil levels.



There's a lot more to checking the dipstick than just noting the oil level. The appearance of the oil is at least as important as its quantity.


What I discovered was that oil consumption (and the oily mess on the airframe) was drastically reduced if I maintained the oil level at around 8 quarts on the dipstick. Since then, I've avoided filling the sump to more than 9 quarts, and I normally do not add make-up oil until the dipstick reads about 7.5 quarts. (This still gives me a 1.5-quart "cushion" above what the engine needs to operate reliably in all flight attitudes.)

You might wonder why TCM puts a 12-quart sump on an engine that requires only 6 quarts. The answer is that FAA certification requirements demand that the engine be designed to hold twice as much oil as it actually needs:


FAR §33.39 Lubrication system.

(a) The lubrication system of the engine must be designed and constructed so that it will function properly in all flight attitudes and atmospheric conditions in which the airplane is expected to operate. In wet sump engines, this requirement must be met when only one-half of the maximum lubricant supply is in the engine. [Emphasis mine]


The TCDS for my TSIO-520-BB engines states that maximum acceptable oil consumption is about one quart per hour. If my engines actually used that much oil, then I'd need to fill the sumps nearly to their maximum capacity to ensure that I had enough oil to make a 5-hour flight without risking oil starvation. But since I know from long experience that my engines use about 0.1 quart per hour, there's no reason for me to carry anywhere near that much oil.

Every aircraft engine installation has an optimum oil level at which oil consumption is minimized and the engine is happiest. I would encourage you to experiment to determine what oil level works best for your airplane. Your engine will operate properly at 50% of its maximum oil capacity -- guaranteed. As long as you keep the oil level a quart or two above the 50% point, your engine will be happy.

The best time to get an accurate dipstick reading is just prior to the first flight of the day. If you check the oil level shortly after the engine has been run for awhile, the dipstick reading will be noticeably lower because a significant quantity of oil remains adhered to various engine components. Another reading taken 24 hours later will often show an oil level that is 0.5- to 1-quart higher.
Written by an A&P/ Pilot, not me


I hate to muddy the water with trivial type certificate details.........



Many piston engines have an operating range related to length of flight and oil consumption. It isn't just one number.

schwanm

What are the thoughts on putting cold oil in a hot engine? Bad?

Rama

To expand on Jungle's earlier post, big jet engines such as the JT-9 specify checking oil within 30 minutes of shutdown. The sump is located fairly high on the engine and the oil will tend to drain down into the case and give an inaccurate reading after it sits a for a while. At this time you would motor the engine for about 2 minutes to get the oil circulating and then you would get an accurate reading. The JT-9 was also famous for eating (leaking) oil so you would always fill it all the way to the top. 1 quart an hour was not an unusual consumption rate on the earlier engines.

Qtip

Follow the manufacturers recommendations for when the oil level should be checked.

Too much oil in an engine may cause internal problems. Excessive oil may also vent overboard causing an impression of an actual leak. You will also get a messy aircraft.

rickair7777

If you want to get really overloaded on oil theory, check out this web site...

Bob Is The Oil Guy