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While I was going through the FCOM's SOP, normal procedures chapter, preliminary cockpit preparation section I came across the term oil thermal expansion. In the image below it is mentioned in the note section that

IF THE ENGINES HAVE BEEN SHUT DOWN FOR MORE THAN 60 MINS, DECREASE THE ABOVE QUANTITIES BY 3 QTS TO TAKE INTO ACCOUNT OIL THERMAL EXPANSION.

What exactly is this phenomenon and why does it require us to reduce the oil quantities mentioned when carrying out the engine oil quantity check?

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  • $\begingroup$ As the oil heats up it expands and takes up more room. When the oil cools down it shrinks in size. This is a material property seen in almost all liquids, gasses and solids and called thermal expansion. You have to fill in less when the engine is cool because once it heats up the oil will expand and all of the sudden you have too much oil. $\endgroup$ – Jan May 23 at 11:59
  • $\begingroup$ @Jan then why isn't such a check mentioned on engines of different manufacturers. In my company we have IAEs as our CEOs and PWs on the NEOs. On both these engines the SOP doesn't call for such an adjustment. I have asked it also specifically in the question for the LEAP 1A. $\endgroup$ – Jai May 23 at 12:11
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    $\begingroup$ Depending on engine make/model, this may or may not be a critical issue. Some engines just don't care about these things. $\endgroup$ – Jpe61 May 23 at 13:38
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When oil heats up, its volume expands (and its density goes down). This means that if you measure the level of oil in a gearbox when it is hot, the level will read high, and if you come back later and measure it after it has cooled back down to ambient, it will read low.

So if you were to fill a sealed gearbox with cold oil without taking its thermal expansion into account, when it then gets hot it might expand enough to overpressurize the gearbox and blow out its seals.

I had a tractor engine in which overfilling it with cold oil would then cause it to pump the box containing its ignition points full of engine oil once the engine had gotten hot. This would then foul the points and stop the engine.

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