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Someone told me that servicing the engine oil on a Bombardier CRJ900 aircraft with the APU on could lead to the particular engine being overserviced. He then advised that the engine-oil service be done within 20 minutes of aircraft shutdown.

What's the reason for this?

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    $\begingroup$ You should edit the title to transform it in a question (this is a Q&A website). It will help browse questions about oil services without opening each question $\endgroup$
    – Manu H
    Commented Dec 4, 2019 at 8:51
  • $\begingroup$ Pure speculation, but could draining the oil from an engine leave the APU with insufficient oil? I've no idea what 'over serviced' would mean. $\endgroup$
    – Robin Bennett
    Commented Dec 4, 2019 at 10:17
  • $\begingroup$ Please clarify: do you wish to service the APU oil with the APU running, or service the engine oil while the APU is running? $\endgroup$
    – Ugo
    Commented Dec 4, 2019 at 12:19
  • $\begingroup$ Engine Oil Service with the APU running. $\endgroup$
    – Jamiu NINIOLA
    Commented Dec 5, 2019 at 19:35
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    $\begingroup$ @RobinBennett, the usual meaning of “over servicing” in this context is overfilling, i.e. adding more oil than you should. $\endgroup$
    – Jan Hudec
    Commented Dec 28, 2019 at 21:32

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When the engine is running, it sucks up the oil from the reservoir and puts it in all the galleries, ie the oil's all in the right places to lubricate the engine.

When you shut the engine down, the oil starts returning to the reservoir by gravity. So there's a gentle drip-feed back into the reservoir, which means that the apparent oil quantity measured at the reservoir is increasing.

So the problem is WHEN should you measure the oil quantity - when just before the oil starts returning to the reservoir, or after that process has completed? Well, it can take a while, so the general convention is to read the oil quantity, and to add oil as necessary, before the oil has gone back to the reservoir.

(Don't know why the APU has anything to do with this.)

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    $\begingroup$ That shouldn't be a matter of convention, but rather be clearly indicated in the user or maintenance manual. $\endgroup$
    – bogl
    Commented Dec 4, 2019 at 9:32
  • $\begingroup$ I understood the OP has asked about the service while the APU is running. So it's like a third engine which also would affect (decrease) the oil level in the reservoir. $\endgroup$
    – PerlDuck
    Commented Dec 4, 2019 at 9:41
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    $\begingroup$ @PerlDuck, I am somewhat confident each engine has a separate reservoir and so does the APU. Because if one leaks out, you don't want it to disable all of them. $\endgroup$
    – Jan Hudec
    Commented Dec 28, 2019 at 21:17
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The Aircraft Flight Manual or Flight Crew Operating Manual will specify the timeframe during which engine oil may be checked after engine shutdown. For the CF34-3B engine on a Challenger 605, checking must be done between 15 minutes and 2 hours following shutdown. If done outside of this window, the engine must be dry-motored (cranked without introducing fuel) for at least 30 seconds. Otherwise the oil level may (incorrectly) show too-low (the oil tank is not at the bottom of the engine), which could lead to overfilling. Overfilling is said to be worse for jet engines than operating them with slightly too-low oil levels.

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I don't know the specifics for that plane, but in general you do oil changes with the oil warm so that it is less viscous, so there's a minimum amount of dirty, old oil left inside the engine (and so you don't have to wait hours for it to drain).

However it's generally a bad idea to do it while the engine is still at operating temperature, because then the oil will be hot enough to cause serious burns as you remove the drain plug.

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  • $\begingroup$ Which explains the second half, but not the first. $\endgroup$
    – Jan Hudec
    Commented Dec 28, 2019 at 21:14

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