Definition
Engine oil that remains trapped in the engine, oil cooler, lines, and other internal passages after the oil sump has been drained, and which therefore cannot be removed through normal draining procedures. For weight and balance purposes, undrainable oil is considered part of the empty weight of the aircraft.
Plain English
The leftover oil that stays inside the engine even after you drain it. It still has weight, so it counts as part of the aircraft's basic empty weight rather than something added later.
Context Anchor
Seen in weight-and-balance records, especially when figuring what weight is already built into the airplane before adding people, baggage, and usable fuel.
Derivation
From 'un-' (not) + 'drainable' (able to be drained out). The term simply describes oil that cannot be drained — but the weight and balance significance is what makes it a defined item rather than just leftover residue.
Why Pilots Care
It adds a fixed amount to the aircraft's empty weight and must be included for accurate loading and performance calculations.
Intuition Check
Do not read "undrainable" as meaning the oil can never be removed by maintenance. Here it means the oil is not removed by normal draining, so it is still counted in the airplane's weight.
Example Sentence 1
The aircraft's empty weight in the POH includes undrainable oil, so the pilot only added the weight of the eight quarts of usable oil when computing the loaded weight.
Example Sentence 2
Before takeoff the pilot adds the undrainable oil weight to the empty weight to get a correct starting total.