Definition
The portion of engine oil that can be removed from an aircraft engine and its oil system through the normal drain points during a routine oil change. It does not include the oil that remains trapped in passages, galleries, the oil cooler, filter, and engine internals after draining.
Plain English
The oil that actually pours out when you open the drain plug. It is only part of the total oil in the engine — the rest stays inside coating parts and sitting in lines and the cooler.
Context Anchor
Seen in weight-and-balance discussions, especially when determining what is included in an aircraft’s empty weight.
Derivation
From 'drain' (Old English 'dreahnian,' to draw off liquid) plus the suffix '-able' meaning 'capable of being.' Together: the oil capable of being drawn off. The term exists because not all the oil in an engine can be drawn off — so a specific word is needed for the part that can.
Why Pilots Care
Tells the pilot or mechanic the exact amount of oil that will leave the engine when drained, so the correct quantity can be added back to reach full capacity.
Intuition Check
Do not assume drainable oil means all the oil in the engine. In this context, it means only the oil that comes out through the normal drain; oil left inside the system is treated differently.
Example Sentence 1
The mechanic noted that the drainable oil came out at about seven quarts, even though the engine's total capacity is eight.
Example Sentence 2
After the oil change, the mechanic measured five and a half quarts of drainable oil recovered from the sump.