Definition
Containers that hold the engine's lubricating oil supply. In a wet-sump system, the reservoir is built into the engine itself, typically the lower crankcase. In a dry-sump system, the reservoir is a separate tank mounted away from the engine, and oil is pumped to and from it through external lines.
Plain English
The tank or sump that stores the engine oil before it circulates through the engine to lubricate moving parts.
Context Anchor
You encounter this term during preflight inspection, engine servicing, and when checking that the airplane has the correct amount of oil before flight.
Derivation
Reservoir comes from the French réservoir, meaning a place where something is kept in reserve. The oil reservoir is simply the place where the engine's reserve of oil is held until it is needed.
Why Pilots Care
Correct oil level in the reservoirs prevents engine damage from inadequate lubrication or overheating during flight.
Intuition Check
Do not think of oil reservoirs as spare oil bottles carried in the airplane. In this context, they are part of the aircraft’s oil system and supply oil to the engine.
Example Sentence 1
During preflight, the pilot opened the cowling access door and checked the oil reservoir to confirm the level was within limits.
Example Sentence 2
During the climb, the oil reservoirs continued to feed the lubrication system without any drop in pressure.