Definition
Fuel tanks installed inside the nacelles of a multiengine airplane — the streamlined housings that surround each engine. Because they sit out on the wings away from the airplane's centerline, fuel burned from nacelle tanks has a noticeable effect on the airplane's lateral (side-to-side) balance and on the bending loads carried by the wings.
Plain English
Fuel tanks built into the engine housings on the wings of a twin- or multi-engine airplane. Their position out on the wings means burning fuel from them shifts the airplane's left-right weight balance and changes the loads on the wing structure.
Context Anchor
Seen in weight-and-balance data, loading charts, and pilot operating handbooks for some multi-engine airplanes.
Derivation
Nacelle' comes from the French word for a small boat or shell-shaped enclosure. In aviation it refers to the streamlined casing that houses an engine. So a nacelle fuel tank is simply a fuel tank built inside that engine housing.
Why Pilots Care
Fuel quantity in these tanks must be accurately included in weight and balance computations because their location affects the aircraft center of gravity.
Intuition Check
Do not assume all aircraft fuel tanks are in the wings. Nacelle fuel tanks are specifically in the engine housing area, and their location matters in weight-and-balance calculations.
Example Sentence 1
The POH directed the pilot to burn fuel from the main tanks first and leave the nacelle fuel tanks full until later in the flight.
Example Sentence 2
During the preflight inspection the amount of usable fuel remaining in the nacelle tanks was checked against the aircraft fuel gauges.