Definition
Additional fuel tanks installed on an airplane beyond the standard main tanks, used to extend range or endurance. Their location and the weight of the fuel they carry must be accounted for in the airplane's weight and balance calculations, since filling or burning fuel from them shifts the center of gravity.
Plain English
Extra fuel tanks added to an airplane on top of the regular ones. They let the airplane fly farther, but the pilot has to include their fuel weight when working out how heavy the airplane is and how that weight is distributed.
Context Anchor
Seen when computing aircraft weight and balance, especially when fuel is carried in more than the airplane’s primary tanks.
Derivation
Auxiliary comes from the Latin auxilium meaning 'help' or 'support.' An auxiliary fuel tank is a helper tank — it supports the main tanks rather than replacing them.
Why Pilots Care
They change the aircraft's total weight, center of gravity, and fuel management procedures, requiring updated performance and loading calculations for safe flight.
Analogy
Like strapping extra jerry cans in the back of a truck for a long cross-country drive.
Intuition Check
Auxiliary does not mean “unimportant” or “emergency-only” here. It means additional tanks that supplement the primary fuel tanks and must still be counted in weight and balance.
Example Sentence 1
Before the cross-country flight, the pilot topped off the auxiliary fuel tanks and recalculated the takeoff weight and center of gravity.
Example Sentence 2
The preflight checklist included verifying fuel quantity and selector position for both the main tanks and the auxiliary fuel tanks.