Definition
Replacement fuses of the correct type and amperage carried on board an aircraft so that a blown fuse in an electrical circuit can be replaced in flight or before further flight. For aircraft using fuses (rather than circuit breakers) to protect electrical circuits, federal regulations require that one spare of each type and rating, or 50% of the number of each type installed (whichever is greater), be carried on the aircraft.
Plain English
Extra fuses kept in the aircraft so that if one burns out and breaks an electrical circuit, the pilot can swap in a new one and restore the affected equipment.
Context Anchor
Seen during preflight equipment checks, especially when reviewing required equipment for night flying in an airplane that uses fuses.
Derivation
A fuse is a small device with a thin wire or strip of metal designed to melt and break the circuit when too much current flows through it, protecting the wiring and equipment. The word comes from the Latin 'fusus', meaning 'melted' or 'poured'. 'Spare' simply means held in reserve. So 'spare fuses' are reserve melt-wire protectors ready to replace one that has done its job and burned through.
Why Pilots Care
A blown fuse can remove power from lights, radios, or instruments; spares let the pilot restore function immediately and continue the flight safely.
Intuition Check
Do not think of spare fuses as random extra parts in a tool bag. In this context, they are the correct replacement fuses for the airplane’s installed electrical fuse holders, and they are different from circuit breakers, which are resettable protective switches.
Example Sentence 1
During preflight, the pilot confirmed that the spare fuses were stowed in the labeled compartment near the circuit panel.
Example Sentence 2
After the landing light fuse blew on rollout, the instructor swapped it with one of the spare fuses kept in the side pocket.