Definition
The minimum amount of fuel a flight must still have remaining after reaching its destination, as required by federal aviation regulations. For day VFR flight in an airplane, this is enough fuel to fly for at least 30 minutes at normal cruise speed beyond the destination. For night VFR, it is 45 minutes. For IFR flight, it is enough to fly to the destination, then to the alternate (if required), and then for an additional 45 minutes at normal cruise.
Plain English
A spare amount of fuel the rules say you must still have in the tanks when you arrive, so you are never running on the last drops. The exact amount depends on whether you are flying by day, by night, or on instruments.
Context Anchor
Used during preflight fuel planning, especially when checking whether the airplane has enough fuel for the route, expected winds, and required extra time.
Derivation
Legal' here means 'required by regulation,' not 'allowed.' 'Reserves' comes from the Latin 'reservare,' meaning 'to keep back.' Together: fuel the rules require you to keep back, untouched, for the end of the flight.
Why Pilots Care
Carrying less than legal reserves violates the FARs and creates a direct risk of fuel exhaustion, a leading factor in general aviation incidents.
Intuition Check
Legal reserves are not the same as a personal comfort margin. They are the minimum extra fuel required by regulation; a careful pilot may choose to carry more.
Example Sentence 1
After running the numbers, she added another 30 minutes of fuel on top of legal reserves to allow for forecast headwinds.
Example Sentence 2
After recalculating headwinds, the crew added fuel to restore the planned legal reserves before departure.