Aircraft Main Fuel Tank

14 CFR  25.981(b)(3)(iii) defines the main fuel tank as a fuel tank that feeds fuel directly into one or more engines and holds required fuel reserves continually throughout each flight. The functions of the main tanks are immediately available and operate without immediate supervision by the flightcrew in the event of failure or inadvertent depletion of fuel in an auxiliary tank. Generally, main tanks are those dedicated to the feed of the engines during engine feed isolation.