Aircraft fuel system

A method of operating an aircraft, comprising:

• supplying fuel to a fuel storage facility of an aircraft fuel system of the aircraft,
• transferring at least some ullage from the fuel storage facility, through a transfer arrangement, to a separate ullage storage facility, based on a pressure input to the transfer arrangement,
• storing at least some of the transferred ullage in the ullage storage facility, and
• transferring at least some of the stored ullage from the ullage storage facility, through the transfer arrangement, to the fuel storage facility, based on the pressure input to the transfer arrangement,
• descending the aircraft to a reduced altitude level, thereby decreasing the pressure in the fuel storage facility relative to atmospheric pressure, and, at the reduced altitude level, transferring at least some ullage from the ullage storage facility, through the transfer arrangement, to the fuel storage facility, based on a pressure input to the transfer arrangement, and in-taking atmospheric air from outside the aircraft into the fuel storage facility at a first altitude and then allowing transfer of the at least some of the stored ullage from the ullage storage facility, through the transfer arrangement, to the fuel storage facility at a second, lower altitude.