B-757 minimum fuel ground
B-757 AMM:
The three identical heat exchangers are in the wing fuel tanks and use fuel as a coolant. The left and light system heat exchangers are at the inboard end of the left and right main fuel tanks, respectively. The center system heat exchanger is adjacent to the left system exchanger. A minimum of 600 gallons (4020 pounds/1827 kilograms) of fuel is required in each main tank to provide hydraulic fluid cIooling.
Is this restriction applied for flight OR both in flight and on the ground?
Thanks!