As far as i can understand there is no limitation regarding the the amount of fuel required, in the center tank to start the engines. How ever we must keep in mind that the CENTER TANK AD states that there must be minimum 2300 kgs of fuel in the Center Tank for T/O and this is only to avoid the fuel pumps from being exposed due high body angle. As we follow the AD procedure of turning the pump off at the first indication of low pressure, if the same is followd on ground, i do not see any reason not to use the center tank for starting engines even if the QTY is pretty low.
