I've just recently bought the 727, recommended by a friend. On a -100 i did a normal load and then cut the engines off. For the flight i was going to make i was told to leave the center tank empty. I made sure that the flow from the left and right tanks is on, but for some reason the aircraft only used fuel from the center tank, which is empty, so i could not start the engines or have them running without fuel in the center tank. Can someone help me solve this issue? thanks