First four planes take off A, B, C, D. At 1/6 of the circle, tanks are 2/3 full. A and B give 1/3 tank to C and D, A and B got 1/3 tank full to fly back. C and D are full. C and D continue to fly to 1/3 of the circle, both got 2/3 tank full. C gives 1/3 tank to D. C has 1/3 tank left and fly back and at the same time plane E takes off. C and E meet at 1/6 of circle, C is empty and E got 2/3. E gives 1/3 to C and both fly back. Now D with full tank at 1/3 of circle and continues flying half a circle to 5/6, and plane F needs to take off in time to meet D at 5/6 of the circle, gives D 1/3 tank and both can fly back to the airport. Total 6 planes