Do you mean In a loop-d-loop? like a plane flying in a vertical circle?
In this case, there is also gravity to consider. The acceleration toward the centre of the circle must always be the same, so when the plane is at the bottom of the loop, it must create more upward thrust to cancel out the force due to gravity.
This is why if you swing a bag round (vertically), it is most likely to break at the bottom of the swing, since the tension is greatest at this point.