An actuation system for a control surface (6c, 8c, 10c) for an aircraft with a wing or stabilizer (6, 8, 10) to which a control surface (6c, 8c, 10c) is hinged, the wing or stabilizer (8, 10) comprises a sub-structure, and the control surface (6c, 8c, 10c) has a trailing edge, in which the actuation system comprises a first, second, third and fourth actuator (18, 20, 22, 24), a first and second bell crank (26, 28), and at least one push pull rod system, each of the first and second bell cranks (26, 28) comprises a first and a second crank arm (90, 98, 92, 100), the first and second crank arms (90, 98, 92, 100) intersect with and are joined to each other at an intersection (93, 101), the first and second crank arms (90, 98, 92, 100) extend from the intersection (93, 101) at an angle to each other, the first bell crank is adapted to be pivotally connected to the sub-structure by a first pivot extending through the first bell crank's intersection, and the second bell crank is adapted to be pivotally connected to the sub-structure by a second pivot extending through the second bell crank's intersection, the first and third actuators (18, 22) have first and second ends, the first ends of the first and third actuators are adapted to be connected to the sub-structure, the second end of the first actuator is connected to the first crank arm of the first bell crank at a first connection position, the second end of the third actuator is connected to the first crank arm of the first bell crank at a third connection position, the second and fourth actuators (20, 24) have first and second ends, the first ends of the second and fourth actuators are adapted to be connected to the sub-structure, the second end of the second actuator is connected to the first crank arm of the second bell crank at a second connection position, the second end of the fourth actuator is connected to the first crank arm of the second bell crank at a fourth connection position, the first and second actuators (18, 20) have the same specifications as each other, and the third and fourth actuators (22, 24) have the same specifications as each other, characterised in that in the first bell crank the distance D1 between the first connection position and the first pivot is greater than the distance D3 between the third connection position and the first pivot, and in the second bell crank the distance D2 between the second connection position and the second pivot is greater than the distance D4 between the fourth connection position and the second pivot, and in that the specification of the first and second actuators (18, 20) is different to the specification of the third and fourth actuators (22, 24). the actuation system comprises a first, second, third and fourth actuator (18, 20, 22, 24), a first and second bell crank (26, 28), and at least one push pull rod system, each of the first and second bell cranks (26, 28) comprises a first and a second crank arm (90, 98, 92, 100), the first and second crank arms (90, 98, 92, 100) intersect with and are joined to each other at an intersection (93, 101), the first and second crank arms (90, 98, 92, 100) extend from the intersection (93, 101) at an angle to each other, the first bell crank is adapted to be pivotally connected to the sub-structure by a first pivot extending through the first bell crank's intersection, and the second bell crank is adapted to be pivotally connected to the sub-structure by a second pivot extending through the second bell crank's intersection, the first and third actuators (18, 22) have first and second ends, the first ends of the first and third actuators are adapted to be connected to the sub-structure, the second end of the first actuator is connected to the first crank arm of the first bell crank at a first connection position, the second end of the third actuator is connected to the first crank arm of the first bell crank at a third connection position, the second and fourth actuators (20, 24) have first and second ends, the first ends of the second and fourth actuators are adapted to be connected to the sub-structure, the second end of the second actuator is connected to the first crank arm of the second bell crank at a second connection position, the second end of the fourth actuator is connected to the first crank arm of the second bell crank at a fourth connection position, the first and second actuators (18, 20) have the same specifications as each other, and the third and fourth actuators (22, 24) have the same specifications as each other, characterised in that in the first bell crank the distance D1 between the first connection position and the first pivot is greater than the distance D3 between the third connection position and the first pivot, and in the second bell crank the distance D2 between the second connection position and the second pivot is greater than the distance D4 between the fourth connection position and the second pivot, and in that the specification of the first and second actuators (18, 20) is different to the specification of the third and fourth actuators (22, 24).