A bi-directional flight control surface mechanism comprises a fixed part V of a vertical stabilizer having leading and trailing edges and a rudder having leading and trailing edges. The rudder is mounted relative to the fixed part of the vertical stabilizer so that the trailing edge of the fixed structure is adjacent the leading edge of the rudder. The rudder is movable from a neutral position in which the rudder lies in line with the fixed part of the vertical stabilizer to define a notional centerline to first and second angled positions in which the rudder is angled relative to the fixed part of the vertical stabilizer at a non-zero angle on one or other side of the notional centerline. An actuator is provided to drive the rudder relative to the fixed part of the vertical stabilizer so as to vary the distance between said part of the trailing edge of the fixed part of the vertical stabilizer and the leading edge of the rudder.