Abstract: An aircraft simulator including means for determining the g-force acting on the simulated aircraft in response to the operator's controls and means for applying to a helmet (3) worn by the operator a vertical force dependent on said g-force. The means for applying a vertical force to the helmet may comprise a pneumatic piston/cylinder arrangement (14, 15, 16) responsive both to a g-demand signal determined by the position of the controls and to means (22) for monitoring the force exerted on the helmet.The piston (15) conveniently acts across adjacent sides (8, 9) of a parallelogram linkage (6, 7, 8, 9) interconnecting a mounting (4, 5) for the helmet and a fixed support (12), enabling horizontal, vertical and rotational movements of the helmet to be made and monitored.