There is disclosed a method of monitoring the health of an abradable seal located on a piston of an actuator, the method comprising the steps of: (i) measuring an initial velocity of said actuator piston while said actuator is maintained in a passive condition at an initial time; (ii) operating said actuator; (iii) measuring a subsequent velocity of said actuator piston while said actuator is maintained in a passive condition at a subsequent time; (iv) repeating steps (ii)-(iii) and either: recording or outputting the measured subsequent velocities over time; or determining a health status of said actuator when said subsequent velocity has increased above a predetermined amount.