A method of monitoring the performance of an aerodynamic surface of an aircraft. The aircraft is operated during a non-perturbed measurement period such that the flow of air over the surface is at least partially laminar. A parameter is measured which is indicative of the drag of the surface during the non-perturbed measurement period to provide non-perturbed drag data. Air flow is perturbed temporarily over the surface in a perturbed measurement period so that it undergoes a transition from laminar flow to turbulent flow. The parameter is also measured during the perturbed measurement period to provide perturbed drag data. The degree of laminar flow during the non-perturbed measurement period can then be estimated in accordance with the difference between the perturbed drag data and the non-perturbed drag data. An ice protection system can be used to perturb the air flow.