The closed-loop properties as well as the simple design procedure for the control of linear time-invariant (LTI) systems, offered by the sliding mode control (SMC) algorithm, are well known. The focus of this paper is in designing an SMC for a fixed-wing aircraft in the presence of control constraints and illustrating that the switching surface parameters decide the initial conditions of the aircraft states from which sliding mode can even be expected to occur. The need to perform a trade-off between the magnitudes of these initial states and the speed of the sliding mode dynamics is highlighted; a procedure to calculate these magnitudes is also proposed. A dynamic model of the longitudinal dynamics of a fixed-wing aircraft, linearised about some trim conditions, is chosen for the study.