ESDU 74008 gives curves (for inviscid incompressible flow) of critical Mach number at a point on the aerofoil surface plotted against the local pressure coefficient and the surface slope at that point. The method is based on first-order calculations but is also equivalent to second-order. If an exact method is used to calculate the pressure coefficient@ a method is suggested of using ESDU 74008 to provide a comparable accuracy for the predicted critical Mach number.