ESDU 91021 gives a method of estimating the boundary beyond which as either lift coefficient or Mach number is increased the turbulent boundary layer on the upper surface separates from the foot of the shock to the trailing-edge. The method is complementary to that of ESDU 81020 which does not predict this form of flow breakdown so well. Features of experimental pressure distributions are found to correlate with the onset of flow breakdown. In both methods it was found that those same features of a pressure distribution calculated by techniques that do not model separation could be used similarly. In this case, the bubble length is found to be a function of local Mach number and momentum thickness at the shock location, while the point of reattachment was found to correlate with features of the adverse rear gradient. Using those features, a step-by-step procedure for predicting the flow breakdown boundary is presented and has been checked extensively against experimental data for aerofoils, and wings with quasi-two-dimensional flow. Two comprehensive worked examples illustrate the use of the method. It is believed to predict the boundary with an r.m.s. error of 0.04 in lift coefficient when Mach number is increasing or within 0.01 in Mach number when lift coefficient is increasing.