T' condition o' crossin' t' sonic barrier, ya bilge rat, from t' initial appearance o' local shock waves (typically around Mach 0.8 t' 0.9), extendin' t' speeds where all local airflow becomes supersonic (typically Mach 1.05 t' 1.2). Well, blow me down! This be t' period o' maximum aerodynamic stress in a supersonic flight.