A unified stability curve was generated for impulsively started pipe entrance flows, comprising the variation of critical Reynolds number with a novel dimensionless velocity profile shape parameter. Time dependent regions of stability and instability were obtained, explaining qualitatively the measured almost instantaneous upstream propagation of turbulence in a starting pipe flow. In so doing they provide a formal alternative to the widely held view that the observation of turbulence far downstream under steady‐state conditions is purely from the evolution and washdown of turbulent structures in the unstable entrance region of the pipe, suggesting the possibility that local downstream instabilities and turbulence from an earlier time, play a role.