Critical speed or Mach number is the speed at which the speed of sound is locally reached somewhere along the flow around an airfoil. A supercritical airfoil can tolerate operating speeds slightly above the critical speed (hence the name), which means that a pocket of supersonic flow will show up, most likely on the upper surface. What qualifies an airfoil as supercritical is that the recompression does not need a [shock][1], but can be done shock-free for a small lift coefficient range by clever shaping.

See [this question][2] for more.

Calculation of a supercritical airfoil is best done with a computer program which models the flow conditions around Mach 1 faithfully. Once such program would be [TSFOIL][3]. The picture below shows one result for the BACJ airfoil at Mach 0.8, and you can see that the speed on the upper surface reaches Mach 1.3.

![B747 Mach distribution][4]


  [1]: https://aviation.stackexchange.com/questions/5048/why-does-supersonic-flight-detach-airflow-from-a-wing
  [2]: https://aviation.stackexchange.com/questions/13453/what-is-the-actual-air-speed-over-and-under-a-wing-due-to-bernoullis-principle/13501#13501
  [3]: http://www.dept.aoe.vt.edu/~mason/Mason_f/tsfoil2.f
  [4]: https://i.sstatic.net/Xev7k.png