Abstract: The operation and function of the pressure ratio controller and the air pressure controller in the afterburner fuel control unit (AFCU) are investigated on the basis of the data obtained from flight tests and laboratory experiments of the four AFCU models for turbojet engines. Our preliminary opinions are presented in respect to this moot subject which has caused controversy for years.It is found that there are three operational conditions of the afterburner fuel control system. The fuel flow rate fed to the afterburner is controled independently by the pressure ratio controller only during ground operations or in low altitude flight at high velocities, while the air pressure controller does not function. Within the major range of the flight envelope, both the pressure ratio and the air pressure controllers function and control the fuel flow rate jointly. The air pressure controller may function and control the fuel flow rate alone merely in high altitude (near ceiling) flight at low velocities.