Abstract: A jet engine is controlled by placing time variant total pressure probes at the entry plane and the discharge plane of a jet engine fan. The pressure values at the probes are converted into electrical signals, filtered, and then fed to meters where the root-mean-square (RMS) values of the electrical signals are determined. The RMS values are fed to a computer which computes the ratio of the two RMS values and this ratio is fed to a comparison circuit which generates a difference signal representing the difference between the RMS ratio and a predetermined limit value. If the RMS ratio equals the limit value, then the operating point of a fan is altered by either changing the nozzle area, changing the fan speed, or changing the altitude or Mach number.