The fuel control system for a modern gas turbine engine has to meet far more stringent requirements than demanded by early engines. To achieve greater efficiency the controlling parameters such as pressure and temperature must be measured and acted upon with a greater degree of accuracy. A 5°C change in turbine entry temperature can cause a 200 hour decrease in the life of a turbine with an expected life of 2000 hours. With increased fuel efficiency, a saving of less than 1% in fuel consumption is a vital economic factor. These higher demands can be met in most instances by mechanical or hydromechanical fuel systems but the problems of these systems such as hysteresis, friction, fuel temperatures and coefficients of expansion increase rapidly with the search for higher performance. The complexity and number of moving parts becomes greater and so the system is commensurately less reliable.