The thrust F produced by the engines is of great importance in almost every phase of flight because it counteracts the drag and enables the aircraft to climb if required. The maximum available thrust Fm depends on the height and speed of the aircraft and is limited by the approved ‘rating’ for the appropriate phase of the flight. The three ratings that are important in relation to aircraft performance calculations are those specified for take-off, climb and cruise, and the rated thrust for each of these is the maximum available. In any phase of flight the thrust can of course be reduced by the pilot below the rated value, usually by moving a single control lever which is commonly known as a ‘throttle’ lever, even though it may act on a complex engine control system.

For almost all aspects of aircraft performance calculation it is necessary to know how Fm varies with the speed and height of the aircraft. In addition, for calculations of range, endurance and operating cost, a knowledge is required of the rate of consumption of fuel and the way in which this varies with flight speed, height and engine throttle setting. In this chapter the principles governing these variations will be discussed and approximate equations will be introduced for representing the variations in calculations of aircraft performance. For this purpose the rate of consumption of fuel will be expressed as the ratio of the rate of consumption to either the thrust or the shaft power of the engine.