Effect of altitude

Effect of altitude

Fig. 21-6 The effects of altitude on thrust

and fuel consumption

32. With increasing altitude the ambient air pressure and temperature are reduced. This affects the engine in two interrelated ways: The fall of pressure reduces the air density and hence the mass airflow into the engine for a given engine speed. This causes the thrust or s.h.p. to fall. The fuel control system, as described in Part 10, adjusts the fuel pump output to match the reduced mass airflow, so maintaining a constant engine speed. The fall in air temperature increases the density of the air, so that the mass of air entering the compressor for a given engine speed is greater. This causes the mass airflow to reduce at a lower rate and so compensates to some extent for the loss of thrust due to the fall in atmospheric pressure. At altitudes above 36,089 feet and up to 65,617 feet, however, the temperature remains constant, and the thrust or s.h.p. is affected by pressure only. Graphs showing the typical effect of altitude on thrust, s.h.p, and fuel consumption are illustrated in fig. 21-6 and fig. 21-7.