The fuel system must also supply fuel so that the engine can be easily started on the ground and in the air. The fuel nozzles form part of the fuel system and atomize or vaporize the fuel so that it ignites and burns efficiently. The fuel system must deliver fuel to the combustion chambers not only in the right quantity, but also in the right condition for satisfactory combustion. The engine must operate through acceleration and deceleration without any fuel-control-related problems. The other extreme, lean flame-out, occurs if the fuel quantity is reduced proportionally below the air quantity. Rich blowout occurs when the amount of oxygen in the air supply is insufficient to support combustion and when the mixture is cooled below the combustion temperature by the excess fuel. If the quantity of fuel becomes excessive in relation to mass airflow through the engine, the limiting temperature of the turbine blades can be exceeded, or it will produce compressor stall and a condition referred to as rich blowout. The quantity of fuel supplied must be adjusted automatically to correct for changes in ambient temperature or pressure. However, some turboprop aircraft also use variable-pitch propellers thus, the selection of thrust is shared by two controllable variables, fuel flow and propeller blade angle. In turbine-powered aircraft, this control is provided by varying the flow of fuel to the combustion chambers. It must be possible to increase or decrease the power at will to obtain the thrust required for any operating condition. The fuel system is one of the more complex aspects of the gas turbine engine.
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