Aircraft engines with a fixed exhaust nozzle area have a single steady-state operating or running line which is fixed by fuel flow from idle to maximum speed. Variable vane angles and flow areas (bleed valves) in the compressor don't change the running line at a particular operating point because the angles and valve positions are unique for a corrected speed, that is they are controlled according to a schedule against corrected speed. Over or under-fuelling compared to the fuel flow required for constant-speed running moves the line up or down while the engine is changing speed to a new requirement.