When a compressor is designed it is essential to establish the points at which it is likely to surge. Tests are carried out to determine the relationship between pressure ratio and mass flow at speeds covering the whole working range of the compressor. The results are recorded on a series of curves known as surge lines. To obtain the curves, the compressor is run at a constant speed, the mass airflow is gradually decreased and during this test the pressure ratio is carefully monitored. As the mass airflow reduces, there is an increase in pressure ratio. Eventually the compressor airflow becomes turbulent and the compressor surges. When this occurs, there is a rapid drop in pressure. The tests are carried out at various speeds until the whole working range of the compressor has been covered. During the test the points at which turbulence occurred at the various speeds are plotted. The points are then connected by drawing a line, this line is the surge line of the particular compressor being tested. During normal operation the engine is never allowed to operate beyond the surge line. A safety margin is established and the fuel and airflow control systems are adjusted so the engine will run within the safe limits. Figure 4.15 refers.