Air flow control system – operation:

The stages of the compressor are matched to give the highest efficiency in the speed range maximum rev/min. To extend the range of smooth operation over lower engine speeds, variable-incidence intake guide vanes and/or an air bleed valve are fitted. In the lower speed range the bleed valve opens to allow some of the air to escape from the rear stages of the compressor, thus restricting the mass air flow through the later stages and preventing an unstable flow pattern.

When the bleed valve is open, the guide vanes if fitted are partially closed; at higher engine speeds, when the bleed valve is closed, the guide vanes if fitted move progressively towards the open position. The vanes are operated by a hydraulic ram which incorporates its own control mechanism and which receives a signal of engine speed in terms of hydraulic pressure from the engine speed governor in the fuel pump.