As someone stated the larger Turbo fan engines have VSV's (variable stator vanes) and VBV's (variable bleed valves) to port air into or unload the compressor. In older engines such as the CF6-50 theses were controlled by the MEC, and moved according to fuel pressure, with lines coming of the MEC(main engine control) to actuators. Today on newer engines the MEC has been replaced by a HMU (hyro mechanical unit), but info to move the vsv's and vbv's is now electronic inputs. The thing I found really interesting was rigging the VSV's and VBV's on the old CF6-50, the first time I participated in it I was amazed at the precision with which it could be rigged. I have only changed one HMU and no rigging was required, I think it's all done automatically via the EEC and the HMU. Has anyone had to rig something like a CFM-56?