Large gas turbine domain
A Hydro-Mechanical Unit (HMU) [alternatively a Fuel Metering Unit (FMU)] is the assembly used to house most (if not all) electrical control to fuel valve interfaces. By this (my) definition, the Hydro-Mechanical Unit must therefore contain: -
a metering valve, with electrical drive and position feedback
a spill valve (return excess HP to the LP fuel system), usually with no electrical interfaces
a high pressure shut-off valve (HPSOV), with electrical drives and position feedback.
The high pressure shut-off valve (HPSOV) is usually designed for both FADEC (e.g. Autostart) and aircraft functions, i.e. related engine run/cut-off switch and fire switch operation. These particular aircraft function examples must be reasonably isolated from the FADEC fuel metering role, so that simple failures can not propagate into an inability to shut-down when the metering system is faulty.
The fuel delivery system has to work over a very wide range of fuel flows and (engine burner required) delivery pressures, whilst some of the valves require high end pressures to satisfy servo force margin designs. It is therefore normal for the Hydro-Mechanical Unit assembly to include a pressure raising function, this can be arranged as one of the following: -
a Pressure Raising Valve (PRV) that acts independently and without electrical interfaces, or
a Pressure Raising Shut-Off Valve (PRSOV), i.e. merged function with the above SOV, or
a Pressure Raising Spill Valve, i.e. merged function with the above spill valve.
In my experience an IDG is an Integrated Drive Generator, this unit provides electrical power exclusively for the aircraft, i.e. it does not directly power the HMU or FADEC. The Boeing 787 combines this generator role with the engine starter motor role, in a starter/generator – this is discussed elsewhere in PPRuNe due to some
battery interactions!