Milt
Are the B-777's high pressure fuel pumps gear type or swash plate piston ?
If gear type how is their output varied to control engine thrust/n?
The PAG (pump and govenor assy) has two pumps. The LP pump is a centrifugal type and the HP pump is a gear type.
As has (probably
)been covered before. Fuel is fed from the tanks by the boost pumps rated at 12 psi for the wing tank pumps and 36 psi for the CWT pumps from there it passes to the LP side of the engine pumps, then via the FCOC which incorporates a bypass and a diff pressure s/w (for the EICAS indication) it passes to the HP (gear) pump where the pressure is raised for supply to the burners. From the HP pump it passes through the FMU (a lovely engineered piece of kit that is only the size of a small shoebox!)
The FMU contains the pressure drop and spill valve assy, the FMV and the HPSOV. From here the HP fuel is fed to the F/F transmitter and through to the burner rail via a small HP filter. (the HP filter being there to catch debris from a failed HP pump).
The pumps are driven directly from the HP gearbox and as such their output depends on ultimately (n3) shaft speed. Therfore unused fuel that is not required for combustion is returned to the inlet side of the HP pump via the P&D valve.
As a guess and from RB211 experience the output press. is in the region of 1200 psi at takeoff and less than 300 psi at crz.