Backup speed control is accomplished by the simultaneous operation of hydro-mechanical (PGA) and electronic (EG) governing systems. To achieve backup control, the electronic system must be reverse-acting (an increase in fuel is called for by a reduction in current to the actuator portion of the PGA-EG).
setting the speed of the mechanical governor slightly higher than that of the electronic governing system causes the mechanical portion of the PGA-EG to always seek a fuel setting higher than that being controlled by the electrical part of the PGA-EG. Electrical control thus controls the fuel position because of the least-fuel-selection feature.
Should the electronic signal to the PGA-EG drop to 0, due to some problem in the system, the electric actuator will call for an increased fuel position above the setting of the mechanical governor. At this point, the mechanical speed setting is lower and the ballhead controls the engine-fuel setting.
Although the speed will be slightly higher under mechanical control, no gross overspeed or shutdown occurs due to a loss in signal from the electronic control.
Direct-acting PGA-EG controls are available. These units will go to minimum fuel on loss of electronic control signal.