1 includes four generator control units 5 a 5 b 5 c 5 d each generator control unit is connected to a respective electrical generator although for the sake of clarity only one generator 8 is illustrated it will be appreciated that the number of generators and generator control units will vary from aircraft to aircraft and the system shown in fig1 is simply a representative example each generator control unit 5 controls the power output by its respective generator in particular the frequency phase and magnitude thereof the power control unit 2 comprises an auxiliary power control unit 3 connected to an auxiliary generator not shown and or a ground power control unit 4 connected to one or more external ground generator sources not shown the ground power control unit 4 is also connected to each of the generator control units 5 a 5 b 5 c 5 d onboard the aircraft by means of a communications cable 6 the communication cable 6 may be a bi directional rs485 data link with a data transfer rate of 100 kb per second although other suitable data links may be used the generator control units 5 a 5 d each receive a frequency reference signal in general this signal is provided by a reference unit 7 which may be within the power control unit 2 this unit can also be used to supply a reference signal to the auxiliary power control unit 3 however where the ground based power unit is provided to initiate aircraft operation the control unit associated with the ground based generator may act as the master for the frequency reference signal in such an arrangement the auxiliary power control unit 3 on the ground based generator must.