1. Apparatus, for synchronizing the speed of engines on an aircraft, the engines being of the type having an electronic engine control (EEC) unit and a fuel control unit (FCU), the EEC unit having speed rating logic (SRL) circuitry and speed governing loop (SGL) circuitry, the SRL circuitry including a signal processor with signal memory for storing signals representing an engine speed algorithm, the processor using the algorithm to calculate the magnitude of an EEC command speed signal (N sel) in response to sensed operating data received from the aircraft, the SGL circuitry receiving the EEC command speed signal at an input thereof for providing a command fuel flow signal (Wf) to the FCU at a magnitude dependent on the magnitude of the EEC command speed signal, the FCU controlling fuel to the engine in dependence on the magnitude of the command fuel flow signal, the apparatus comprising: synchronizer reference unit (SRU) means, responsive to the sensed operating data from the aircraft, and having SRL circuitry for calculating a SRU command speed signal at a magnitude substantially equal to that of the EEC command speed signal; and electronic switch means, one associated with each EEC unit, each switch means receiving the EEC command speed signal from the SRL circuitry of the associated EEC unit at a first input thereof, and each switch connected at an output thereof to the input of the SGL circuitry of the associated EEC unit, each switch further receiving the SRU command speed signal from the SRU means at a second input thereof, each said switch means being responsive to operator control for selectably connecting one of said inputs to said output, whereby the SGL circuitry provides the command fuel flow at a magnitude dependent, alternately, on the magnitude of the EEC command speed signal and on the magnitude of the SRU command speed signal.