FADEC is a sophisticated system that controls the engine parameters by digital signals generated by a Control unit. In essence, and in a simplified form, the FADEC communicates via computer to the Flight Management systems, and will command the thrust output to deliver the correct power for the current mode of flight. During engine start a FADEC system will monitor critical engine parameters such as EGT and N1 N2 etc. In modern FBW types such as A320 and B777, the system will operate in an active way, and will shut down engines and abort the start sequence if any abnormalities or parameter exceedences occur.
Hope that helps.
Tailwinds
