From various technical sources:
WHAT IS FADEC?
To be a true, 100%, Full Authority Digital Engine Control, there must not be any form of manual override available. This literally places full authority to the operating parameters of the engine in the hands of the computer. If a total FADEC failure occurs, the engine fails. If the engine is controlled digitally and electronically but allows for manual override, it is considered solely an Electronic Engine Control (EEC) or Electronic Control Unit (ECU). An EEC, though a component of a FADEC, is not by itself FADEC.
FADEC : LIMITATIONS
Pilot cannot override the FADEC Control. In the event of complete FADEC Failure pilot left with no other option than having to land safely with least performance.
With modern FADEC systems there are no mechanical control rods or mechanical reversions,
Still the system is not void of disadvantages. During critical conditions the system does not provide with manual override.
