What can an expired CFI certificate not be used for until renewed?

An expired Certified Flight Instructor (CFI) certificate cannot be used for conducting flight training until it is renewed. A CFI's primary role is to provide instruction in flight maneuvers and aeronautical knowledge to pilot students, and this requires a valid, current certificate. When a CFI’s certificate has expired, they are not authorized to log flight instructor time or conduct training sessions, which is essential for both the safety and legality of flight instruction.

In contrast, giving ground school instruction or answering student inquiries can often fall under different regulations, as they may not directly involve the act of flight. Additionally, logging flight hours can usually be accomplished in various contexts that do not strictly require a valid CFI certificate, such as logging hours as a pilot under other ratings. However, for flight training specifically—where hands-on instruction and hands-on flight activities take place—having an active CFI certificate is a regulatory requirement.