excellent read.

"It is important to note in the FAAs preamble and clarifications, it does not delineate “fatigue” as the only issue surrounding the PICs decision to extend. The extension decision should be made based not only on fitness for duty of the PIC, but on the totality of the situation (e.g. fitness for duty of the entire crew, operational environment, transitions into and out of the WOCL, weather conditions, outside stressors, and many other factors that tend to be additive in most situations that lead to the necessity for an FDP extension). When making this extension decision late in the duty period, it is also important to note NASA’s observation that when an individual has fatigue onset, this is when
they are most likely to not recognize their fatigue state during their decision making."

"The FAA agrees that the responsibility for determining whether a FDP needs to be extended
rests jointly with the pilot in command and the certificate holder. This ensures that one
party is not taking excessive action over another party, and that proper considerations are
factored into the decision-making.

Since it is prior to takeoff, once the certificate holder becomes aware of such issue, the
certificate holder and pilot-in-command have discretion to evaluate the situation and
determine whether it is permissible and appropriate to extend the applicable FDPs and continue with the flight or whether it is more appropriate to replace the affected flightcrew
member."