What is the definition of a duty day in relation to Sp and P?

The definition of a duty day in the context of Sp (Scheduled Part) and P (Part 135 or 121 operations) correctly begins with the first scheduled activity of the day. This means that the duty day encompasses the entire time from the initiation of scheduled duties—such as briefings, pre-flight checks, and flight operations—up until the last activity concludes.

Understanding this definition is crucial for both pilots and operations managers, as it clarifies the expectations regarding work time, rest periods, and scheduling. Recognizing the start of the duty day allows for better management of flight crew schedules, ensuring adequate rest and compliance with regulations concerning flight time limitations.