What must be filed before flying a Part 135 leg?

Filing a flight plan is an essential step before conducting a Part 135 leg. This requirement ensures that air traffic control (ATC) is aware of your intended route and destination, which facilitates safe and efficient traffic management. By filing a flight plan, you provide critical information regarding your departure time, route, and estimated arrival, helping ATC maintain situational awareness and ensure adequate separation from other aircraft.

While operational checklists, maintenance reports, and weather briefings are important components of flight preparation, they are not specifically mandated as a formal filing requirement before the flight under Part 135 regulations. An operational checklist serves as a guide for pre-flight procedures and aircraft readiness. Maintenance reports are necessary for tracking the aircraft's mechanical status and compliance with safety standards. Weather briefings are crucial for assessing the conditions you will encounter during the flight but do not need to be filed with ATC. The act of filing a flight plan directly connects to regulatory compliance and safety protocols within the context of Part 135 operations, making it the correct answer in this scenario.