What action must a pilot take if a flight plan is filed and there are changes during the flight?

When a flight plan has been filed and changes occur during the flight, it is crucial for the pilot to notify Air Traffic Control (ATC) of the changes. This action ensures that ATC is aware of the current position and intentions of the aircraft, which is vital for maintaining safety in the airspace system. ATC provides essential services in managing air traffic, and any changes in route, altitude, or other significant factors need to be communicated to avoid potential conflicts with other aircraft and to ensure a smooth transition through various phases of flight.

Keeping ATC informed helps them provide you with the necessary support and information, such as traffic advisories or changes in weather, to facilitate a safe flight. Simply ignoring the changes would not only jeopardize safety but could also lead to violations of air traffic regulations. Filing a new flight plan is usually not required for changes that can be communicated effectively in-flight, and while documenting changes for personal records is a good practice, it does not replace the need for immediate communication with ATC. Thus, notifying ATC of any changes is the appropriate and required action.