Final answer:
The FAA controller handbook indicates that an imaginary line is present on every final approach course when radar vectoring to an instrument approach is being conducted, serving as a guide to ensure safe and efficient landing alignments.
Step-by-step explanation:
The FAA controller handbook specifies that an imaginary line exists on every final approach course whenever radar vectoring to that instrument approach is in progress.
The final approach course is a flight path prescribed for an aircraft to make its final descent to land on a runway. This path is the last segment of an approach to an airport which aligns the aircraft with the runway for landing. When air traffic controllers are using radar to vector an aircraft for an instrument approach, they ensure that the final approach course is maintained with precision and safety, thus the importance of an imaginary line. This imaginary line acts as a guide to align traffic and manage the flow of aircraft safely and efficiently as they come into land.