Why does Part 107 set a visibility minimum at all, given that the aircraft must remain within visual line of sight?

It seems redundant at first: if the aircraft must stay within visual line of sight, why also require three statute miles of visibility? The answer is that seeing the aircraft is only half the job. The remote pilot must also see other aircraft in time to yield, and traffic approaching at even modest speed covers ground quickly — a light aircraft at 100 knots crosses a mile in 36 seconds. Detecting it at half a mile leaves no useful time to recognise the conflict and manoeuvre clear. The visibility minimum exists to preserve that detection distance, which is why it is assessed from the control station rather than from a weather station: the question is what the pilot can see from where they are standing. Understanding this also explains why the requirement holds in Class G airspace and at low altitude, where a candidate might reason that traffic is unlikely. Unlikely is not the same as absent, and the margin is what the rule protects.

The pilot must see and avoid other aircraft, not merely see the unmanned aircraft, and detecting approaching traffic in time to yield requires a usable detection distance.

Part 107 requires a minimum flight visibility of three statute miles assessed from the control station, supporting the pilot's ability to see and avoid other aircraft as well as to maintain visual contact with the unmanned aircraft.

See the Traffic, Not Just the Drone. Visual line of sight is about your aircraft; visibility is about everyone else's.

The requirement applies in every airspace class including Class G. Low traffic likelihood does not relax it.

• •Reasoning that visual contact with the aircraft satisfies the requirement
• •Assuming Class G relaxes the minimum
• •Measuring visibility from a reporting station