Part 107 permits a visual observer to be used and places the responsibility for that observer's effectiveness on the remote pilot in command. The observer is a safety function rather than a credentialed role: no certificate is required, no employment relationship is required, and the observer need not be aided by anything. What the rule does require is that the observer be able to see the aircraft and the surrounding airspace, that the pilot and observer maintain effective communication throughout, and — critically for this question — that the remote pilot in command ensures the observer understands the aircraft's operational limitations and the procedures to be followed, including emergency procedures. That briefing is what converts a bystander into a functioning crew member, because an observer who cannot recognise that the aircraft is near its limits, or who does not know what to say when something goes wrong, adds nothing to safety. The requirement to maintain visual line of sight can be satisfied by the pilot or the observer, so it is not the observer's unique obligation.
