Where it breaks
Night and weather
Darkness, cloud and mountain wind ground the helicopters and slow the teams. Yet night is when a thermal camera sees a person best, warm against cold ground.
Big areas, slow feet
In a test with an Irish mountain rescue team, five searchers took two hours on average to find a subject in one square kilometre. The search area in our scenario is 50 square miles.
One drone, one pilot, many warm rocks
Today's search drones fly one at a time, within the pilot's line of sight, and every warm spot on the thermal screen needs a human to go and look.
Source for the ground-team figure: DJI study with an Irish mountain rescue team, reported by DroneXL.