Why do drones near airports necessitate cancelling flights? From Wikipedia's page on the Gatwick drone incident:


  Between 19 and 21 December 2018, hundreds of flights were cancelled at Gatwick Airport near London, England, following reports of drone sightings close to the runway. The reports caused major disruption, affecting approximately 140,000 passengers and 1,000 flights.


I assume that drones can crash into aircraft, negatively impacting the safety of flights.


Suppose airport staff report sightings of a drone. What should the airport do?
Is this considered an emergency? Why or why not?
Can other planes request clearance to avoid the area?
Have any documented instances of drones resulting in injury to or loss of crew, passengers, or plane?
Re the title question, a plane hitting a crow is enough to take out an engine, as shown in the following video.


                
            

Crows weigh about 500&ndash;600g. A drone such as a DJI Phantom weighs two to three times as much and isn't nearly so squishy.