A plane crash is where a plane is traveling above the city which loses control and starts spinning eventually crash landing somewhere in the city. Small fires usually breakout around the crash site which can burn large areas if left alone.
In SimCity, if the player chooses the air crash option in the absence if an airport, this will only create an ordinary explosion and notifies the player of the explosion that happened.
The plane crash disaster was removed in SimCity 4 and SimCity (2013), presumably due to the terrorist attacks that occured on September 11, 2001. The disaster option was also removed from edited editions of SimCity 3000 following the attacks.