Airlines often retire flight numbers associated with tragic accidents or incidents out of respect for the victims and their families. Some examples of retired flight numbers include:
American Airlines flight AA11, United Airlines flights UA93 and UA175: These three flight numbers were retired following the September 11, 2001, terrorist attacks.
Malaysia Airlines flight MH370: The flight number was retired following the mysterious disappearance of the aircraft in 2014.
Malaysia Airlines flight MH17: The flight number was retired after shooting down of Flight MH17 over Ukraine in July 2014.
TWA flight TW800: The flight number was retired after the aircraft exploded shortly after takeoff from New York JFK in 1996, killing all 230 passengers and crew onboard.
Germanwings Flight 4U9525: Retired after the aircraft crashed into the French Alps in a deliberate act by the co-pilot in March 2015.
Asiana Airlines Flight OZ214: Retired after the Boeing 777 crashed while landing at San Francisco International Airport in July 2013.
Air France Flight AF447: After the tragic crash in the Atlantic Ocean en route from Rio de Janeiro to Paris in June 2009, Air France retired the flight number.