National Airspace System: Summary of Flight Delay Trends for 34 Airports in the Continental United States (GAO-10-543SP, May 26, 2010), an E-supplement to GAO-10-542

Airline-Reported Sources for Canceled Flights at All 34 Airports, 2009

	Extreme Weather	National Aviation Sytem	Airline	Security
Number of Cancelled Flights	24,608	10,868	22,652	13
Percent of Cancelled Flights	42.3	18.7	39.0	0.0

DOT Definitions of Airline-Reported Sources of Canceled Flights

Extreme weather: Cancellations caused by serious weather conditions that prevent the operation of a flight, such as tornadoes, snow storms, and hurricanes.
National aviation system: Cancellations attributable to a broad set of circumstances affecting airport operations, heavy traffic volumes, and air traffic control. This includes any nonextreme weather condition that slows the operation of the system, such as wind or fog, but does not prevent flying.
Airline: Cancellations that were within the control of the airlines, such as aircraft cleaning, baggage loading, crew issues, or maintenance.
Security: Cancellations caused by the evacuation of an airport, reboarding due to security breach, inoperable screening equipment, and/or long lines at the passenger screening areas.