A relief airport is an airport that is built or designated to provide relief or additional capacity to an area when the primary commercial airport(s) reach capacity. In some cases a relief airport is an existing one that is designated to handle a specific class of aircraft such as general aviation. They provide capacity gains by moving general aviation from a commercial airport to a different facility. By removing aircraft with lower capacities and slower speeds, an airport is able to operate more flights with larger aircraft and handle more passengers increasing efficiency with minimal additional cost in facilities at the older and generally larger facility. This also spreads out the aircraft over a wider area generally improving air traffic in the entire community.
To qualify for this designation, an airport must have 100 or more based aircraft or 25,000 annual itinerant operations