A civil enclave is an area allotted at an airport belonging to the armed forces, for the usage of civil aircraft and civil aviation related services.
Civil enclaves are common in countries like India, Sri Lanka, and Pakistan where a part of an airbase, almost invariably a legacy of World War II vintage, is allotted for domestic air traffic instead of building a new civilian airport. These airports usually have a curfew (mostly after sunset) during which civil aircraft are not permitted to operate. Many reserve morning hours for military flight training. Some civil enclaves adopt an altitude restriction, i.e. one below which a civilian aircraft cannot descend while over-flying the enclave. The curfew system may result in airport congestion while the altitude restriction can cause long detours and greater fuel consumption.
Air traffic control at civil enclaves is usually entrusted to the armed forces or it may be a joint civilian-military crew. In some countries security is the responsibility of military personnel; in others, civil security authorities such as the Transportation Security Administration in the United States retain responsibility for all civilian aviation security. It is generally accepted that the military receives revenues from civil use of ATC services if they own and operate them, but in some countries it is not clear as to whether capital expenditure for improvements such as runway expansion is the responsibility of the military or civilian authorities. One example in India is Goa's Goa International Airport at Dabolim.
Civil enclave Wikipedia
The term "civil enclave" is rarely used in the United States, which nonetheless has several shared military-civilian facilities, usually referred to in Federal Aviation Administration records as shared-use airports or joint-use facilities. One example is Northwest Florida Regional Airport (a joint-use facility), which utilizes the runway and ATC services of Eglin Air Force Base. Charleston International Airport uses the runways and services of Charleston Air Force Base. Both civil airports, however, operate their own passenger terminals and taxiways. Charleston also has private fixed-base operators on the civilian side of the field catering to the general flying public.
The obverse, a military enclave, is quite common in the United States, though never referred to as such in that country. Many Air National Guard operations take place at smaller regional airports run by civil authorities. Airports containing ANG enclaves include McGhee Tyson Airport, Harrisburg International Airport and Kalamazoo-Battle Creek International Airport among many others; virtually every state without a major Air Force base has military operations present in at least one civilian airport.
Ahmedabad's Sardar Vallabhbhai Patel International Airport in India and Zia International Airport in Bangladesh are examples of South Asian military enclaves.
In Singapore, while not officially referred to as such, Changi Air Base is effectively a military enclave of Singapore Changi Airport, the main international airport of Singapore, and shares its runway facilities.