This is not as preposterous as it sounds if the Department of the Interior were a nation, the size of its air force would make it a mini-power.

But if Interior Secretary Cecil D. Andrus decided to pit his fleet against a traditional ally, the Department of Transporation, he'd have a fight on his hands.

Transportation Secretary Brock Admas can put a formidable air force of his own into the sky. With the exeption of Andrus, Adams commands more aircraft than any other director of a federal civilian agency.

Interior and Transporation obviously aren't going to war, but they are the kingpins of a huge fleet of aircraft owned leased and operated by civilian agencies of the federal government.

Nobody knows how many planes there are in the fleet or how much it costs to keep them airworthy because the government has no uniform reporting and cost accounting system.

The best available figures, published last month by the General Accounting Office, indicate the agencies own more than 650 aircraft with a value of at least $340 million.

The agencies also lease, charter or rent several thousand more planes every year, with each agency filling its own needs without regard to the overlap and underuse the system generates.

Interior, for example, owns 70 fixed wing aircraft and eight helicopters. But it leases 72 other planes and 201 helicopters for use by its agencies at a cost of $16 million. Charters of $8 million lifted Interior's total flying costs to $28 million last fiscal year.

Transporation weighs in as an aviation biggies because two of its agencies are the Federal Aviation Administration and the Coast Guard.

None of which is to say that the civilian agencies do not need or do not make good use of their air fleets. The question posed by GAO is whether they are efficiently managing what they have.

As the largest property overseer in the country. Interior uses its planes for fire fighting, geological studies, range management, park supervision and other official tasks, mostly in the West and Alaska.

Transportation's FAA uses 73 aircraft to test and monitor air navigation facilities around the country and to develop air traffic control procedures. The Coast Guard's 172 aircraft are used for search and rescue missions, law enforcement and marine environmental protection work.

The size of these two departments' private air forces becomes more awasome when they are compared with those of some Third World nations.

For instance, according to the Institute for Strategic Studies in London, Colombia has an air force of 67 planes and 77 helicopters; Rhodesia, 73 planes and 16 helicopters; Zaire, 103 planes and 31 helicopters; Chile, 150 planes and 261 helicopters.

With no one paying very close attention, the size of the federal civilian air fleet has increased greatly with the purchase of everything from tiny, single-engine planes to a Boeing 747 for space-shuttle tests by the National Aeronautics and Space Administration.

The only approximation of control is exercised by Congress, which must consider each aircraft acquisition by an agency, and by the Office of Management and Budget.

But the GAO, the investigative arm of Congress, said each acquisition is dealt with separately, without adequate consideration of alternatives to buying or leasing planes and without cosideration of the idea of various agencies sharing an aircraft.

The 11 federal agencies surveyed generally came back with a stock answer: The needs and mission of each are so different that tighter controls would not work.

"Someone must take the lead if economies and efficiencies are to be made in the civilian agency aircraft program," GAO said. The logical leader, GAO said, should be the OMB.

But even OMB had doubts, saying GAO had not sufficiently documented the case for a centralized aircraft management operation. OMB suggested more study.

Other agencies surveyed by GAO justified their private air forces as essential to their work.

NASA flies equipment and personnel with its 101 planes (20 borrowed from the military). The Customs Service has 73 aircraft (11 from the military) for patrolling borders against smuggling operations.

GAO's survey came up with these other air fleet figures: Agriculture, 123; Justice, 46; Environmental Protection Agency, 10; Energy Research and Development Administration, 14; National Oceanic and Atomspheric Asministration, 9; National Science Foundation, 7; Tennessee Valley Authority,16.