In a sweeping proposal that will move government-mandated drug testing into the private arena for the first time, the Transportation Department yesterday announced plans to seek a rule permitting random testing of an estimated 500,000 pilots, flight attendants, mechanics and other airline employees.

"There is compelling evidence we have a drug problem in America today, and aviation is not exempt," Transportation Secretary James H. Burnley IV said in unveiling a proposed federal rule.

Burnley acknowledged that he has no clear evidence of a drug problem in the aviation industry. But, he added, "I'm not going to wait until we've got a midair collision because a pilot is impaired."

He said the rule is needed to protect millions of American travelers who trust their lives to pilots and other airline employees every time they board a flight. General aviation pilots would not be tested.

The Air Line Pilots Association, which represents 40,000 pilots around the country, immediately denounced random testing as "inappropriate and unconstitutional" and criticized Burnley for "headline-grabbing."

"General aviation on occasion causes accidents," said Roger D. Hall, first vice president of the airline pilots' union. "How you can exclude them from this is a puzzle."

Matthew Finucane, safety director of the 23,000-member Association of Flight Attendants, voiced opposition to both random and post-accident testing without reasonable suspicion.

The Air Transport Association, which represents the largest airlines, indicated tentative support for the new rule, although a spokesman said the carriers want to carefully review the costs associated with the program and provisions that require rehabilitation programs.

"To our knowledge, there has never been a single major commercial accident attributed to drug use," said Stephen D. Hayes, spokesman for the airline lobbying group.

The plan to broaden random testing to the private sector comes amid conflicting court cases brought to challenge drug testing's legality. On Tuesday, the Army was ordered by a federal district court judge to stop random tests on 9,400 civilian employees on grounds that the tests were an unreasonable search prohibited by the Constitution. Last month, the 9th U.S. Circuit Court of Appeals in San Francisco found post-accident testing unconstitutional, except in cases of reasonable suspicion.

And, in what is expected to be the crucial decision on the issue of drug-testing, the Supreme Court agreed on Monday to review a ruling by the 5th U.S. Circuit Court of Appeals in New Orleans that upheld drug testing of U.S. Customs Service employees who want promotions.

"We can't sit around and wait for weeks and months while these cases wind themselves through the courts," Burnley said. "I'm confident this does pass the constitutional muster."

The rule, which will take effect after what may be a lengthy rule-making procedure, would require a wide range of testing including: pre-employment, random, and post-accident tests, periodic testing for those subject to annual physicals and testing in circumstances when an employee is believed to be using drugs.

The most controversial part of the program -- random testing -- is essential, Burnley said, because it acts as a deterrent to drug use.

Burnley said the proposal to test the aviation industry is the first of several drug-testing rules he plans for transportation industry jobs. Last year, the department began administering random tests to its employees in sensitive and security-related jobs. So far, Burnley said, 1,200 employees have been tested. Of those, eight tested positive.

Legislation requiring random testing of airline and rail employees and truck drivers is tied up in a House-Senate conference committee.

"You have to ask why the administration wants to rush these rules into effect when the core and constitutional issues have yet to be decided," said Allan Adler, legislative counsel to the American Civil Liberties Union. "One of the things that's at issue is: are the Reagan administration's policies likely to be erased by the next administration?"

Under the rule, airlines will be asked to develop drug-testing programs and submit them to the Federal Aviation Administration for approval within 120 days and begin testing within 180 days.