The federal Urban Mass Transportation Administration (UMTA) found a way yesterday to save face while changing its controversial rule requiring that all new buses bought after next September be fully accessible to the elderly and handicapped.

The rule, proposed in May 1977 by Transportation Secretary Brock Adams, UMTA's boss, has been widely attacked in Congress by bus builders and by public transit authorities nationwide as setting unrealistic, untested requirements.

Chief among them was the requirement that all buses purchased with federal aid after Sept. 30, 1979, would have to have low floors and ramps that would extend to the curb so that the elderly and wheelchair-bound people could board the buses easily. Virtually all big-city bus systems purchase their fleets with 80 percent federal aid.

The low-floor, ramped bus is known as Transbus, and is a popular demand of the increasingly well-organized handicapped lobby.

UMTA Administrator Richard S. Page announced yesterday that the Transbus deadline of Sept. 30, 1979, would remain in force, but left himself an out. If, as the three major U.S. bus manufacturers claim, Transbus cannot be delivered in less than 54 months, then the 1979 deadline will be reassassed.

Furthermore, UMTA's new rule changes two other controversial requirements. The ramp - which bus operators and manufacturers have charged would not work in suburban areas without curbs - can be substituted with a mechanical lift for wheelchairs. Such lifts, yet to be proven reliable, are being installed on a number of other kinds of buses nationwide and will appear soon on some buses in Washington.

Secondly, UMTA said, Transbus will have a double axle in the rear. The original rule had mandated a single axle, but that was found to be incompatible with the low-floor requirements and, in many cases, with single-axle weight limits of many states and localities.

The costs of Transbus are unknown, but industry sources say that an individual bus will probably run at least $15,000. "I'll be shocked if it comes in for any less," a knowledgeable source said.

The Washington Metropolitan Area Transit Authority paid $82,000 each for buses in its most recent purchase, which was one of the last purchases made of the old standard-design bus. An interim-design bus - one that has many new features but falls short of the low-floor Transbus design - is selling for between $90,000 and $100,000.

Manufacturers, most notably General Motors, have fought the Transbus requirements, partly because they wanted to sell their interim-design vehicle. AM General, one of the leading bus manufacturers, closed its production line on standard buses to wait and see what would happen.

The House Public Works and Transportation Committee got into the act by requiring, in legislation the House is due to vote on next week, a new study of Transbus before it can be required. Page said, and committee sources confirmed, that he has assurances that the new rule yesterday will result in an amendment on the floor that would strike the requirement for more study.

While all this jostling was going on, three cities - Miami, Los Angeles and Philadelphia - formed a consortium to buy 530 Transbuses, then found they had to wait. Page said yesterday that the consortium has been encouraged to seek bids.

The promised delivery dates that result from the bidding on the consortium's contract will be key in determining whether the 1979 deadline will remain, Page said.

The three major U.S. bus manufacturers - GM, AM General and Grumman Flxible - all said yesterday that they would have to study the new specifications in detail before making substantial comments. GM called the new rule "a positive step in the development of a low-floor bus."

Frank Bowe, executive director of the American Coalition of Citizens with Disabilities Inc., called the new rule, "excellent, exemplary."

Page said that other cities are prepared to form consortia and thus guarantee to manufacturers that there will be a longterm market for Trans-buses.