The Grumman Flxible Model 870 transit bus and its only U.S. competitor, the General Motors RTS II, might well be called the Federal Buses.

They were designed to meet specifications developed by a committee, they are almost always purchased largely with federal money and since their introduction about three years ago they have been beset with irritating problems.

Structural difficulties, so far as is known, are unique to Grumman Flxible.

The most commom complaint has been that air conditioning and heating systems do not work properly, a critical problem because both buses were originally available only with sealed windows. Both GM and Flxible have redesigned the air conditioning.

Transmissions and engines have not been as reliable as similar components in older buses. GM subsidies manufacture the transmissions and engines for both the GM and Flxible models, and GM says it has solved the problems and extended warranties for transit authorities. Other complaints have been less critical.

Both GM and Flxible officials claim transit authorities who buy the new buses get vehicles that are more comfortable (when air conditioning works) and protect passengers better in low-speed accidents.

The sealed window, like the rest of the specifications for the bus, came from a committee. Here is how it happened:

The federal Department of Transportation, which pays 80 percent of the cost of a new transit bus, felt obliged to guarantee competition. It wanted to make sure local transit authorities did not write bus specifications so as to exclude potential bidders.

Transit authorities were invited to work on a standard specification through the American Public Transit Association, and they did. The subject of windows came up, and it was worrisome.

There had been many lawsuits from people who claimed to have been injured when their arms were hanging outside windows. Riders sometimes defeat air conditioning by opening windows, and, in some cities, it was (and is) common practice for boarding passengers to keep transfers, then hand them out the rear windows to friends, who then boarded without paying. The sealed window was born.

When the air conditioning didn't work and people roasted, or when cities like Seattle and Monterey insisted they had no need for air conditioning, the federal government permitted an amendment to the standard specification. Eighteen amendments have now been permitted and others are being considered.

Fewer than 5,000 transit buses per year are purchased in the United States, and an increase would require a major jump in federal transit aid. There is a question whether 5,000 is enough to encourage competition and assure a better bus.

"That damn market is so small, you wonder about that," said E.R. Stokel, director of public transportation for General Motors. "We have averaged about 3,500 units in the last 10 years, and we have the capacity here alone to build 5,000 a year."

However, two European manufacturers have set up factories in the United States, and one, NeoPlan, has won a contract from Atlanta.