The International Trade Commission refused yesterday to impose stiff, 44 percent tariffs on a Brazilian-made plane that Fairchild Industries of Germantown, Md., accused of being able to undercut its competing airplane because of heavy government subsidies.

The Fairchild complaint added another irritant to the increasingly strained trade relations between the United States and Brazil, which is fighting hard for export earnings to cut its mounting deficits.

In another closely watched case yesterday, the ITC split 2-2 on a petition by American mushroom canners to restrict imports from the People's Republic of China. The final decision now goes to the White House, which is seen by trade specialists as unlikely to act in view of the commission's tie vote and the danger of angering mainland China.

The commission, however, will meet again this afternoon to decide what form of help, if any, it should recommend that the White House offer the American mushroom canners, who accused China of "dumping" mushrooms to capture a larger share of the United States' $400-million-a-year market. China counts mushrooms among its top 15 exports to the United States.

The 19-seat, Brazilian-made Bandeirante, popularly known as the Bandit, has become increasingly popular with America's regional, feeder airlines that are proliferating because of government deregulation. Fairchild has no backlog of orders for its 19-passenger competing plane, Metro III. Yet, the Brazilian company, Embraer, has 92 Bandits that have been purchased or ordered by U.S. airlines and another 82 already in operation with them.

A Fairchild spokesman said the company has not yet decided whether to appeal.

"We sought relief from Embraer's predatory financing policies," which gave the Brazilian company "an unfair advantage in the marketplace," said Fairchild Senior Vice President Anthony J. Spuria.

Fairchild had charged that Embraer, 51 percent of which is owned by the Brazilian Air Ministry, receives a 44 percent subsidy from the government of Brazil. This allows the Brazilian firm, formally known as Empresa Brasileira de Aeronautica S.A., to provide American purchasers financing terms superior to anything Fairchild can offer.

The commission staff, though, reported at yesterday's meeting that American airlines were buying the Brazilian plane because it meets their needs better than the Fairchild product. Among other things, the Metro III is pressurized, which adds to its purchase price and maintenance costs without providing a needed option for short-haul airlines. The Bandit is not pressurized.

In the mushroom case, American canners from Michigan and Pennsylvania said underpriced imports from China are ruining the market for the domestic product. Chinese mushrooms are capturing an increasing share of the U.S. market, going from one-tenth of 1 percent in 1979 to almost 16 percent last year.

John K. (Jack) Kooker Jr., executive director of the American Mushroom Institute of Kennett Square, Pa., expressed disappointment at the commission decision, which he said will make it harder for President Reagan to support his association's position.