A recent decision to uphold the firing of a non-striking air traffic control supervisor who spoke approvingly of the strikers' actions has caused a harsh split at the Merit Systems Protection Board and has prompted charges that its Republican chairman manipulated the case to get the opinion he wanted.

Richard Emery, an attorney for the fired supervisor, has charged that MSPB Chairman Herbert E. Ellingwood, a Reagan appointee, intentionally delayed the decision of the three-member board in the case until the term of a board member opposed to the firing expired.

Ellingwood, who was on annual leave, could not be reached for comment. Lon Anderson, a spokesman for the board, said Ellingwood has denied any wrongdoing.

"I looked at him eyeball-to-eyeball and he told me that at no point did he instruct anyone to delay it whatsoever," Anderson said of Ellingwood yesterday.

In May, 13 days after the appointment of new Republican board member Maria L. Johnson, the board voted 2 to 1 to uphold the firing of Harold Brown, a 25-year government employe, who was fired after he urged air traffic controllers at a union meeting to "stay together" during the August, 1981, illegal strike by the Professional Air Traffic Controllers' Organization.

Emery says that Ellingwood should have excused himself from the case because he worked as an attorney at the White House when President Reagan decided to fire the air traffic controllers.

In an appeal of the board's decision filed in a federal court here last month, Emery also contends that Ellingwood delayed issuing the board's decision until May when Republican board member Ersa H. Poston's term expired. MSPB spokesman Anderson said Poston had signed an earlier draft opinion ordering the Federal Aviation Administration to rehire Brown.

Had the board issued its opinion before Poston left, Emery contends, it would have ordered Brown reinstated. The remaining board member, a Democrat also appointed by Reagan, wrote a blistering dissent to the decision issued by the board.

In his 25-page dissent, Dennis M. Devaney accused his fellow board members of "effectively stripping federal employes of the free speech protections accorded by the First Amendment."

The Brown decision is considered significant because of its narrow interpretation of federal court rulings on government employes' free speech rights.

According to records in the case, Brown attended a union meeting in New York a day after the strike began. When he arrived, he said, some striking controllers urged him to go on a stage where strike leaders were speaking. Once there, he said, "I'm so happy. Stay together, please, because if you do, you'll win." The remarks were broadcast by ABC television news.

Joseph K. Riotto, the MSPB administrative law judge who heard the case, said Brown was protected by the First Amendment because he was speaking about an issue of national significance and voter importance and because the FAA failed to prove that his actions harmed the government.

But Ellingwood and Johnson said Brown was directing his remarks to striking controllers, and that they "did not contain any information . . . which could be regarded as . . . of significant interest to the public in terms of either the safety of air travel or . . . the strike."

They also said his acts "can clearly be said to have encouraged the strike's continuation . . . . "

The FAA appealed Riotto's Aug. 23 ruling last summer. The MSPB, however, took no action until Emery went to U.S. District Court Judge George Pratt, charging the board with undue delay. The board then agreed to reach a decision by April 29.

The board staff's first draft decision supported Brown and ordered the FAA to take him back. Poston signed it in early April and forwarded it to Ellingwood. The chairman then returned it to the staff, saying they needed to consider an April 20 Supreme Court decision which placed a greater burden on public employes to prove they were fired for exercising free speech.

A second draft recommending Brown's reinstatement was returned directly to Ellingwood May 3, the spokesman said.

On May 6, Johnson replaced Poston. Meanwhile, Pratt ordered the board to appear before him May 20 to explain why it hadn't issued its opinion. On May 19, the board voted to sustain Brown's firing, making the hearing moot.