A government investigator's conclusions and opinions are not "hearsay" and may be used as evidence in federal trials, the Supreme Court ruled Monday.

The justices reversed a federal appeals court ruling that had ordered a new trial in a wrongful-death lawsuit against the makers of a Navy airplane in which two people crashed six years ago.But Monday's 7-2 vote appeared to make a new trial necessary, because the justices also ruled that the plaintiffs' lawyers wrongly were limited in presenting their case.

Federal trial rules excluding hearsay as evidence have an exemption for governmental investigatory reports containing "factual findings."

Monday's ruling extends that exemption to conclusions and opinions contained in such reports.

The court's opinion was written by Justice William J. Brennan, who was not on the bench Monday to announce the decision. He was hospitalized last week for treatment of pneumonia.

Lt. Cmdr. Barbara Ann Rainey, a Navy flight instructor, and Ensign Donald Knowlton, her student, were killed while practicing landings and takeoffs at Middleton Field in Alabama on July 13, 1982.

Their spouses, John C. Rainey and Rondi Knowlton, sued in Florida, alleging that a defect in the design or manufacture, or both, of the T-34C Turbo-Mentor aircraft the two were flying caused it to lose power.

The suit named Beech Aircraft Corp., Beech Aerospace Services Inc. and Pratt & Whitney Canada Inc. as defendants.

After a trial, a federal jury ruled for the defendants. But Rainey and Knowlton appealed,contending that the presiding judge wrongly had allowed some evidence and wrongly excluded some evidence.

The 11th U.S. Circuit Court of Appeals in 1987 agreed and ordered a new trial.

The appeals court said the judge should not have let the jury consider as evidence the conclusions of a Navy officer who investigated the accident. He concluded, among other things, that pilot error was "the most probable cause" of the crash.

***** The court also:

- Barred the government from retroactively applying rules limiting reimbursement paid to hospitals treating Medicare patients.

- Having already ruled that privacy rights do not extend to garbage left at curbside for pickup, refused to rule differently on garbage discarded closer to home.

- Agreed to study a university's attempt to withhold confidential information from a faculty member who says her denial of tenure was due to illegal discrimination.

- Refused to reinstate a $4.5 million libel award won by a former prosecutor against the Philadelphia Inquirer.

- Rejected an appeal by a family seeking damages for the death of a Marine helicopter co-pilot.