Defense contractors won a major victory today as a federal appeals court ruled that companies are immune from liability claims for defects in aircraft built for the military.

A three-judge panel of the 4th U.S. Circuit Court of Appeals threw out damage awards totaling $4.7 million in three cases brought by the families of pilots killed in crashes. The families claim the aircraft were defective.

"It's a sad day for military families and military personnel," said John Green, an Odessa, Tex., lawyer who represented two of the families. "It destroys a valuable right: to sue for wrongdoing."

Lawyers for both the families and the contractors said today's rulings are likely to be reviewed by the Supreme Court, especially because they are at odds with a recent decision of another appellate court that said defense contractors could be sued under some circumstances.

"You can't give manufacturers carte blanche" in providing equipment to the military, said Louis S. Franecke, a San Francisco lawyer who represented one of the families.

"This says they're not responsible, even for their own negligence," said Washington lawyer Michael Pangia, who represented another one of the families. "Our position is they're responsible for anything sold to specifications, be it a rivet or a can of beans."

The importance of the cases to the defense establishment was acknowledged by several major military contractors not directly involved in the cases, including Lockheed Corp. and McDonnell Douglas Corp., which joined in the appeals.

L. Neal Ellis Jr. of Richmond, who represented the contractors, said today's ruling "upholds a principle adopted by at least four other circuits," but agreed that "the time may be ripe" for the Supreme Court to rule on the questions raised by the suits.

He said the public is protected "by the back-and-forth" contract negotiations, in which "the government makes sure the specifications are met." The criteria "very plainly don't allow selling defective goods."

Fred J. Meier of Los Angeles, a lawyer for LTV Corp., one of the contractors, called the decision a "vindication of a necessary policy." He said military suppliers have to "push technology more than you would with consumer products, yet make them as safe as you can."

The fact that the government approved the specifications and accepted the equipment "is all that is required" for the contractors to be shielded from suits, just as is the federal government, wrote Judge J. Harvie Wilkinson, who was joined in the unanimous opinions by Judges Donald Russell and K. K. Hall.

Donna K. Dowd, whose husband was one of two pilots killed when their helicopter crashed, said the ruling "basically tells men in the military and their families that they do not have any rights." She said that, if her husband "had been flying the same helicopter as a civilian, I would have won this case."

Army Maj. Douglas Dowd, 38, and Navy Lt. Robert S. Ellis, 29, were killed when the helicopter they were flying crashed during a flight-instruction session May 21, 1981, at the Navy Test Pilot School at Patuxent River, Md.

Mrs. Dowd said her husband, a 15-year Navy veteran who flew Huey helicopters on rescue missions in Vietnam, was demonstrating the Bell as part of Ellis' graduation as a jet fighter pilot.

The plaintiffs contended that the crash occurred because of a phenomenon known as "mast bumping," in which the rotor blade strikes the support for the rotor shaft when the helicopter dips at an extreme angle, and in this instance, sliced through the cockpit.

A federal court jury in Baltimore deliberated 10 hours before agreeing that the rotor system was "unreasonably dangerous." It ordered the Bell helicopter division of Textron Inc. to pay the families of Dowd and Ellis $3.65 million. Mrs. Dowd and her two teen-aged children live in Atlanta; Catherine Ellis and her two young children live in Springfield.

An award-winning newspaper series in The Fort Worth Star-Telegram reported that "mast bumping" was implicated in 68 crashes in which 244 servicemen were killed.

The appeals court found, however, that even though a 1973 Army study found 46 instances of mast bumping on Bell helicopters between 1967 and 1972, the company was protected from liability.

Mrs. Dowd said, "The Army did not design that helicopter, Bell did, and they sold it for a profit." She said "they admitted in court that there was a design problem, and they did not fix it.

The appellate court, however, found that Bell had recommended three design changes to eliminate mast bumping, but that the Army rejected one of them and had not yet installed a second at the time of the crash. It said Bell was immune from liability because it had "warned the United States about dangers involved in the use of the equipment."

Another case involved Marine 1st Lt. David Boyle, 26, a helicopter copilot from Edwardsville, Ill., who died in the crash of a Sikorsky helicopter in the Atlantic Ocean off the coast of Virginia Beach on April 27, 1983.

All four members of the crew survived the impact, and three of them escaped through emergency exits, but Boyle did not, and drowned.

During the trial before U.S. District Judge Richard L. Williams in Richmond, attorneys for Boyle's family argued that the Sikorsky division of United Technologies Corp. had designed a defective escape hatch for the copilot, and that an improperly repaired steering device had caused the crash.

A jury awarded Boyle's survivors $725,000.

The appeals court, however, found that there had been enough "back-and-forth discussions between Sikorsky and the Navy . . . to establish government approval of the design."

The third case involved Navy Lt. Cmdr. Eliot Tozer, who was killed in 1980 when his RF-8G reconnaissance plane crashed off the coast of California as it was executing a low-altitude, high-speed fly-by of its carrier, the USS Kitty Hawk.

At a trial in Baltimore before U.S. District Judge Herbert F. Murray, attorneys for Tozer's wife and children contended the plane had crashed because a panel known as a "Buick hood" had come off, causing him to lose control.

A jury had awarded $350,000 to his widow, Joan S. Tozer of Crofton, Md., and $50,000 to each of his two daughters.