A U.S. District Court jury awarded $500,000 in damages yesterday to a Vietnamese child who was injured when an Air Force jet transport carrying orphans on a mercy mission from Saigon to the United States crashed into a marshy rice paddy five years ago.

The jury of five women and one man deliberated for almost 18 hours before it made the award to 6-year-old Michael Moses Schneider, who now lives with his adoptive parents in Denver.

The boy's case was the first of 63 lawsuits to come to trial in the U.S. court here in behalf of orphans who survived one of the last tragedies of the Vietnam war.

According to Air Force figures, 135 of 330 persons aboard the flight were killed, including 76 of 226 Vietnamese orphans on their way to new homes in the United States and Europe.

The damage award will be paid by the Lockheed Aircraft Corporation, which manufactured the jumbo C5A aircraft, and the U.S. government. The terms of an agreement to share the damage costs are secret.

Both Lockheed and the government agreed prior to the trial that they would assume liability for the crash, leaving the jury to decide only the amount of damages -- if any -- to be awarded.

According to court records the boy will immediately receive $150,000 of the damage award, which Lockheed has agreed to pay whether or not the company appeals the jury's decision. The money is nonrefundable, the court records said.

During a four-week trial before Judge Louis F. Oberdorfer, the boy's lawyers had argued that a temporary loss of oxygen during the accident and the lunging of the aircraft as it plunged to the ground left the boy with epilepsy and "minimal brain dysfunction."

The injury resulted in a number of serious problems including learning disabilities, impaired motor skills and emotional problems, the boy's lawyers told the jury.

Lockheed had contended that the boy's disabilities were not caused by the accident but were due to poor condidtions at an orphanage in a northern suburb of Saigon where the child spent the first 19 months of his life.

The jury found yesterday that Schneider's injuries were not directly caused by the crash, but that his physical condition at the time was aggravated during the accident by loss of oxygen, the crash impact, and the psychological traumma of the disaster.

Schneider, who was named Nguyen Phi Khanh in Vietnam, was one of about 145 infants strapped two and three to a seat in the troop compartment of the transport when it took off April 4, 1975 -- the first official orphan airlift ordered by then President Gerald R. Ford. The plane began its climb and headed out over the South China Sea.

Fifteen minutes after takeoff from Tan Son Nhut air base, a cargo door blew out and control cables and hydraulic lines were severed. The pilot turned back for Saion, but the aircraft fell into marshy land about two miles short of the air base. It skidded for about 1,000 feet, bounced into the air momentarily, crossed the narrow Saigon River and then hit the ground a second time, breaking into four huge chunks and countless small pieces.

Two babies who were cuddled in seats like Schneider died of injuries suffered in the crash. A group of 100 older children were strapped to a lower-level cargo deck. All but a few of those children were crushed when the aircraft bellied into a rice paddy.

Lockheed argued during the trial that the children in the troop compartment showed no visible signs of injury or oxygen loss after the accident.

"They [Lockheed] would have you believe that was a joyride," Oren R. Lewis Jr., lead counsel for Schneider, told the jury during his closing argument last Thursday.

Instead, Lewis argued, the huge aircraft broke up and disintegrated "like a piece of junk."

A nurse in the troop compartment with the infants testified during the trial that after the crash the babies appeared lethargic and almost unconscious due to a lack of oxygen. Lockheed's chief lawyer, Carrolle E. Dubuc, told the jury, however, that other witnesses had testified that the same children showed no such adverse effects.

"The accident, although tragic, may not have been the biggest tragedy in Michael Schneider's life," Dubuc told the jury.

Dubuc cited testimony from a doctor and a Catholic relief worker who said the orphanage -- run by French priests -- where Schneider lived until he was 19 months old had inadequate food and medical care a high incidence of disease and a "very high morality rate."

Poor nutrition and the emotional trauma that comes with abandonment could cause the same injuries that Lewis said were due to the accident Dubuc argued.

In addition to the Schneider case two other cases for individual orphans who survived the crash have been scheduled for trial before Judge Oberdorfer and the same jury. The second trial is to begin tomorrow. As in most aviation disaster cases, it is expected that the lawyers then will use the jury verdicts as a gauge to reach out-of-court settlements in some of the remaining cases.

Lawsuits against Lockheed for the benefit of the surviving orphans initially were filed in 1976 by Friends for All Children, a Colorado-bases relief organization that ran a nursery in Saigon and helped assemble the children for the ill-fated evacuation flight. The agency hired Lewis and his Arlington law firm to represent the children.

Two years laer, Lockheed challenged the agency's right to bring the suits for the children. THE BIG MANUFACTURER ALSO ACCUSED THE Lewis firm of improperly soliciting cases after the firm wrote to adoptive parents and suggested that they substitute themselves as the children's representatives, according to court records of the case.

At the same time, the attorney general received a complaint from actor Yul Brynner -- who adopted two of the orphans -- charging that the letter from the Lewis firm failed to fully disclose the parents' options in the case, in vilation of the American Bar Association's Canons of Professional Ethics.

At that point, Oberdorfer asked Washington lawyer Charles R. Work, a former president of the D.C. Bar, to investigate the charges for the court. In January 1979, Work cleared the Lewis firm of any wrongdoing and suggested that the court appoint a guardian to protect the children's interest in the outcome of the lawsuits.

Oberdorfer agreed and named Work, along with other members of his law firm, to be the childrens' guardian for the duration of the lawsuits. Work decided that the Lewis firm, which is experienced in aviation disaster litigation, should stay on the case as lawyers for the individual children. Work, as the children's advocate, has monitored the litigation and is expected to set up trust funds for children who received damage awards or settlements as a result of the lawsuits.