A U.S. District Court jury, which two weeks ago awarded $500,000 to a Vietnamese orphan injured in the 1975 crash of an Air Force jetliner near Saigon, yesterday declined to award any damages to a second orphan who was a passenger on the same plane and who contended he suffered similar injuries.

The jury reached its verdict in the case of 6-year-old James Mathew Zimmerly after about five hours of deliberation following a two-week trial before Judge Louis F. Oberdorfer.Zimmerly now lives with his adoptive parents in St. Louis.

The same jury of five women and one man awarded $500,000 to Michael Moses Schneider, 6, of Denver last April 14. The lawyers and the judge in both cases were the same and most of the evidence presented to the jury by each side was similar.

Zimmerly's case was the second of 63 lawsuits considered in behalf of the orphans against Lockheed Aircraft Corp. which manufactured the jumbo jetliner for the U.S. government. Lockheed and the federal government have agreed to share the cost of any damage awards.

Lawyers on both sides of the case yesterday refused to discuss the jury's verdicts.Five of the six jurors are scheduled to decide a third case growing out of the crash. One juror was replaced by an alternate after yesterday's verdict.

Zimmerly and Schneider were among passengers in the jumbo C5 A's troop compartment when the aircraft crashed into a rice paddy just outside Saigon. The plane was on a mercy mission from Saigon to the United States after the end of the Vietnam war. The Air Force said that 135 of 330 persons aboard the flight were killed, including 76 of 226 Vietnamese orphans.

In both the Zimmerly and Schneider cases, their lawyers contended that the boys suffered brain damage when the aircraft suddenly lost oxygen just before the crash. That injury, called "minimal brain dysfunction," resulted in a range of impairments, including learning and emotion problems, lawyers for the orphans argued.

Lockheed argued during the Zimmerly trial that if the boy suffered from any illnesses they were caused by malnutrition and other factors related to his treatment in Vietnam, not to the aircraft accident. Lockheed's lawyer, Carroll E. Dubuc, made a similar claim in the Schneider case.

The orphans are represented by Oren R. Lewis Jr. and attorneys from his Arlington law firm.

Zimmerly's lawyers also contended that his brain injuries also caused a condition known as hydrocephalus, or water on the brain, that could result in future brain damage.

Before the jury began deliberations the hydrocephalus issue was removed from the case after Lockheed agreed to pay Zimmerly a $30,000 settlement on that issue. The settlement was reached out of the presence of the jury.

In the Schneider case, the jury considered both the claim of brain dysfunction and the contention that Scheider suffered from a form of epilepsy following the accident. In the verdict, the same jury determined that while the accident had not specifically caused Schneider's injuries, it had aggravated a pre-existing condition.