The Air Force said yesterday that loss of oil pressure in an engine seal caused the errant orbit of a $100 million communications satellite launched last April from the space shuttle Challenger.

"We considered 50 different possible causes of failure, and there is a 99 percent probability that we have pinpointed the cause of failure," Brig. Gen. Donald W. Henderson, chairman of a joint investigative board of officials from the Air Force and NASA, said at a news conference.

The seal was about the size of an automobile tire's inner tube and was filled with oil to act as a hydraulic control on the second stage rocket engine guiding NASA's Tracking Data and Relay Satellite to its destination.

The destination was a circular orbit 22,300 miles above Earth near the equator and over the South Atlantic Ocean, where it would hover and serve as a radio relay for all future shuttle flights and all civilian and military satellites in Earth orbits below 800 miles.

After intensive tests and examination of photographs taken by a secret Air Force camera in the mountains of New Mexico, the investigative board concluded that the seal broke, lost all oil pressure and lost its control over the engine, which swung into the wrong position and locked there while it was still firing. The board added that excess engine heat probably caused the broken seal.

Though NASA was able to get the TDRS satellite into proper orbit last month, the space agency and the Air Force have postponed two shuttle flights that were to use the suspect Inertial Upper Stage Engine, which was built for the Air Force by the Boeing Co.

A second TDRS satellite, due to be put into orbit next month on the eighth shuttle flight, has been put off until the 12th flight next March. The Air Force also was to use the IUS engine late this year to rocket a secret satellite into geosynchronous orbit 22,300 miles high, but has delayed this until the 15th flight late next year. CAPTION: Picture, Brig. Gen. Donald W. Henderson uses a model to describe problem investigators found on $100 million communications satellite. AP