McDonnell Douglas Corp. gave spectators in California's Mojave Desert a glimpse into the future of commercial aviation with a 20-minute test flight of an aircraft propelled by an ultra high bypass engine.

The fuel-efficient UHB engine, which was built by General Electric, was installed in place of a regular turbofan engine on the left side of an MD-80 jetliner. The aircraft flew for 20 minutes, reaching an altitude of 13,000 feet, before landing at Edwards Air Force Base.Long Beach-based Douglas Aircraft Co., a subsidiary of McDonnell Douglas Corp. of St. Louis, has high hopes for the UHB propulsion system. The unducted fan engine is expected to burn 25 percent to 40 percent less fuel than the most advanced turbofan engines in use.

The company is designing two aircraft, the MD-91X and MD-92X, around the UHB engine. These aircraft would be an evolutionary development of the MD-80 jetliner. The MD-80s could, however, be retrofitted with UHB engines.

Jim Worsham, corporate vice president, said Monday's test was cut short

from a planned two-hour flight because a trouble light malfunctioned.

"It was a nuisance instrumentation failure," he said. "We expect to fly again Thursday."

When the trouble light went on, the pilot decided it was best to land the aircraft at Edwards Air Force Base sooner than originally planned. After examining the aircraft, engineers determined nothing was wrong with the engine and it was the light itself that malfunctioned, Mr. Worsham said.

The fan blades on the UHB engine are contoured to enhance aerodynamic efficiency. The blades measure as much as 13 feet in diameter, much larger than those in regular engines. Two sets of blades on the UHB engine rotate in opposite directions, with the second set smoothing the air flow created by the first row, reducing drag.

Mr. Worsham said design studies indicate UHB engines are best located on the aft fuselage of a jet aircraft. MD-80 aircraft already being constructed by McDonnell Douglas are powered by aft-mounted turbofan engines, so these planes easily could be retrofitted with the UHB propulsion systems.

The built-in capability for retrofitting the MD-80 family of aircraft puts McDonnell Douglas "far out in front of the competition" (Boeing Corp. and Airbus Industrie) in the race to launch an aircraft with UHB engines, Mr. Worsham said.

He said the GE engine will be tested during 60 to 80 hours of flight time over the next four months, and then will be removed and replaced in the fourth quarter of 1987 by the 578-DX UHB engine being developed by Pratt & Whitney and the Allison Division of General Motors.

McDonnell Douglas is testing both engines because it normally prefers to give customers a choice of power plants, Mr. Worsham explained.

If the tests proceed well, McDonnell Douglas will begin extensive talks with airlines to determine their specifications for UHB-powered aircraft. Mr. Worsham said he is aiming at certification and launch of UHB aircraft in the early 1990s.

The MD-91X and MD-92X jetliners that McDonnell Douglas is designing will have seating capacity of about 114 and 155, respectively. They will thus be sized slightly smaller and larger than the existing 142-seat MD-80.

Mr. Worsham said McDonnell Douglas eventually will apply UHB technology to the long-distance, wide-body MD-11 aircraft that is planned for launch in the early 1990s, but the initial focus will be on the smaller MD-91X and MD-92X aircraft, as well as in retrofitting MD-80s.

If the price of fuel continues to rise, he added, airlines will have an added incentive to retrofit their MD-80s with UHB engines.

Monday's first test flight of the UHB engine occurred several weeks ahead of schedule. McDon nell Douglas originally planned to run the first test around June 10.