Monday was a kind of milestone for Lockheed-California.

It was the seventh anniversary of the aircraft manufacturer's first L-1011 TriStar widebodied jetliner in commercial service, and also was the day the first of the latest-technology planes in the L-1011 family-called the Dash 500-arrived in Lomden for British Airways.

Six firm orders for the long-range plane from British Airways and 20 more, including an even dozen from Pan Amercian World Airways, have made Lockheed officials optimistic that the L-1011 program, its onlu commercial airliner, will be in the black for the first time next year.

"We're now in a position where we expect to reach an annual cash-flow breakeven point in 1980 and be profitable," D. L. Dudas, Lockheed's vice president for plans and requirements, said in an interview at the company's headquarters i Burbank, about 60 miles southwest of the assembly plant here. "That means the investment Lockheed has made in the program will began to pay off, and it should pay off handsomely," Dudas said.

Nearly 160 of the various versions of the L-1011s put together at Lockheed's assembly and flight test center here at Palmdale are aready in service around the world, but aircraft manufacturers typically defer development costs of a plane until sales begin so the company can match costs incurred against revenues realized. Losses on the L-1011 program were in the $120 million range in each of the last three years, and Lockhees just reported that losses had increased in the first quarter because of higher-than-anticipated costs on aircraft currently in production. This principally is the result of an accelerated production rate and of the introduction of the Dash 500 model. Lockheed has been amortizing the costs of the L-1011 program at the rate of $50 million a year, and it still has $350 million to go. (McDonnell Douglas Corp. still has a substantial amount left to amortize for its DC-10, one of the L-1011's competitors.)

"It's a high-stake business," says Dudas, who is in charge of marketing the plane as well as assuring the cost and schedule objectives of the L-1011 program. "There are high development costs, but if you can market your product successfully, the rewards are high.

"We think the L-1011 is now moving into the harvest years."

The Dash 500 incorporates what Lockheed believes is the most advanced technology available. The first one delivered to British Airways last Monday, for instance, made what Lockheed says was the first fully automatic passenger-carrying flight between California and England. The plane was put under automatic control immediately upon takeoff ny BA Captain John D'Arcy, and neither he nor his crew touched the control wheel or rudder pedals until after the automatic landing 5,500 miles and 9 hours and 35 minutes later at London's Heathrow Airport, Lockheed reported last week.

A recently development flight management system in which a computer adjusts the throttles to permit maximum fuel economy-which Lockheed says saves 3 percent of the fuel of any given flights-was also on broad.

British Airways plans to use the new plane on a route between London and Abu Dhabi beginning this month; another TriStar to be delivered soon is to be used on a London-Singapore route.

The Dash 500 is the fifth derivative and the latest in a step-by-step evolution of the initial L-1011-1, which had a 3,300-mile range. The first variation was the same size and shape as the basic model but had a larger fuel tank, increasing its range to 4,500 miles. Another derivative submitted Rollsroyce engines with 14 percent more power than the original Rolls engines to give the plane more boost and performance, particularly in hot weather. The next derivative had increased take-off weight.

The Dash 500 is 13-1/2 feet shorter than the earlier L-1011s, has higher-thrust engines and has additional fuel capacity for nonstop flights up to 6,100 miles with a full load of passengers. (Although the plane can hold a maximum of 330 seats, BA's plane will carry 232 to 202 in coach and 30 in first class-and other orders are in the 240 seat to 250-seat range)

The initial L-1011 could take off fully loaded at 409,000 pounds with 158,000 pounds of fuel on board; the Dash 500 can take off fully loaded at 496,000 pounds with 212,000 pounds of fuel. The first models carried fuel only in their wings while the new plane has a center-section fuel tank as well. Except for the shorter length-164 feet- the other dimensions of the plane are the same, including the 155-foot wing apan.

A "house" Dash-500 parked near the plant is altered as new technology becomes available for testing; the house model now sports 4 1/2-foot extended wing tips and a new control system that Pan Am's versions-delivery starts next year-will have. The changes are designed to save another 3 percent fuel per flight.

Lockheed also is experimenting with a number of other technological changes, including the use of new structural materials-composite graphite-that will weigh less than metal but have the same strength. "We don't see the L-1011 being made obsolete by any new technology we can see in the next 10 years," says one Lockheed official. Like other new planes, the L-1011 is considerably more fuel efficient than the ones it is replacing in airlines' fleets, and it also meets the newer, more stringent noise standards Lockheed contends the L-11011 is the "quietest" of the large widebodies available.

So far, Pan Am is the biggest customer for the Dash 500, having ordered 12 at a cost of $500 million, including spare parts. The first two cost $85 million. If it picks up on its options for 14 more, the total package will mean $1.66 billion to Lockheed.

The company is talking with a number of airlines about other proposed derivatives for shorter routes, but thinks its models now can complete with other aircraft makers for the medium-range and long-range routes. Lockheed contends that the L-1011 has the perfect size and operating characteristics for the long-range, low-density routes that the larger Boeing 747 could not operate profitably because the 747 is economical on long routes only when it's filled. Lockheed's promotional material says the Dash 500 has the lowest breakeven load fctor of all the long-distance widebodies, enabling it to fully cover operating expenses with fewer passengers at all distances.

The L-1011s are assembled here in a $50 million plant built in 1970 just for the plane's production. The facility was built on the edge of the Mojave Desert, with the snow-capped peaks of the San Gabriel and San Bernadino mountains in the background, on some of the 677 acres of Lockheed-owned land located adjacent to an Air Force installation and air strip. It is a 20-minute plane ridem or a 60-mile drive, northeast from Lockheed's offices at the Hollwood-Burbank Airport.

Lockheed brings all the parts of the L-1011 here to Palmdale where they are assembled into planes in a large building the size of 9 football fields. The wings of the plane come on the oversize railroad cars on a rail spur alongside the plant. The rolls-royce engines come three at a time by cargo plane from Britain. Flight testing of the planes takes place right here, using the AF strip, for which Lockhees pays landing fees. CAPTION: Picture 1, D. L. DUDAS . . . TriStar payoff nears; Picture 2, Workers at Lockheed's Palmdale, Calif., plant work on a cockpit. Lockheed Corp.