Engineers at Boeing Co.'s jumbo aircraft plant are starting work on detailed design of the new 747-400 freighter, but late delivery of passenger models has convinced the manufacturer not to offer early freighter delivery.

Executives for Boeing's 747 operation said at a briefing last week that the company's new airplane planning group last month started turning over its conceptual outlines for the new freighter to engineers, who will draw up detailed construction specifications.Derived from two planes already flying, the 747-400 will have the advanced aviation electronics and cockpit of the new 747-400 but the body of a 747-200. Boeing designers said the lighter 747-200 body will be used because most cargo lines would not be able to use the 747-400's stretched upper deck efficiently.

In the past, Boeing teams developing derivative models have been able to trim as much as 18 months off the standard four-year period required to move a new jetliner from the conceptual stage to first delivery for commercial service.

But the company said its effort to trim the introduction time for the 747-400 passenger and combination freight-passenger models to 30 months resulted in so many delays that Boeing will stick to a 48-month schedule with the 747-400 freighter.

''We learned our lesson," said Elizabeth Reese, Boeing spokeswoman for 747 models.

That means the first 747-400 freighter would not be delivered until 1994, even though Boeing recognizes there is an enormous demand for all-cargo aircraft, she said.

Deliveries of passenger models of the 747-400 were running three months behind schedule last year when a strike by Boeing's assembly line workers led to further delays. Since the strike, 747-400 production has been pushed to record levels in an attempt to catch up.

Bob Davis, a Boeing engineering executive, said the company is confident it can avoid those kinds of delays with the new freighters.

''We wouldn't think there'd be delays or problems," he said. "It's fairly straightforward - we're just taking the 747-400 avionics and putting them in a freighter airplane, which is a more simple airplane than the passenger model."

Sixteen airlines are currently flying 57 of the new jumbos. More than half of those are in the fleets of British Airways, Northwest Airlines, Singapore Air, Qantas, United or Japan Air Lines. Four airlines, KLM, Lufthansa, Malaysia Airlines and CAAC, are flying 11 747-400s as combis, with freight on the main passenger deck.