Pilot error was the probable cause of the crash of Air Florida Flight 90 into the 14th Street bridge last Jan. 13, the National Transportation Safety Board ruled yesterday.

The crew of the twin-engine Boeing 737 took off in a heavy winter storm despite the presence of snow and ice on the wings, failed to use engine anti-icing devices and did not abort the takeoff despite signs of trouble on cockpit instruments, the board concluded after a seven-month investigation.

The board further said that the crew, even after taking off under those circumstances, could have prevented the crash if it had taken proper corrective measures.

Among lesser contributing factors in the accident, which killed 78 people, was 737s' "known inherent . . . characteristics" to "pitch-up" or rise suddenly at the nose when "even small amounts of snow or ice" are on the wings' leading edges, the board said.

Other contributing factors were the 49-minute delay between takeoff and the time that the plane was cleared of snow and ice at the gate, during which time more snow or ice built up on it, and the two-man crew's limited experience flying jetliners in winter weather, according to the board.

Air Florida General Counsel Robert Silverberg declined comment yesterday on the board's conclusions but pointed out that the airline's submission to the board had laid primary blame for the crash on what he called "uncontrollable" pitch-up by the 737 after liftoff.

An official of The Airline Pilots Association also blamed pitch-up as the prime cause of the crash and rejected the board's findings of pilot error.

Twenty-three other incidents of 737 pitch-up in icy weather, none of which resulted in accidents, have been reported since 1968, and Boeing, the plane's manufacturer, has issued three bulletins advising airlines how to cope with it.

Seventy-four people aboard the jet and four motorists died after it plowed into rush-hour traffic on the 14th Street bridge's in-bound span and plunged into the Potomac River at about 4 p.m. Four passengers and a flight attendant were pulled from the river alive.

From the start, the safety board concentrated on theories that accumulations of ice or snow had damaged the wings' ability to generate lift and that the cockpit crew inadvertently had set their throttles too low.

The crew set power at about 80 percent of the desired level, the board concluded yesterday, because ice had blocked thrust gauges in the jet's two engines, causing artificially high thrust readings in the cockpit. If engine anti-icing devices had been turned on, the ice blocking the gauges would have melted and correct readings would have resulted, the board said.

Neither one of these factors alone should have resulted in a crash, the board said.

Despite the combination of the two, the plane could have remained airborne if appropriate controls had been applied to bring the nose down and thrust had been added right after liftoff, the board said. "While the flight crew did add appropriate pitch control, it did not add thrust in time to prevent impact," the board concluded.

The board concluded that the airplane was properly equipped and maintained, and that the two pilots, Captain Larry Wheaton and First Officer Roger Alan Pettit, were properly certified and qualified.

American Airlines personnel, working under a ground service contract with Air Florida, applied different mixtures of de-icing fluid to the plane's right and left sides as it was parked at the gate. This was deficient and "not consistent with American Airlines' own procedures," the board ruled.

Neither Air Florida's maintenance chief at National Airport nor Wheaton verified that the plane was free of ice or snow before it left the gate, according to the board.

"Contrary to flight manual guidance, the flight crew used reverse thrust in an attempt to move the aircraft from the ramp," the report said, and this resulted in the engines' blowing snow into the air, "which might have adhered to the aircraft."

"The flight crew was aware of the adherence of snow or ice to the wings while on the ground awaiting take-off clearance, according to the report. Federal Aviation Administration regulations forbid pilots to take off if there is ice or snow on their planes.

First Officer Pettit, who was flying the plane, was aware of "an anomaly in engine instrument readings or throttle position" during the takeoff roll. "Although the first officer expressed concern that something was 'not right' to the captain four times during the takeoff, the captain took no action to reject the takeoff," the board concluded.

Due to the low thrust, the plane took an additional 15 seconds and 2,000 feet of runway to reach takeoff speed. After liftoff, the report said, "snow and/or ice contamination on the wing leading edges produced a nose-up pitching . . . . " A cockpit warning device activated in the cockpit to warn the pilots the plane was about to stall or lose lift due to insufficient airflow over the wings.

The board also concluded that a tower controller "erred in judgement and violated ATC air traffic control procedures when he cleared Flight 90 to take off ahead of arriving Eastern Airlines Flight 1451 with less than the required separation." This "jeopardized safety and created a hazardous situation."

The board ruled that the FAA had allowed National to become congested the day of the accident, resulting in delays between de-icing and takeoff. The airport's emergency plan met federal standards but "had not been tested for use in ice-covered waters and it proved ineffective."

The board does not determine liability for financial damages in the accident and its report is not admissible as evidence in court. However, board experts can be deposed for civil litigation.