An American Airlines supervisor whose crew de-iced Air Florida Flight 90 testified yesterday that he twice cautioned its pilot to delay de-icing until the last minute, telling him that antifreeze solution sprayed onto the plane would prevent new ice or snow buildup for only 10 to 15 minutes.

The de-icing was finished about 3:10 p.m. on Jan. 13 during a heavy snowfall. The jet remained at the gate until about 3:35, witnesses have said, and took off at about 4 p.m., close to 50 minutes after the de-icing. After 30 seconds in the air, the plane crashed into the Potomac River, killing 78 people.

Investigators believe that snow or ice on the wings may have damaged the jet's ability to stay in the air.

The testimony on de-icing came from George R. Lynch, the American Airlines crew chief, during the third day of hearings by the National Transportation Safety Board at an Arlington hotel. Flight 90 was serviced by American under a ground-maintenance contract.

Much of the questioning of Lynch and other witnesses tried to ascertain weather conditions, the content of weather reports that Air Florida's crew received, airport procedures and how much snow, if any, was on the Boeing 737 when it took off.

Two American Airlines employes sprayed the plane with a mixture of water and ethelene glycol, a de-icing chemical, Lynch said. After clearing snow accumulation, they sprayed the jet a second time to prevent new buildups, he said.

The Associated Press reported last night that investigators believe the de-icing solution used on the plane was substantially weaker than thought by the ground crewmen applying it.

Citing documents presented to the board, the AP said that tests conducted by the FBI and by American Airlines, showed that nozzles on the de-icing sprayers produced a weaker solution than the nozzle settings indicated.

When the nozzles were set to produce a 25 percent glycol solution, the FBI found that only 13 percent was applied, while American Arlines found that a 25 percent setting produced a 15 percent solution, according to AP.

Yesterday, Lynch testified that a 25 percent concentration was standard until sometime after the accident, when American Airlines issued a directive that a 40 percent glycol solution be used in de-icing.

Lynch declined to speculate on why the order was issued. But an American Airlines spokeman present at the hearings, called it "an extra precaution" and said "we felt it's a good idea . . . ."

Meanwhile, the Air Florida Pilots Association said that it would consider proposing that de-icing facilities be set up close to the runway so that planes could take off immediately after being sprayed. National Airport had such a facility some years ago but closed it, Federal Aviation Administration officials said.

Transcripts of conversations between pilots and the airport tower show that crews of many planes, including Flight 90, asked when they would be able to leave their gates. Testifying at the hearings, National controllers have said that in busy conditions there is no precise way of determining when a particular plane will be able to leave. Consequently, pilots may not be able to gauge the optimum time to de-ice.

In other testimony, tower chief Harry Hubbard defended his controllers' handling of Flight 90 and an Eastern Airlines jet that was cleared to land on the same runway. Investigators are trying to ascertain whether the two planes came dangerously close and whether Eastern touched down before Air Florida lifted off.

Hubbard said that an internal investigation he conducted showed that no "system error," a violation of federal separation limits, had occurred.