The ground crew responsible for de-icing Air Florida Flight 90 washed it down unevenly with unusually diluted fluid and in at least one procedure violated the airline's own maintenance regulations, testimony at federal hearings indicated yesterday.

The fourth day of hearings by the National Transportation Safety Board on the Air Florida crash was dominated by the de-icing question and brought to light widespread disagreement within the airline industry on how to de-ice planes and how much protection antifreeze chemicals actually provide against new snow build-ups.

Flight 90 crashed into the Potomac River in a blinding snowstorm on Jan. 13, 30 seconds after taking off from National Airport. Seventy-eight people died. Investigators have devised a wide range of theories on the cause of the crash, but ice on the wings has emerged as a prime suspect.

Pilots say that de-icing is crucial to safe flying in bad weather because ice can change the shape of a wing's airfoil and reduce its ability to create lift. The safety board has cited wing ice as a factor in two air crashes in recent years.

Testifying yesterday, the two American Airlines ground crewmen who de-iced Flight 90 and American's National Airport maintenance chief all said they did not know what Air Florida's manual says about de-icing. American services Air Florida planes at National under contract.

Robert F. McKeon, a mechanic, told the hearing that he sprayed the jet's left side twice, setting his sprayer to produce a solution of 30 percent ethelene glycol, an antifreeze agent, and 70 percent water. McKeon said that he raised the glycol setting from the standard 25 percent. "I've never de-iced any other way," he said.

The second mechanic, Sylvester R. Thompson, who took over from McKeon to do the right side of the Boeing 737, told the hearing that he used hot water for the first treatment and then switched to a 25 percent glycol setting for the second spraying, which was intended to forstall the buildup of new snow on the plane. He described that as standard procedure.

Laboratory tests conducted by the FBI using the de-icing truck that the two men used indicate that the nozzle gave false mixtures, actually delivering a 13 percent mixture when it was set to give 25 percent.

Air Florida's manual specifies that engine inlets should be plugged before de-icing. But the two mechanics testified that they did not cover them, saying they were not familiar with Air Florida's manual. American Airlines does not operate 737s.

Air Florida's procedure reflects concern that de-icing spray can damage the insides of an engine. Other airlines, including American, direct maintenance crewmen to be careful in directing the de-icing spray but do not require the engines to be covered.

Investigators have been examining the possibility that thrust sensors located in the engines of the Air Florida jet may have been blocked by ice at takeoff, giving the pilots false instrument readings that caused them to set their throttles too low.

McKeon said that after de-icing Boeing 727s, which have three engines mounted high in the tail and on the rear fuselage, he normally checks the engines from the truck's cherry-picker basket to make sure they are clear of ice. However, he said, a 737's two engines are underneath the wings and he did not check the engine on his side.

Thompson said that after the de-icing, when a tractor was unable to pull Flight 90 away from the gate, its pilots put their engines into reverse to try to back the plane out. "I had to duck out the way because the snow was swirling around so much," he testified.

Many airlines caution against using reverse thrust in icy weather because blown snow can melt and then refreeze on contact with the plane's body. Thompson said he made a quick check of the plane afterward, but it remains unclear whether anyone gave it a thorough look for new ice before it taxied out for take-off.

In a statement submitted to the board, Air Florida maintenance supervisor Juan Cruz said that as the plane pulled away from the gate, he remarked: "I would not be surprised if this turkey returns," a reference to the heavy snow falling and the long line of planes awaiting take-off.

Much of the testimony during this week's hearings has indicated fundamental disagreements or ignorance within the aviation industry over de-icing procedures and chemicals.

At certain temperatures, American Airlines' manual calls for ground crews to clear away snow using a glycol-and-water solution. But the Boeing manual for the 737 specifically warns against using a solution for snow removal, saying it can mix with the snow and refreeze.

Trump Inc., the Nebraska-based manufacturer of the de-icing truck used on Flight 90, recommends clearing snow away with jets of hot water, then spraying dilutedglycol coating onto the cleared plane as an anti-icing agent to block new build-up, as Thompson did on Flight 90's right side.

But Union Carbide, which investigatators believe manufactured the glycol used on Flight 90, recommends clearing ice with anit-ice protection, if needed. n spraying on an undiluted covering for anti-ice protection, if needed. "Do NOT use diluted de-icing fluid for anti-icing treatment of ice-free aircraft," a company brochure says.

An Eastern Air Lines captain, Dean E. Martin, who was on the runway the day of the crash, told the hearing yesterday that Eastern believes its anti-icing procedures give a plane about 40 minutes' protection from new ice. But a Union Carbide glycol specialist, Dr. Sam Livengood, said his company has no idea how long anti-icing treatment lasts.

Yesterday, American Airlines assistant vice president Norman S. Rice said the airline had reversed a decision to use a 40 percent glycol solution on planes. As a precaution, the airline switched from the standard 25 percent after the accident while de-icing trucks were examined, Rice said, but no problems were found and the old procedures have been restored.