Federal safety investigators were focusing yesterday on the possibility that ice on the wings caused an Air Florida jetliner to crash into the Potomac Wednesday afternoon, killing 78 persons.

The National Transportation Safety Board on Wednesday impounded the de-icing chemical spray that was used on the wings of the plane shortly before it took off from Washington National Airport. Francis B. McAdams, one of the five members of the investigating board, confirmed that the board would be looking particularly at icing as one of the prime suspects in the crash.

The investigating team held its first session yesterday at the Twin Bridges Marriott Hotel in Arlington. Only a few hundred yards away, federal and military personnel were busy organizing the effort to obtain the cockpit-voice and flight-data recorders from the tail of the plane, which lay submerged in 20 to 27 feet of water.

Amid freezing temperatures and snow that fell for a second straight day, divers began the slow, arduous process of recovering the plane and more than 60 bodies that may still be trapped inside..

As of 5 p.m. yesterday, when operations were suspended for the night, 11 bodies had been recovered from the river--nine from the plane and two persons swept off the northbound span of the 14th Street bridge by the plane before it plunged into the Potomac. Two more persons, injured on the bridge Wednesday, died yesterday afternoon.

About half the passengers on the plane were from the Washington area. Five persons who were on the plane survived as did four persons injured on the bridge.

Recovery of the plane and the bodies is the joint responsibility of the D.C. police and the National Transportation Safety Board. McAdams toured the crash site yesterday and said that the salvage would be "rather difficult." The divers who descended into the murky water yesterday had to feel their way about gingerly to avoid being cut by jagged metal as they tried to sketch the position and condition of the fuselage. The goal is to help the salvage crew determine how badly the plane was broken up.

The tip of the tail section of the twin-engine Boeing 737, where the two recorders were located, is visible. It is believed that the fuselage is in two parts. McAdams said about four divers, out of a total of six, would be underwater at any given time. Still to be decided, according to McAdams, is whether to recover the bodies before or after the plane is brought up and whether to attach cables to lift the plane or attempt to use flotation devices to bring it up.

Recovery of the recorders is is expected today and is regarded as essential to help establish what happened to the aircraft. Investigators stressed that they were early in their work and that nothing--from a pilot's heart attack to a mechanical problem to weather--had been ruled out as a possible cause. Also high on the list of the many factors the board will study is whether there was a problem with the engines.

Nonetheless, several investigators agreed privately that ice on the wings was receiving special attention because it is a logical explanation for what happened: a plane falling out of the sky almost immediately after taking off in a blinding snowstorm.

Air Florida officials said that de-icing--a process by which the wings and tail surfaces are sprayed with a special solution--was started at least twice Wednesday and completed the second time.

The first time was interrupted when the runways at National were closed for snow removal and sanding between 1:38 and 2:53 p.m. The plane, Flight 90 from Washington to Tampa and Fort Lauderdale, was scheduled to leave at 2:15, but did not pull away from the terminal to taxi to the runway until about 3:40. The plane was cleared for takeoff and started to roll at about 4 p.m.

"I assume," said Dave Mulligan, vice president for operations at Air Florida, "that the final de-icing occurred just before the plane left the terminal."

Mulligan said that a de-icing treatment in weather like Wednesday afternoon's should be effective for a maximum of an hour. One source said that the Federal Aviation Administration, after examining aircraft-control tapes, determined that about 20 minutes elapsed between the time the plane left the gate and the time it took off.

When safety board officials on Wednesday took custody of the de-icing fluid that was used on Flight 90 they impounded the fuel truck to check its contents for contaminants. Both steps again point to the areas of early suspicion.

The de-icing solution itself was applied to the Air Florida plane by American Airlines equipment, because American has a contract to provide ground maintenance at Washington National for Air Florida. Such arrangements among airlines are common.

Ice or snow on the wings is a problem because it can add weight to the plane and dramatically limit the lifting ability of the wings.

In recent years icing has been blamed partially in at least two fatal crashes in the United States. A commuter plane in Clarksburg, W.Va. crashed on takeoff on Feb. 12, 1979, killing the copilot and one passenger. In February 1980 an air cargo plane crashed near Boston, killing seven of eight persons on board.

On Nov. 14, 1980, the safety board issued a set of recommendations to cope with icing problems. It recommended that the FAA advise "operators of the danger of wet snow forming after airplanes are de-iced with diluted ethylene glycol solution," that anti-icing agents be studied and that detailed information on de-icing and anti-icing be published.

Last month the Airline Pilots Association's in-house publication, Engineering Notes, warned pilots that certain types of de-icing solutions give inadequate protection.

One pilot noted that ethylene glycol, when diluted with water, only removes ice and provides no protection against formation of new ice. So-called anti-icing, pure ethylene glycol, prevents formation of new ice but is more expensive. The composition of the de-icing solution used on the Air Florida jetliner is not yet known.

Visibility at the time of takeoff was between three-eighths and five-eighths of a mile. Mulligan said that it was legal and permissible for the flight to leave with visibility of one-fourth of a mile. At the time of takeoff, he said, the plane was carrying 26,000 pounds of fuel or 3,714 gallons of kerosene. Its total weight, including passengers and luggage, was 102,000 pounds. The maximum permitted weight, according to Mulligan of Air Florida, was 110,000.

Mulligan said safety board officials were in Air Florida's Miami headquarters checking maintenance records on the airplane and that everything was in order.

The runway itself, according to several sources, was wet but clear, although there may be been some slush buildup. In the cleaning process, snow was pushed and blown to the side with a plow, a giant broom or street sweeper ran over it, and the runway was treated with special sand to 50 feet on each side of the center line--a total of 100 feet. The runway is 150 feet wide.

After the runway was reopened, more than a dozen aircraft took off and a number of flights landed before Air Florida's Flight 90 crashed while taking off.

One underlying issue that has already been raised is the safety of National Airport, with its short runways and midcity location. Federal Aviation Administrator J. Lynn Helms addressed those questions Dec. 11 in a letter to Eastern Airlines President Frank Borman when Helms ruled that the wide-bodied A300 would not be permitted to use National.

Because of the short runway and the short unpaved area at the end of the same runway Flight 90 used, Helms wrote, "I cannot confirm that in every case sufficient margin would exist for proper clearing of the railroad bridge . . . ," a bridge close to the 14th Street highway bridge.

Officials confirmed yesterday that Helms, while studying the A300, also tested a 737 because it is similar in configuration and has the same number and location of engines, although it is much smaller. "He determined that the 737 is safe to take off from National and not limited by runway length," according to Linda Gosden, spokesman for the Transportation Department.

Gosden also confirmed that Air Florida had paid a civil penalty to the FAA sometime in 1981 for violating safety regulations, but said the records of the violations were not available yesterday because the pertinent federal offices were closed.

Officials familiar with that investigation said the violations concerned the maintenance of some Boeing 737s Air Florida had leased from Frontier Airlines, then returned to Frontier. Upon investigation, those officials said, most of the violations appeared to be of the paperwork variety and not of substance as safety issues.

Air Florida yesterday revised its tabulation of the number of persons aboard Flight 90 to 70 ticketed passengers, 1 employe, 3 infants and 5 crew members. Five persons were rescued from the plane in the river after the crash, leaving 74 persons presumed dead. Another four persons were fatally injured when the plane struck the bridge.

McAdams said board investigators had been told that the roof of one of the automobiles on the bridge carried tire imprints from the plane's landing gear, which apparently was still extended, almost a mile from the end of the runway.

Aviation experts agreed yesterday that, in a normal flight, the gear should have been raised by the time the plane reached the bridge and the plane should have been at least 500 feet above the bridge.

The diving teams, one from the Navy and one from the Coast Guard, were briefed by safety board experts yesterday. The Coast Guard team is to concentrate on recovering the recorders, the Navy team on the bodies of the victims.

The salvage operations continued to tie up both the north and southbound spans of the 14th Street bridge, which was closed to traffic and will remain so indefinitely. An eerie sort of quiet, broken only by the sound of aircraft taking off and flying overhead, prevailed at the scene yesterday. Police set up tight security around the salvage site to keep curiosity seekers away. Reporters were confined to the northbound span of the bridge, only yards away from the place where the plane severed the guard rail before plunging into the icy river.

At National Airport yesterday, Air Florida's Flight 90 took off with 53 passengers on board; the plane can hold 125. Although airlines sometimes change flight numbers after a crash, an Air Florida official at the gate said no such change had been considered for Flight 90.

According to airport operations officer James F. Kastner, National was temporarily shut down twice yesterday for sanding and plowing runways--once in the afternoon and the other in the early evening.

Washington Post staff writer John Burgess contributed to this report.