Federal investigators decided yesterday to give special attention to whether the unusual takeoff and other flight requirements of National Airport played a role in the crash Wednesday of Air Florida Flight 90, according to informed sources.

An airport investigating team was being organized last night by the National Transportation Safety Board as other specialists continued to concentrate on whether Flight 90 left National with too much ice and snow on its wings and fuselage. Seventy-eight people died when the plane, a Boeing 737, struck the 14th Street bridge and plunged into the Potomac River.

The decision to examine the airport comes after years of allegations that it is not safe. The charges have been leveled by politicians, pilots, tourists and area residents, who point to National's short runways, its winding approach and takeoff patterns and the lack of unpaved overruns at the end of some runways.

The airport investigating team will address those issues, sources said, and will include industry officials as well as safety board specialists. The Federal Aviation Administration, which owns and operates National Airport, has regularly denied that the airport has safety problems.

Meanwhile, investigators learned yesterday that 45 minutes elapsed between the time the Air Florida jetliner received de-icing treatment and the time it took off.

Safety board member Francis B. McAdams said that a Braniff Airlines pilot who saw the plane taking off amid heavy snow "noticed ice, not only on the wings, but also on the fuselage.

"If he was able to notice ice under those conditions, perhaps there was quite a buildup," McAdams said.

Federal aviation regulations state, "No person may take off an aircraft when frost, snow or ice is adhering to the wings, control surfaces or propellers." Last April 10, the FAA published an Air Carriers Operators Bulletin--a nonbinding advisory--stating that airlines "should emphasize that a visual inspection of the aircraft should be made just prior to takeoff by the pilot if any delay is involved following de-icing of the aircraft."

Boeing officials said that a pilot could see only part of the wing from the cockpit window. McAdams, in a briefing last night, said the board does not yet have Air Florida's manual and does not know if the pilot is required to look out the passenger-area windows to check the wings for snow and ice.

The problem in taking off with snow and ice on the wings, safety experts agree, is not so much the additional weight but the effect they have on the way air flows over the wings, which lifts an airplane and holds it aloft. A less critical effect is sluggishness of controls.

Wings on modern jetliners are designed by computers to give maximum lift. Changes in the contours of a wing--and a chunk of ice would be a change--mean that an airplane cannot fly as efficiently. At a critical moment in flight--and none is more critical than takeoff--a loss of efficiency is potentially catastrophic.

The Boeing 737 has equipment that thaws ice as it forms on the forward edges of wings during flight. That equipment, however, will not clear snow and ice that builds up when the plane is on the ground.

The potential effect of loss of lift is a stall, which means that the flow of air around the wing is insufficient to keep the plane airborne. A warning sign of an impending stall is a violent shaking of the aircraft while its nose is pitched upward. That description is consistent, according to experts, with the accounts surviving passengers gave of events preceding the crash.

Air traffic control records, still being examined by officials, show that the plane never rose high enough off the runway to be picked up for more than eight seconds by the radar.

Investigators continued to caution yesterday that they have not declared icing as the cause of the accident, only that it is one of the prime suspects.

They were also awaiting the salvage from the river of the cockpit-voice and flight-data recorders that, they hope, will tell them what the crew members were saying and how the plane was behaving technically. The devices, the board has confirmed, are in the tail section. Most of the left wing was brought up late yesterday; it was the first major structural section of the plane recovered.

Meanwhile, investigators began to check such items as the visibility of the runway lights at National Airport, and the de-icing treatment the Air Florida plane received.

The runway lights at National Airport are color-coded to tell the pilot where he is. One of the things a pilot must know when taking off is his speed in relationship to his position on the runway. National's runway, at 6,870 feet, is shorter than most major airport runways, but well within the capabilities of the aircraft. McAdams said that, under normal conditions, the plane would have left the ground after accelerating over 3,900 feet.

McAdams said that Flight 90 was de-iced twice Wednesday, the second time with a mixture of 40 percent ethylene glycol and 60 percent water.

McAdams said there is no hard evidence as to whether the landing gear was up or down when the plane crashed. There are no doors covering the gear on a 737, he said, and so tires could have hit a car on the bridge (as they apparently did) "and leave a mark with the gear fully up."

Investigators also interviewed other pilots who flew out of National on the afternoon of the crash to find out about runway conditions.

Don McGuire, a vice president of Piedmont Airlines, said one of its 737s took off, almost fully loaded, eight minutes before the Air Florida flight and had no difficulty. It had visited the de-icing truck twice. Another Piedmont flight was in line behind Flight 90, but returned to the terminal when the runway was closed after the crash, and the pilot told his company he regarded the runway as being in fine shape for a takeoff.

One airplane--a private jet--took off after Flight 90 but before the crash was discovered.

FAA officials in Oklahoma City, where the records are kept, said yesterday that the plane that crashed had experienced two problems during flight in the past five years, neither regarded as critical. Last May 1, when the plane landed at Tampa, a part of the landing gear known as the "left main lower torsion link" failed, and was replaced before the plane was returned to service.

In July, also at Tampa, one of the plane's two engines failed shortly after takeoff. The plane returned to Tampa and the engine was changed.

Records at FAA headquarters show that last April Air Florida paid a $25,000 civil penalty for violations of various safety regulations in 1979 and 1980. Most of the violations, the documents show, involved delays in changing plane parts that are supposed to be replaced on a regular schedule. Other violations included flying a plane without a fully operational automatic pilot.