The crash of Air Florida Flight 90 looks more and more like a classic aviation accident, one in which a number of long-recognized problems -- any three or four of which the plane could have survived -- combined for disaster.

It will be months before the National Transportation Safety Board reaches its final conclusion about what caused the Jan. 13 accident and the deaths of 78 people, but investigators and other experts feel strongly that all of these factors played a role:

* The actions of the cockpit crew in wretched weather. It is probable that the pilot and copilot did not fully appreciate the danger of their situation until it was too late.

* Snow and ice on the wings, which could have kept the plane from flying once it left the ground.

* The Boeing 737's history of pitching up suddenly after takeoff if the front edge of its wings is covered, even slightly, with ice and snow.

* Engine instrument sensors may have been frozen, which could have misled the crew into thinking their engines were working harder than they really were.

* Snow and ice on the runway, which could have slowed the acceleration of the aircraft on takeoff.

* The shortness of the runway at National Airport, which reduced both the time the crew members had to make decisions and the options available to them after they started their takeoff.

* The absence of a requirement that pilots taking off monitor not only the speed of the plane, but the time and runway distance they have used in reaching that speed.

* The absence of markers along the runways at civil airports telling pilots how much runway is left. The Air Force requires such markers.

In short, the accident investigation has turned into a series of "what-ifs?" What if the pilot had done this? What if federal regulations had required that? What if the airport had longer runways? What if airlines did not try to fly when it was snowing?

Regardless of where blame ultimately is placed, the weather was a dominant factor in the crash. Statistics developed by the safety board show that weather was a cause or a factor in 46.5 percent of all airline accidents between 1970 and 1979 and 46.2 percent of all fatal airline accidents. Aviation literature is full of warnings.

Donald W. Madole, a Washington lawyer who specializes in aircraft accident litigation, said in an interview, "I think that a takeoff on any runway-limited airport, in low visibility with slush and snow, is a test flight with passengers aboard." Madole is one of the lawyers representing Joseph F. Stiley III, a surviving passenger of Flight 90.

Very little is known so far about what Capt. Larry Wheaton and First Officer Roger Alan Pettit were saying to each other as they planned for the takeoff and in the short seconds of their flight, but it is on their shoulders that responsibility ultimately rests, assuming there was no unforseeable mechanical failure. Federal regulations are clear: "Each pilot in command has full control and authority in the operation of the aircraft, without limitation . . . "

It is known that 43 minutes elapsed between the plane's last de-icing treatment and the takeoff and that between .7 and 1 inch of new snow fell in that period. The safety board has said since the accident that no more than 20 minutes should pass if it is snowing. The FAA regulations say, "No pilot may take off an airplane that has frost, snow or ice adhering to" engines, windshields, wings or control surfaces.

Ice and snow are dangerous because they can alter a wing's contours and reduce the wing's ability to lift the airplane. That can mean a plane will stall -- lose the ability to remain in flight -- at a higher speed than normal. But if the pilot believes he is going too fast to stall, he may take steps that only compound the problem.

Despite precautions, such as de-icing treatments and FAA warnings to all pilots, wing ice continues to figure in plane crashes. Two recent crashes led the safety board to recommend in November 1980 that the FAA formally remind airplane operators that ice still can form on wings after treatment with diluted de-icing fluids and that it distribute information on de-icing fluids and their use. The FAA concurred with the first recommendation, but said the second was a job for fluid manufacturers.

A surviving passenger of Flight 90 recalled that ice on the plane was heavy enough to obscure the view out of windows, and that at least one window was still obscured when the plane left the terminal, though the ice fell away as the plane moved.

The captain of a Braniff plane that saw Flight 90 as it raced down the runway later has said he noticed ice on the fuselage and wings; his copilot recalls seeing it only on the fuselage. Other pilots reported noticing no ice on the plane.

The Air Florida crew ordered de-icing treatments twice before the flight. The first was interrupted when the main runway at National Airport was closed for plowing and sanding. The second, from a truck operated by American Airlines, began at about 3:15 p.m. A worker on the truck, using a hose and a nozzle, sprayed the plane's fuselage, wings and control surfaces. The treatment should have been 40 percent ethylene glycol and 60 percent water, according to investigators.

Wing ice never has been blamed for a 737 crash, but over the years pilots have reported that on takeoff in icy conditions the 737 can pitch up and roll without warning. Boeing has issued two bulletins warning airlines of this peculiarity and recommended cold-weather flight procedures to avoid it.

"The importance of maintaining clean, smooth wing leading edges must again be emphasized since a properly maintained wing would eliminate the need for any other actions," an article in Boeing's internal "Airliner" magazine warned late last year. Further, the article said, if there is frost on the wing leading edge, "the airplane must be accelerated an additional 10 knots on the ground."

In other words, if Flight 90's crew members did not add substantial speed to their takeoff calculations to compensate for possible ice or frost on their wings, they were at risk.

If the 737 was carrying enough ice to damage lift, it might still have got off the ground due to a phenomenon pilots call ground effect. A cushion of air builds beneath a plane as it gains speed on a runway. That cushion provides extra lift while the plane is close to the ground, but dissipates as the plane climbs. The highest altitude the Air Florida plane reached was 337 feet, investigators believe, before it crashed into the 14th Street bridge. Jets frequently climb 1,000 feet or more by the time they cross the bridge.

Investigators have said the 737 took an abnormally long time to accelerate to takeoff speed -- 47 seconds as opposed to the standard 30 seconds. That fits another crash theory in which the villain is ice, this time in the engine.

Because ice can damage the interior of high performance jet engines, the engines include devices that, when activated, divert a small amount of thrust to warm the inlet in the same way that a car heater channels motor warmth to the passenger compartment.

One part that is warmed is a sensor on the engine's nose. It measures the pressure of air going into the engine, and matched with the reading of another sensor in the back of the engine, produces an "engine pressure ratio" reading in the cockpit that shows how much thrust the engine is producing.

If the forward sensors on the Air Florida jet's two engines were frozen, the pilot could have seen artificially high readings on thrust, leading him to set the throttle too low. That is one possible explanation for the longer than normal takeoff roll.

However, there are three other instruments reporting conditions in the engines that would also be monitored by a prudent pilot, according to industry sources, and the readings on those three instruments should tell the crew either they had a bad gauge or to suspect engine performance, those sources say.

It is a preliminary conclusion of the safety board that the cockpit toggle switch that would give the engine anti-ice protection, and thus thaw the instrument sensors, was in the "off" position during takeoff. The Boeing 737 flight manual says that "Engine anti-icing should be turned on during all ground operations, takeoff and climb when icing conditions exist or are anticipated."

It is unclear, however, whether according to Air Florida procedures the engine de-icing should have been on. According to the safety board, Air Florida's 737 manual requires its use in wet snow conditions but not with dry snow. The contents of the last weather report to the crew, including whether the snow was considered "wet" or "dry", is unclear. A written weather report Wheaton and Pettit had in the cockpit was destroyed in the crash.

Slow acceleration on takeoff could also be the result of heavy ice and snow buildup on the runway. Studies going back to 1960 show that water, slush and ice on the runway substantially slow jetliners on takeoff. An FAA advisory says that "takeoffs should not be attempted when standing water, slush or wet snow greater than 1/2 inch in depth covers an appreciable part of the runway."

Pilots have said that between 1/4 and 1/2 inch of snow was on the runway at the time of Flight 90's takeoff.

Air traffic controllers cleared Flight 90 for takeoff at 3:59:24 p.m. It was the 16th plane to take off since the runway had been reopened after plowing. Other pilots reported no problems, but the board is studying their flight data recorders to check the performance of those aircraft against that of Flight 90.

Survivor Joseph Stiley told reporters that Flight 90 seemed to falter as it sped down the runway and it seemed as if the pilot was trying to abort the takeoff. Witnesses said Flight 90 finally lifted off near the intersection with Runway 33, about 1,300 feet farther than the 3,900 feet the plane should have needed for take-off.

Despite slow acceleration, the plane reached speeds that would be adequate for takeoff and flight -- if everything were normal. However, the plane began to lose speed quickly as it climbed, pitched nose-up, some say as high as 30 degrees, then began falling.

If there were ice on the wings, frozen instrument sensors and a snow-packed runway, everything was not normal. Furthermore, the runway at National Airport, perfectly safe by the agreement of almost all aviators for good weather operations, becomes a little riskier when it is wet.

The pilot computes the takeoff speeds based on a chart that considers runway length, weight of the airplane, and other factors, including weather. The critical computations are called V1, the go, no-go speed, and accelerate-stop distance -- roughly defined as the amount of runway it will take to safely stop the airplane if the takeoff is aborted at V1. Correction factors must be included for rain, snow and ice.

Once the plane starts rolling on takeoff, however, the only thing the pilot monitors is V1. If the runway is wet and icy, and he has not included enough allowance for how much that retards acceleration rates, and if his engine is not developing as much thrust as he thinks it is, he conceivably could run out of runway before reaching V1--and still be conforming.

If the runway was longer (the runways at Dulles, National's critics point out, are almost twice as long) the pilot would have more room to recover from a mistake.

The Air Force puts up big signs beside its runways that tell pilots how much room is left. The safety board and pilot groups have suggested to the FAA that it require similar signs at civil airports. The FAA proposed a rule requiring such signs in 1975, then withdrew it in 1977.

"In general," the FAA noted in its withdrawal notice, "opposition centers around the high cost of installation and maintenance of the markers when compared to what many believe are limited benefits to be derived from their installation."

Further, the FAA said, "the use of runway markers during takeoff might result in a potentially unsafe situation since the pilot not in control would be required to monitor passage of the markers. This would divert his attention from adequately monitoring engine and flight instruments . . . during a most critical period of the takeoff."

The withdrawal notice was signed by R.P. Scully, then associate administrator for flight standards, and now senior vice president for operations at Air Florida.