Air Florida has made major changes in its icy-weather takeoff procedures for Boeing 737s since the Jan. 13 crash of Flight 90 during a blinding snowstorm, according to documents made available yesterday at federal hearings into the crash.

The airline's chief 737 pilot ordered the new procedures five weeks after the plane, a 737, struck the 14th Street bridge seconds after lifting off from National Airport and plunged into the Potomac, killing 78 people.

A Feb. 19 bulletin instructed 737 pilots to accelerate to higher-than-normal runway speeds before liftoff in icy conditions, which would help offset any loss of lift caused by ice on the plane.

Another bulletin, issued Feb. 15, reemphasized the dangers of ice to a plane's body and engines and stressed the importance of pilots' making sure their planes are ice-free before takeoff.

Many of the new instructions reflect recommendations that the National Transportation Safety Board made after the crash, but some came on Air Florida's own initiative.

Before the crash, Boeing, the 737's manufacturer, had cautioned airlines that the plane can pitch up or roll without warning in icy flying. Air Florida's position is that its new instructions to its pilots grew out of increased awareness of those characteristics, and reflect no judgment on the performance of Flight 90's pilots or the airline on Jan. 13.

Disclosure of the changes came as federal investigators closely questioned three senior Air Florida officials on safety procedures during the eighth day of hearings at an Arlington hotel. The hearings are directed by the safety board, which is heading the investigation.

Probable cause of the crash will not be officially determined for months. But one major theory that has emerged in testimony is that the Air Florida jet took off caked with ice or snow which damaged its wings' ability to generate lift and keep the plane in the air. Taking off with ice on the wings violates federal rules and basic principles of safe aviation.

The 737s tendency to pitch up and roll could have compounded any lift problem, according to investigators.

Ice figures prominently in another crash theory which holds that the pilots did not turn on engine anti-ice devices, causing thrust sensors in the engines to freeze. That resulted in erroneous thrust readings in the cockpit, which in turn led the pilots to set their throttles abnormally low, the theory goes.

Air Florida's new bad weather flight procedures provide for the following:

* Pilots of 737s now assume that their planes are 8,000 pounds heavier than they really are when taking off in icy conditions. With a load of 91 passengers this would require pilots to accelerate to about 168 mph, about 8 mph faster than usual, before they lift up the nose and make the plane fly.

The instructions to the pilots note "increasing concern in the aviation community" that wing ice can be more dangerous than previously thought. The increased takeoff speed goes beyond steps Boeing has recommended to airlines in three warning bulletins issued between 1974 and 1981. Air Florida officials have suggested that Boeing has not done enough to counter the problem.

* Pilots are required to turn on engine anti-icing if temperatures are 46 degrees or less and any type of snow is falling. This supersedes Air Florida's manual, which appears to say that engine anti-icing is not necessary if the snow is "dry." Many pilots feel that either wet or dry snow can cause thrust probes to freeze and engine anti-icing should be used in any type of snow. The issue has surfaced repeatedly during the hearings.

Examination of Flight 90's wreckage indicates the engine anti-icing was off. Outside temperatures were about 24 degrees, which may have led the pilots to conclude that the snow was dry and engine anti-icing was not needed.

* Pilots must assure that their planes get a detailed inspection after being de-iced, or treated with anti-freeze chemicals to remove snow or ice. It also requires a visual reinspection of the plane if snow is falling and more than 20 minutes passes between de-icing and takeoff. This was based on safety board recommendations.

* Crews are reminded to monitor all engine gauges at take-off, another safety board recommendation. Tapes from Flight 90's cockpit voice recorder indicate that the pilots saw troublesome readings on at least some of their gauges. But the frozen probe theory holds that they relied on the erroneous thrust readings, even though other gauges--temperatures, engine rpm--were giving unusually low readings.

Much of yesterday's questioning appeared to be designed to ascertain who was responsible to see that Flight 90 had been properly de-iced.

Air Florida operations vice president David Mulligan denied that the airline's National Airport maintenance chief, Juan Cruz, held any responsibility for the inspection. Ultimate responsibility rested with the captain, he said, though he also had a "right to rely" on the American Airlines ground crew which de-iced the plane under contract.

Later, an Air Florida attorney told the hearing that the airline recognized that parts of its manual relating to such responsibility appear to be ambiguous and will be reviewed.

Air Florida training director Edward Cook testified that the airline's pilots receive annual retraining on special techniques for winter flying, using slide presentations and films. Pilots were given copies of the Boeing advisories on the 737's pitch-up problems and received a newsletter outlining the special problems in icy flying, according to testimony.

Cook said that the airline exceeds FAA requirements in training its pilots. Air Florida has burgeoned in size since the airline industry was deregulated, growing from 30 pilots in 1977 to close to 300 today. Investigators are studying closely how the airline has trained those new pilots.

The hearings are adjourned today and tomorrow and are scheduled to resume Friday.