The nation's system for keeping planes flying safely in wintry weather is fundamentally flawed because pilots cannot determine at takeoff whether their planes are free of nearly invisible but potentially lethal ice, pilots and safety officials say.

The system for preventing airline accidents as a result of icing has come under intense scrutiny following the crash Sunday of a USAir jet at New York's LaGuardia Airport. Icing of the wings is a prime suspect in the crash, which killed 27 people.

Even traces of ice on the leading edge of a wing can cause disaster bydramatically reducing the lift that keeps a plane aloft. Under Federal Aviation Administration regulations, it is the responsibility of the pilot to assure before takeoff that the airplane is clear of ice and snow. Critics of the FAA regulations say that responsibility is impossible to fulfill because ice cannot always be detected by flight crews making observations from the cockpit or cabin windows.

According to airline pilot Dennis Petretti, New York chairman of the Allied Pilots Association, it is impossible to say with certainty if it is safe to take off.

"If you're sitting in the cockpit you don't have the data," he said. "In the end you're just not sure. There can be a thin layer of ice."

"You tell me how you can see clear ice on the wing," he said. And even if a pilot can see the wing, he can't see other crucial areas from the cockpit. "I can't see the tail surface. Who the hell knows what's on the fuselage."

Pilots are not the only ones critical of the current rule. Safety experts agree that the responsibilities assigned to the pilots are impossible to fulfill under current procedures.

John P. Galipault, president of the non-profit Aviation Safety Institute, said it is unfair that pilots be forced to make life and death decisions based on inadequate information. "Everyone is putting the gun to the pilot's head," he said. "You just can't let him be in a situation of too many unknowns."

Pilots waiting in line to take off have to weigh whether the de-icing fluids have continued to keep the plane clear of ice. At that point, the best a pilot can do is look out the cockpit window or walk back to the passenger compartment for a closer look at the wings. "Do we want him to go back in the cabin, lean over somebody's grandmother and look out a scratched up oily window in the dark and expect him to see any ice, which may be transparent?" Mr. Galipault asked.

He doesn't think these kinds of visual inspections are adequate. The rule putting responsibility on the pilot to determine the presence of ice is unworkable, he said.

The FAA defends its standard.

Paul Steucke, the manager of the FAA's news division, said the agency remained convinced that pilots inside their planes can determine with absolute certainty whether snow or ice is on their aircraft.

"Yes you can. Pilots do it all the time. They determine on a regular basis that the aircraft they are in requires de-icing."

It is, however, a tricky business at best. For example, Fokker 28s, the kind of plane that crashed at LaGuardia, come equipped with a black stripe that wraps over the leading edge of each wing, the most critical area for ice build-up. That stripe is designed to appear duller if ice forms over it, offering an additional visual cue to assist the pilot in spotting ice. These planes are also equipped with lights that shine on the wings and the stripe to make spotting ice easier.

But even those special aids may not have been enough to help the USAir crew that crashed at LaGuardia. According to the co-pilot, he and the captain repeatedly checked the wing before deciding to head down the runway.

"The co-pilot said he checked for ice on the wings at least three times, perhaps as many as 10, and he noticed the captain checking as well," said Ted Lopatkiewicz, spokesman for the National Transportation Safety Board, which is investigating the accident.

In fact, despite its insistence that visual inspection is adequate, the FAA in January began to require hands-on checks for ice before takeoff of DC-9-10s.

In its directive, the FAA noted that the McDonnell Douglas DC-9-10 had been involved in five accidents involving "contamination" of the wings by ice.

The FAA observed, "Although Federal Aviation Regulations . . . prohibit takeoff with ice, frost, or snow on the wings, in each reported accident, it was apparent that the ice buildup on the wings was not noticed or detected by the flight crew prior to takeoff."

The DC-9-10 is considered especially vulnerable to ice problems. Its wings do not have devices known as slats that increase a wing's lift during takeoff and landing.

The Fokker 28 also lacks wing slats. That fact has raised concern that these planes may also be particularly prone to loss of lift because of ice contamination. The FAA's Mr. Steucke said his agency had been considering applying the hands-on inspection rule to the Fokkers before the LaGuardia crash occurred.

Robert Hawk, a spokesman for Fokker Aircraft USA Inc., said his company's planes are no more susceptible to icing problems. The issue, he said, is not the absence of slats but the presence of ice, regardless of the airplane. "Wings and ice don't mix," he said.

Steve Morris, a commercial pilot who flies corporate jets, said the FAA's decision to require hands-on inspection of the DC-9-10s represents a tacit admission that visual inspection of planes is inadequate to detect ice.

"If the wing is painted white, how do you see the snow on it or the clear ice? The only way you can tell it's there is to rub your hand over it and feel for it," he said. "In essence what they're saying is other planes may be able to get away with it while the DC-9-10 can't."

"That's what it all comes down to, a judgment call," he continued. "Sometimes that's going to work out. Others, it's not."

It's also a judgment call pilots are forced to make in very difficult circumstances. Wallace J. Majure, the pilot of the Fokker 28 in the LaGuardia crash, was already two hours late when he reached the runway. The airport was still open. Other pilots, operating in identical conditions, were taking off ahead of him. Others were behind him waiting their turn. He had already had his plane de-iced twice.

The disincentives to turn back can be powerful. Winding your way back to the terminal through a long line of planes is difficult, Captain Petretti said. And once you have been de-iced again you may find yourself in the same situation.

Captain Petretti is critical of a system that puts the pilot in such a difficult spot. "They lead you out there. They kind of get you there," he said. "You can see where the system, it kind of starts piling up on you."

He believes the solution is for the FAA or airport operators to conduct ice inspections at the end of runways in bad weather. An inspector operating from truck with a cherry picker hoist could make a careful search for ice. Armed with accurate information about the state of the aircraft just before takeoff, the pilot would then know it was safe to take off.

"It's an easy fix," he said.

Asked if ice inspections at the end of the runway would be practical, one safety inspector replied, "No, it's not practical, but it may be necessary."

Mr. Galipault, of the Aviation Safety Institute, said that while he doesn't know of any simple solution, the LaGuardia crash demonstrates the need to take action. "That's enough. We've killed enough people, destroyed enough airplanes, destroyed enough confidence," he said. "We've got to help the pilot."