Federal Aviation Administration chief J. Lynn Helms said yesterday that his agency will propose requiring that airliners' emergency exits be marked with floor-level lights so they will be easier to see in a smoke-filled cabin.

Helms made his announcement at the FAA's research headquarters near Atlantic City as he led a media tour designed to counter criticism that the government agency has moved too slowly on tougher standards for preventing airplane fires and other hazards.

His statement came a day after he told a congressional panel that the FAA plans to require airlines to put fire-resistant sheets beneath seat covers to slow the spread of flames to seat padding, which is a major source of smoke and toxic fumes in airliner fires.

Helms denied that the two measures were prompted by a fire aboard an Air Canada DC9 this month, which killed 23 people and reopened a debate on the flammability of cabin materials. Helms said the steps were part of a comprehensive research program he initiated after becoming administrator in 1981.

In a news conference, he declined to comment on why years of research under previous FAA chiefs on many of these same problems had not led to action.

However, talking with reporters at the close of the tour, Helms suggested that "I'm more active in safety research than my predecessors have been."

Helms said the FAA is also developing seats that are more crash-resistant, additives to help prevent jet fuel from turning into an explosive mist on impact, standards to make cargo compartments more flame-resistant and other steps. "Over the next 12 to 24 months," he told reporters. "I think you'll see significant regulatory actions."

If implemented, he said, the safety measures would be "the most dramatic single step forward that we have made in the past quarter century." Rules on the floor lights and the flame-resistant seat materials, which will be formally proposed within 30 days, might be in place within a year, Helms said. After that, airlines would have three years to install them.

The seat covers would cost between $3 and $7 each, he predicted, and they would also raise fuel costs by increasing the plane's weight.

The other innovations will become common aboard airliners in the late 1980s, Helms predicted.

Helms' announcement on the exit markers was delivered 10 years after the National Transportation Safety Board recommended that the lights and other exit markers be installed.

The board's recommendation grew out of a fiery ground collision involving a DC9 at Chicago's O'Hare International Airport in which 10 people died. Some passengers were unable to find the exits because rising smoke hid the lights, the board found.

Helms said FAA researchers now believe that the most serious hazard in many fires is a phenomenon known as flash-over, in which gas and smoke gather at a cabin's ceiling as heat increases, then ignite into a fireball that envelops the cabin and completely cuts off visibility.

This differs from the thrust of much past research, in which crash experts stressed how much smoke and toxic fumes were being given off as seats, carpeting and wall panels caught fire.

Helms said the new seat covers could delay flash-over by about 40 seconds, giving passengers much more time to escape.

Currently, all planes are supposed to be capable of being evacuated within 90 seconds with half of the doors closed.

Next year, the FAA and NASA plan to crash a remote-controlled Boeing 707 loaded with sensors and dummies to test new materials and concepts now being developed at the Atlantic City center, Helms said.