Federal safety officials say airplanes frequently leave the gate with emergency slides that would not work if they were needed, and regulators are trying to correct the all-too-common occurrence.

The National Transportation Safety Board (search) estimates that slides fail in about one-third of all evacuations. A board study four years ago found that at least one slide failed to deploy in seven of 19 evacuations during a 14-month period.

"We need to find out why so many slides fail during actual evacuations," said the board's chairman, Ellen Engleman Conners.

Passengers and crew have about 90 seconds to evacuate a plane in an emergency, said Paul Hudson of Aviation Consumer Action Project (search). Any longer, he said, and "there's a strong probability they'll die of fire or smoke inhalation."

American Airlines Flight 1128 was pushing back from the gate at Dallas-Fort Worth International Airport last fall when fumes and smoke filled the cabin.

The captain declared an emergency and ordered an evacuation. Flight attendants rushed to deploy four slides so passengers could exit safely to the ground. The tail slide failed to inflate, however, and eight of the 94 people aboard were injured during the evacuation.

Faulty emergency chutes brought trouble this month to another carrier, United Airlines, when the government recommended a $1 million fine for operating a Boeing 777 (search) on 263 flights when the plane did not have working slides.

Inspectors from the Federal Aviation Administration (search) found that United's maintenance workers failed to remove a pin from the slides despite an attached streamer with instructions to "remove before flight." The pins made the slides inoperable.

The NTSB (search), which makes safety recommendations, wants the FAA to require the airlines to test 10 percent of all evacuation slides to identify common problems. The FAA has been reluctant to do that.

Each carrier now must test some of its slides, though the rate is much lower than one in 10.

Testing would be more costly for airlines and would require that more planes be taken out of service.

"We are actively working to develop an alternate approach that would satisfy the intent of the safety recommendation," FAA spokesman Les Dorr said.

The agency recommended last year that airlines ensure maintenance manuals were updated and check parts of the slide assembly such as the pressure in the tank that inflates the device.

The FAA previously had allowed airlines to count inadvertent or emergency slide deployments toward the number of inspections required on slides each year. Last week, the agency said that no longer would be allowed.

A slide can fail because a cable is not attached properly, a hook on the cover catches on something, a regulator valve malfunctions or for other reasons.

In one case, a change in an airline's maintenance manual left it unclear who was supposed to conduct daily checks of inflation bottles that fill slides in an emergency. In a second case, slides were labeled incorrectly as automatic when they were manual.

The safety board does not keep track of how many injuries result from slides that deploy improperly.

Grant Brophy, an air safety investigator and director of flight safety and security programs at Embry-Riddle Aeronautical University (search) in Daytona Beach, Fla., likened getting on a plane with a malfunctioning slide to being at sea without a life jacket.

"Nine times out of 10, the boat's not going to sink, but there's always that one time," Brophy said. He thinks the board does not go far enough in recommending testing for 10 percent of slides.

"You're going to test more than 10 percent to statistically prove if there's an issue or if there is no issue," Brophy said.

In the case of American Flight 1128, investigators found the slides had been rigged or modified improperly by maintenance workers.

The carrier later inspected its entire MD-80 fleet, about 300 jets, and found 47 slides had rigging problems that would have resulted in failure to inflate. The airline fixed the problems; the slide manufacturer suggested all MD-80 operators consider a one-time inspection of that model.

Last month, the safety board recommended that the FAA require that all airlines using that model check the slides and fix any problems.

The manufacturer of those slides, Air Cruisers Co. (search) of Belmar, N.J., says it makes 60 percent of all aircraft evacuation systems.

The FAA is considering the request.