Four years after the Federal Aviation Administration ordered airlines to retrofit their fleet with a safety feature designed to prevent accidents like what happened to TWA Flight 800, carriers said they may not get the job done on time.

Airlines for America, a trade group with members from major U.S. carriers, notified the federal agency that the airlines will not be able to install the safety feature, a flammable-suppression system, to half of their fleet by 2014 and complete the retrofit by 2017 as required.

The reason, the group said, is that Boeing has not certified the kits and service instructions needed for certain models of its aircraft, like the one that killed everyone aboard the Paris-bound Boeing 747 flight in 1996 when it exploded in midair.

"As you may know, the development and approval processes for FRM retrofit kits is significantly behind schedule," the trade group wrote in a letter dated March 26. "Carriers cannot begin the installation process until retrofit systems have been approved by FAA."

Miles Kotay, a spokesman for Chicago-based Boeing, said the company has complied with the FAA requirement for two of its models and is installing the devices on planes currently in production.

"Boeing has since provided the service instructions to the FAA concerning the out-of-production aircraft that are the subject of the proposed penalty," Kotay said in an email.

The federal rule, announced on July 16, 2008 -- 12 years after the downing of TWA Flight 800 -- was one in a string of safety regulations aimed at preventing explosions in and around fuel tanks located at the center of certain models of Boeing and Airbus planes.

TWA Flight 800 exploded in midair and crashed into the Atlantic Ocean just off Moriches Inlet at 8:31 p.m. on July 17, 1996, killing all 230 passengers and crew. The plane exploded 12 minutes after it took off from Kennedy Airport.

An extensive investigation by the National Transportation Safety Board concluded that the cause was the ignition of oxygen in the jet's partially empty fuel tank. Before taking off, the plane had been sitting on the tarmac, under the sun, for hours.

The FAA rule covers 2,730 aircraft or about 55 percent of the nation's passenger airliner fleet. They include 1,830 Boeing planes and 900 Airbus planes built before 1991. Boeing and Airbus are the only aircraft manufacturers to use the center-fuel-tank design.

The cost of installing the new devices was estimated to be about $92,000 to $311,000 per aircraft.

On Friday, the FAA imposed a $13.57 million fine against Boeing because it said the company failed to meet two deadlines. The agency had given Boeing and Airbus until Dec. 27, 2010, to submit to the agency service instructions that explain how to replace the oxygen in fuel tanks with nonflammable nitrogen gas. Toulouse, France-based Airbus met the deadline.

Jay Carven, of Chevy Chase, Md., who lost a sister, Paula, a flight attendant, and a nephew, said he's not surprised by the delay. "It's probably more cost effective to delay the implementation of the fuel-tank inerting. The longer the delay, the fewer planes they need to deal with," he said. "They have been fortunate that in the last 16 years another plane hasn't exploded."

The delay affects 383 Boeing aircraft registered in the United States, according to the FAA.

"We take this matter very seriously," Michael Huerta, acting FAA administrator, said in a statement. "We have issued hundreds of directives to eliminate fuel ignition sources over the past 16 years, and this step will add another layer of safety."

The FAA said it will not extend the 2017 deadline, when all 2,730 aircraft used by U.S. air carriers must have the flammable-suppression system. The agency, however, will consider extending the interim deadline of 2014 if an individual airline demonstrates need.

"Based on our discussions with manufacturers, operators and industry associations, we expect that most, if not all, operators will meet both the 2014 and the 2017 deadlines despite receiving service instructions later than anticipated," Margaret Gilligan, FAA's associate administrator for aviation safety, told the trade in a letter dated Friday.

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