Airlines asked the Federal Aviation Administration to postpone a requirement for installing equipment aimed at preventing fuel-tank explosions on passenger jets.

The approval for kits required under a 2008 regulation is "significantly behind schedule," making it impossible to meet FAA deadlines without having to ground some aircraft, the trade group Airlines for America said in a March 28 letter to the agency.

The Washington, D.C.-based group, which represents large carriers such as Southwest Airlines and Delta Air Lines, released the letter Monday.

The FAA ordered plane manufacturers and airlines to install the kits to prevent explosions such as the one that destroyed TWA Flight 800 off Long Island, N.Y., on July 17, 1996. All 230 people aboard died in the second-worst U.S. aviation accident since 1989.

The tank between the Boeing 747's wings became heated, generating an explosive mixture of fuel vapors and air, the National Transportation Safety Board ruled. Similar explosions caused fatalities in three other cases, and the FAA estimated that four additional U.S. jets would crash over 35 years without more protection.

Passenger carriers have until Dec. 26, 2014, to equip half of their fleets with devices that force oxygen out of fuel tanks and replace it with nitrogen, which prevents explosions, the trade group's letter said.

FAA approval for the kits has fallen "well behind the pace needed" to meet that deadline, according to the letter.

The agency is evaluating the trade group's request and believes it is important to work "with the aviation community to improve safety," the FAA said in an emailed statement.

"This rule provides an important safety net and as a result, fuel tanks are far safer today than they were in 1996," the agency said in the statement.

The delays affect kits to retrofit Boeing models, according to the trade group. Equipment for Airbus jets has already been approved.