The Federal Aviation Administration Thursday proposed mandatory changes in the fuel system wiring and vent systems of early model Boeing 737s, the most commonly used commercial airplane in the world.

The proposed airworthiness directive, which grew out of the TWA Flight 800 crash investigation, would require installation of electrical surge suppression systems or the shielding and separation of wiring routed to the plane's fuel tanks from nearby wiring.In addition, the directive would require the installation of flame arrestors and pressure relief valves in the plane's fuel vent system, steps aimed at keeping flames outside the 737 from entering the fuel system via wingtip fuel vents.

The proposed directive, which would require compliance within one year, has now entered a 45-day comment period. The FAA estimated the cost of the changes at $36,000 per plane.

"We believe it is raising the safety bar," a senior FAA official said, speaking on the condition of anonymity. "We believe a year to do that is appropriate. If there was an immediate problem that required an immediate action, we would propose an immediate directive."

FAA Administrator Jane Garvey added in a statement: "This is just one of the many efforts the FAA has under way to reduce or eliminate explosive fuel-air mixtures and potential ignition sources in fuel tanks."

Both orders stem from the investigation into the explosion of TWA Flight 800 and mirror a proposed airworthiness directive the FAA issued for older Boeing 747s in November.