In the second emergency airworthiness directive in four days, the Federal Aviation Administration reduced the time allowed for inspecting the slat downstop assembly from 24 days to 10.
Both last Saturday's directive and the superseding one issued late Tuesday were based on findings about the fire that destroyed a China Airlines 737 in Japan last Monday.
The FAA told CBS News correspondent Nancy Cordes that the inspections it ordered Saturday have turned up four 737s with the same problem that caused last week's explosion.
Last Thursday, investigators in Japan found that a bolt from a right wing slat had pierced the fuel tank of the Taiwanese jetliner that caught fire after landing on the Japanese resort island of Okinawa. All 165 people aboard evacuated safely seconds before the plane exploded.
A fuel leak through that hole likely caused the fire on the China Airlines Boeing 737-800, said Kazushige Daiki, chief investigator at Japan's Aircraft and Railway Accidents Investigation Commission.
Slats slide out the front edge of the main wings during takeoff and landing to stabilize the aircraft, along with flaps that come out of the wings' rear edge. The downstop limits how far the slats can slide out.
The new directive said owners and operators could do either the detailed inspection ordered initially or use a borescope, an imaging device that can get into closed areas. Dorr said the goal was to ensure all parts were in place, particularly a washer crucial to holding a nut on the bolt. If no repairs were needed, airlines could take the full 24 days to retighten the nut and bolt to specifications, the order said.
The orders apply to 783 U.S. airplanes but will likely be imposed by other countries on the entire worldwide fleet of 2,287 newer 737s. The order covers all 737-600, -700, -800, -900 and -900ER series planes, the first of which entered service in January 1998.
Southwest Airlines, whose entire fleet is made up of 737s, says it has already inspected all 280 of its late-model jets but will not say what it found, Cordes reports. Alaska Airlines has checked 18 of its jets, and has 37 to go. American, Continental, Delta and Aloha also fly the newer 737s, adds Cordes.
The FAA says it is unclear if this is a design flaw or a maintenance issue, Cordes reports. Boeing has been aware for at least two years that the bolts can loosen and routinely warns airlines to check them.