The manufacturer of the Boeing 747 that crashed in Japan Monday and killed 520 people told the world's airlines yesterday that they "may wish to inspect" the tail areas of their 747s, and major carriers said they are doing so.
An undefined structural failure in the tail is the prime suspect in the world's worst single-plane aviation disaster.
Meanwhile, Federal Aviation Administrator Donald D. Engen established a 14-member working group to monitor the Japanese crash investigation closely and react as necessary.
The FAA has not ordered U.S. airlines to perform extraordinary inspections of the 160 U.S.-flown 747s, but the four major U.S. operators -- Pan American World Airways, Northwest Orient Airlines, Trans World Airlines and United Airlines -- all said they are doing so. They said schedules will not be disrupted.
The British Civil Aviation Authority ordered a "precautionary mandatory inspection" of British-flown 747s within 10 days.
About 615 Boeing 747s are in service worldwide. They are usually used for long, overwater routes, especially across the Pacific.
Investigators at the mountainside crash site were concentrating yesterday on a theory that the jumbo jet's aft cabin bulkhead -- the rear wall of the passenger compartment -- gave way explosively as the plane was pressurized, then tore out sections of the tail and vital hydraulic fluid and instrument systems.
The fragmented bulkhead has been found amid the wreckage, an official of the government investigating committee said. Tests will be conducted to see if it collapsed because of a failure in the bulkhead or its attachments or because of the impact when the plane hit the ground. Crews continued removing bodies from the remote crash site.
The tail section behind the bulkhead is not pressurized; thus a significant air pressure differential develops as a plane gains altitude and the outside air becomes thinner while the cabin air is maintained at a comfortable level. Controls, antennas, hydraulic fluid lines, electric instrument sensors and other airplane vitals are in the tail.
While it is unclear whether the bulkhead collapsed first or as an aftereffect, it appears that the crash had its origin somewhere in the tail section. Major pieces of tail structure have been recovered along the flight path. U.S. specialists from both the FAA and the National Transportation Safety Board are assisting the Japanese investigation.
Japanese authorities ordered inspections of their own planes on Thursday, with emphasis on the major spars, or beams, that hold the tail assembly together. Hinges and other attachments were also targeted for inspection. Boeing transmitted the Japanese inspection order in its advisory yesterday.
Boeing said: "Until the cause of the accident is determined, operators may wish to inspect the vertical fin and rudder structure. Due to the reported decompression, operators may also wish to visually inspect the external and aft portion of the pressure shell structure . . . ."
The vertical fin, the up-and-down portion of the tail, contains two movable sections, or rudders. They steer the plane through the air as a ship's rudder steers it through the water.
There are other ways to steer a 747 if the vertical fin is missing, but they are tricky and would depend on hydraulic fluid remaining in the control system. All four redundant hydraulic systems on the 747 have lines in the tail, and some experts have theorized that the damage included penetration of the hydraulic lines, rendering the plane unflyable after it ran out of hydraulic fluid. The pilot radioed about 14 minutes into the flight that he could not control the aircraft.
Anthony J. Broderick, associate FAA administrator for aviation standards, set up the working group as Engen directed. He was a key player in the FAA task force that recertified the McDonnell Douglas DC10 when it was grounded for 30 days for a suspected design deficiency after a Chicago crash in 1979 that killed 273.
"We're reaching back into our minds on what we did right and what we did wrong," Broderick said. The FAA has been criticized for that grounding action, especially after the investigation placed the blame primarily on American Airlines maintenance, not the McDonnell Douglas airplane.
Broderick stressed that the advisory from Boeing "is not a recommendation." Given the lack of certainty about how the accident happened, he said, there is no basis for an FAA order to inspect.
Jeff Kreindler, a spokesman for Pan Am, said, "We had taken action before the advisory to go through the fleet on a one-time basis. We will continue until the fleet is inspected, and it will take a few weeks." Pan Am has 49 of the Boeing jumbos, the largest U.S. fleet.
The crashed plane is a Boeing 747SR, for short range. It was designed for short-haul, high-passenger-load flights, not a normal 747 role. Modifications to the landing gear and wing supports were made to allow for the extra takeoffs and landings the planes would make. The crashed plane had made more than 18,000 flights, higher than any U.S.-flown aircraft. Takeoffs and landings cause the most stress on an aircraft's structure.
The Boeing Commercial Aircraft Co.'s 747 has been in production since 1969 and has an outstanding safety record. Only five other 747s have crashed. Two of them collided on a runway at Tenerife, Canary Islands, killing 582 in the world's worst airline disaster when one pilot attempted to take off in a dense fog without receiving clearance from controllers. Two Air India 747s and one belonging to the Iranian Air Force have crashed, but no causes have been determined, although a bomb is suspected in one of the Air India crashes.