The Federal Aviation Administration yesterday ordered inspections of all Boeing 747 jumbo jets after discovering cracks in the frames of four of the planes.
"Failure of adjacent frames could lead to rapid decompression of the fuselage and possible loss of the airplane," the FAA order said.
Decompression occurs when a plane, flying at high altitude and pressurized for the comfort of its passengers, suddenly loses that pressure through a rupture in the plane's skin or some other cause.
Inspections of the 747 fleet, totaling more than 600 planes around the world, were ordered within the next 25 landings for planes that have made more than 14,000 landings, and within 50 landings for planes with fewer than 14,000.
The FAA has control over about 200 of those planes and has informed other nations of its order. Foreign safety agencies customarily follow the FAA lead on inspection orders for U.S.-built aircraft.
Airline industry and Boeing officials said that they do not expect the inspection schedule to disrupt airline service.
Top U.S. aviation safety officials said yesterday that they see no relationship between the cracks in the frames and the crash in the North Atlantic last June of an Air India 747 in which all 329 people on board died.
There has been considerable testimony before a court in New Delhi recently to the effect that a bomb caused the crash; callers claiming to represent Sikh separatists have said that they placed a bomb on the airplane.
U.S. officials agree that a bomb is the most likely explanation. However, one of them said yesterday: "We have not seen anything that would let us hang our hat on it."
Nonetheless, the officials said they see no reason to continue the extraordinary international salvage effort that was mounted last fall to recover major pieces of the jetliner from the ocean floor.
The FAA inspection order on the 747 frames said that the four reported incidents involved cracking in the braces that support the outer skin of the fuselage under the cockpit, "between the window belt and the main floor."
The cracks were found during inspections, but the FAA said that "these reports have indicated that current inspection intervals are inadequate to assure continued airworthiness."
In one of the four incidents, the FAA said, "three adjacent frames were found essentially severed."
The 747, long regarded as among the safest of airplanes, had two catastrophic accidents last year, the Air India crash and a Japan Air Lines accident that killed 520 people. An improper repair to structural damage from an earlier incident has been pinpointed as the probable cause of the JAL crash.
Cracks in airplane frames and the outer skin are not unusual and are not necessarily dangerous. Regular aircraft inspections are scheduled to discover and repair such cracks, as necessary.