SEATTLE, Washington -Boeing Co said metal-fatigue inspections must be stepped up on older 737 aircraft, the world's most widely flown jet. That's after cracks developed earlier than expected on a Southwest Airlines Co plane that split open last week.

Cracks on the so-called 737 Classic weren't forecast to occur until "much later", after 60,000 cycles of takeoffs and landings, said Paul Richter, Boeing's chief project engineer for older jets. The plane that ripped during an April 1 flight above Arizona had flown 39,781 cycles.

About 175 planes flown by carriers in the United States, Asia and Europe meet the criteria of a directive issued by the US Federal Aviation Administration on Tuesday requiring inspections for cracks on all 737-300s, 737-400s and 737-500s that have flown more than 30,000 cycles. The US Federal Aviation Administration also is taking the rare step of requiring checks again every 500 cycles as a precaution while the probe continues, Richter said.