U.S. airlines must inspect more than 100 Boeing 767 airliners more often than previously required to look for cracks that could cause the engines to fall off, the Federal Aviation Administration ordered Wednesday.

The cracks can occur in the pylons that attach engines to wings. The problem came to light last month when American Airlines found cracks in at least two 767s during normal maintenance.

The FAA safety order affects 138 planes registered in the United States out of a global fleet of 314 planes. Aviation officials in other countries usually follow the FAA's lead on safety of U.S.-manufactured planes.

The order only applies to 767s that have the original pylon design. Boeing changed the design after the problem first became known.

FAA issued a safety order for these planes in 2005 requiring inspections for cracks every 1,500 flights. The new order accelerates that schedule to every 400 flights or every 90 days, whichever is later.

Besides American, other U.S. operators affected by the safety order are Continental Airlines, Delta Air Lines, FedEx Corp., United Airlines, United Parcel Service, US Airways, and ABX, formerly Airborne Express.