Commercial aircraft manufacturers and airlines must take new steps to protect thousands of jets from serious structural fatigue as they age, according to a rule finalized Friday by U.S. aviation regulators.

The Federal Aviation Administration is requiring manufacturers, including industry leaders Boeing Co. and Airbus, a unit of Europe’s EADS, and airlines to intensify and streamline inspections of the metal skeleton and skin of aircraft.

The FAA estimates compliance costs for the industry at $3.6 million.

The regulation has been in the works for years and pulls together related rules and directives issued by the agency on fatigue cracking, which is mainly caused by repeated changes in pressurization during flight.

More than 4,100 planes registered to fly in the United States are affected by the rule.

Structural fatigue and questions about FAA oversight have arisen in a handful of incidents in recent years.

In 2009, Southwest Airlines agreed to pay $7.5 million to settle allegations that it flew Boeing 737s without performing required structural inspections.

In another Southwest case, safety investigators said structural fatigue caused a rupture in a plane’s fuselage during a flight last year. The plane, at 35,000 feet, lost pressure but landed safely.

U.S. safety investigators also blamed faulty maintenance and lax FAA oversight for the crash of an aging seaplane in Florida that killed 20 people in 2005. Fatigue cracks were at the center of that investigation.

The FAA said it is working with European safety officials to coordinate regulations. European Aviation Safety Agency is working on its own fatigue directive.

At issue are tiny cracks, some visible, that often form on a plane as it ages. Individually, the cracks are of little concern. But they can weaken an aircraft’s structure if they spread and link with other cracks.

Fatigue is understood up to a point, the FAA said in raising concern about operating jets beyond a certain age. Many planes are in service for two decades or more, and current inspection methods do not reliably catch all cracks.

Manufacturers have 18 to 60 months to comply with the new FAA rule, depending on the plane involved. Airlines then have another 30 to 72 months to incorporate the changes into their inspection routines.