The Federal Aviation Administration finalized a rule Monday requiring that airlines install new rudder control systems on Boeing 737s.

Problems with the rudder are suspected in two fatal U.S. air crashes -- the 1994 crash of a US Airways jet near Pittsburgh International Airport and a United Airlines crash at Colorado Springs, Colo., in 1991.

The FAA estimates 2,000 U.S. airplanes must be refitted at a cost of $364 million. Another 2,500 737s are in service with foreign airlines.

The rule only applies to U.S. airlines, though foreign air safety agencies often follow the FAA's lead.

The rule gives U.S. airlines six years to install the new system, which adds more backup equipment to guard against failures.

Rudders control sideways movements. In the Pittsburgh and Colorado crashes, the National Transportation Safety Board said problems with the rudders caused the planes to head in different directions than the pilots intended.

The FAA first proposed the rule in November and took public comment before issuing the final requirement.

In June, a Pittsburgh jury found a Boeing Co. supplier was mostly to blame for the 1994 crash, which killed 132 people.

The verdict meant Parker Hannifin Corp., which made a valve in the plane's rudder, had to reimburse the airline for most of the hundreds of millions of dollars US Airways paid to settle lawsuits brought by the families of those killed in the crash.

The Colorado crash killed 25 people.