The company and the FAA may announce plans to replace a controversial hydraulic valve in 737s as early as Thursday.
The 737 "is going to be around for a long time," said FAA spokeswoman Diane Spitaliere. "We feel like this is a long-term solution."
The giant retrofit program, scheduled to take place over the next five to seven years, will eliminate the unusual valve that was the suspected culprit in the crashes of USAir Flight 427 near Pittsburgh in 1994 and United Airlines Flight 585 in Colorado Springs, Colo., in 1991. The National Transportation Safety Board said those crashes "most likely" were caused by a valve jam that made the rudder move the opposite direction from the pilot's command.
Three changes have previously been required in 737 rudder controls, and officials said they have had no reports of rudder problems since they were implemented. They likened the new steps to the ultimate safety improvement.
Boeing plans to pay for the full cost of the retrofit of the 3,200 737s in service, estimated to be about $240 million. The work will be done during regularly scheduled maintenance, so officials expect no serious disruption in airline schedules.
Because of the time needed to implement the redesign, the agency said it would also announce new training procedures for pilots to use if they have rudder problems on the twin-engine jets now in service.
She said Boeing is developing the redesign, which will be submitted to the agency for approval. That is expected to lead to an airworthiness directive sometime next year, ordering the change in all 737s.
The announcement will mark a dramatic change for Boeing and the FAA, which have staunchly opposed a redesign for several years. It is prompted by an upcoming report from an FAA engineering panel that calls for a redesign of the rudder system. The panel was set up last year at the suggestion of the safety board to take an impartial look at the 737's rudder system.
The panel, known as the Flight Control Engineering and Test Evaluation Board, identified new possible failures in the system, including some that could cause the rudder to suddenly deflect fully to one side, which would make the plane difficult to control.
The rudder is the movable vertical panel on the tail that points the plane's nose right or left. The plane's hydraulic system provides the muscle for the rudder, using fluid under extreme pressure. The valve is the key device in the system, controlling fluid that pushes the rudder right or left.
Since the Pittsburgh crash, which killed all 132 people on board, safety board investigators have said they were concerned about the reliability of the rudder valve. Its unique "valve-within-a-valve" design was supposed to provide a backup, but investigators found the design created potentially serious problems because pilots might be unaware of a jam. Safety board officials complained that the valve was not "reliably redundant" and wouldn't meet current safety standards.
Last summer's safety board report on the USAir crash said that despite many improvements in the last five years, "the 737 series airplanes remain susceptible to rudder system malfunctions that could be catastrophic."
Boeing and the FAA have insisted that the valve was safe.
The fixes they have made include one that prevents the valve from pushing the rudder the wrong direction and another that prevents the rudder from suddenly moving to full deflection. But officials opposed a complete overhaul of the rudder system because they said there was no proof that the valve caused the crashes. There were no scratches to indicate a jam and no conclusive evidence on the cockpit tape that the pilots believed they had a malfunction.
Boeing officials blamed the pilots for the USAir crash. Boeing said the USAir co-pilot had been startled and mistakenly stomped on the left rudder pedal.
FAA officials have said the rudder fixes in the last few years have solved any potential problem with the 737 fleet. The FAA required more training for pilots to recover from rudder mishaps and had Boeing fix a trouble-prone device called a yaw damper coupler.
But questions have lingered about whether the government and Boeing had truly fixed the problem. A strange rudder incident involving a MetroJet 737 over Maryland in February 1999 deepened the mystery. Many 737 pilots continue to believe there is a "gremlin" in the valve that has not been found.
The pilot procedure for handling a jammed rudder is being simplified. Instead of the current one, which involves a complicated list of conditions, pilots next month will receive a new procedure that can be memorized more easily.
"We've come up with a new procedure that is a lot simpler," Ms. Spitaliere said. "You don't have to be dragging out manuals."
The 737's new rudder system will be similar to the one on the 757 and 767 fleet. It will have two separate valves that will enable pilots to break out of a jam by stomping hard on a rudder pedal â€“ something they cannot do on the 737 system.
A third valve will act with those valves to provide a backup.
Distributed by Scripps Howard News Service. The Associated Press contributed to this report.