SPRINGFIELD, Va. -- The world's most popular commercial aircraft, the Boeing 737, has a design flaw in its rudder system that could cause the plane to roll uncontrollably and crash under certain conditions, federal safety officials ruled yesterday.
While noting that such incidents are extremely rare, the National Transportation Safety Board recommended that all 737s in use and in production -- more than 3,000 worldwide -- be fitted with new systems to eliminate or overcome a potential rudder failure.
The board also ruled that two fatal 737 crashes and one near-crash were caused by the flawed rudder -- the vertical flap on a plane's tail that controls lateral movement.
In all three cases, a small hydraulic fluid valve jammed and caused the rudders to move in the direction opposite the one the pilots commanded, the board determined.
"There is, it appears, a very rare flaw that does come up at unexpected and rare times that causes problems and a loss of control," said Jim Hall, chairman of the NTSB.
"Our recommendations should ensure that there are no other possible failure modes."
The board's recommendations will be reviewed by the FAA, which has final authority to order new components on passenger aircraft. An FAA official said yesterday that a review panel will be created to study possible changes to the 737's design.
Boeing Co. officials pledged to cooperate, but argued that the airplane's design is already sound despite the NTSB's recommendations.
"What we've done to date we think takes care of all the problems," said Charles Higgins, Boeing's vice president of airplane safety.
Even as board members urged the Federal Aviation Administration to require new rudder mechanisms, they also urged the public not to stop flying 737s. The aircraft still has one of the industry's best safety records, with just one crash for each 1 million flights.
"I fly on them, my family flies on them, and I have no hesitation about it," said board member George W. Black. "It has a good safety history."
USAir Flight 427 was approaching Pittsburgh International Airport at 6,000 feet when turbulence from the wake of another airplane caused it to swivel slightly sideways, or "yaw."
When the captain moved the rudder pedals to correct the plane's course, the rudder swung in the opposite direction and locked in its "hardover" position. The plane rolled onto its back, went into a dive and hit the ground 25 seconds later, killing all 132 people on board.
United Airlines Flight 585 had a similar experience as it approached Colorado Springs, Colo., in 1991, the resulting crash killing all 25 people on board. An East- wind 737 approaching Richmond, Va., in 1996 experienced a rudder hardover, but the crew was able to regain control and land safely.
The NTSB ruled yesterday that all three incidents were most likely caused by a jam in the "servo valve" that regulates hydraulic fluid to the plane's rudder control system.
Boeing has altered the design of 737 servo valves in the past two years, and the FAA has ordered that older valves be replaced by August. Boeing also has installed pressure-limiting devices to prevent the planes' rudders from swinging hard to one side.
Despite the alterations, however, the NTSB voted unanimously yesterday that the 737's rudder system still is not "reliably redundant" -- if the rudder fails, no other system is in place to overcome the failure and keep the plane in the air.
"Rudder design changes and the proposed retrofit of the remainder of the Boeing 737 fleet do not eliminate the possibility of other potential failure modes and malfunctions in the Boeing 737 rudder system that could lead to a loss of control," the board concluded.
A MetroJet 737 fitted with a new servo valve had a mysterious rudder movement last month and made an emergency landing at Baltimore-Washington International Airport. A new Southwest Airlines 737, built with the redesigned rudder components, recently reported unusual yaw movements in Columbus, Ohio. Both incidents are under investigation.
Higgins said Boeing will cooperate as federal officials study what type of changes might need to be made to the 737, but he suggested that the company will contest the NTSB's claim that changes are necessary.
"We have completely eliminated any possibility of a reversal," Higgins said. "Everything we've done to date would suggest that the [rudder] is reliably redundant."
Pilots rarely use the plane's rudder because it can cause passenger discomfort, choosing instead to steer the aircraft in wide curves using the ailerons on the wings. The most common use of the rudder is to counteract crosswinds that cause a plane to yaw, making the rudder particularly important while landing.
Other commercial aircraft defray potential rudder problems with two sets of controls, or by splitting the rudder into two flaps. The 737 has a back-up rudder control mechanism, but it and the primary mechanism are built into one component. The NTSB report said both mechanisms can fail together.