NTSB Advises Rudder Changes for Boeing 737s

The National Transportation Safety Board recommended sweeping changes Wednesday to fix a rare but potentially catastrophic problem in the workhorse airliner of global aviation--the Boeing 737.

Closing its longest investigation ever, the independent watchdog agency confirmed that a sudden failure of the airplane’s rudder--as long suspected--was to blame for two deadly crashes and at least one serious in-flight incident in the last eight years.

Although rudder fixes already mandated by the Federal Aviation Administration are said to improve the margin of safety, the board unanimously concluded that the 737’s rudder controls still do not guarantee that the plane can be landed safely if there is a failure.

“The prudent thing to do for the safety of flight is for the FAA and Boeing to take another comprehensive look at this system,” said Jim Hall, chairman of the safety board.

The NTSB issues dozens of recommendations a year covering every mode of transportation and issues ranging from design to training to operations. This case is exceptional because it involves an elusive flaw that shadows a widely used plane with an otherwise solid record.

More than 3,000 of the medium-range airplanes are currently in use, from Taipei, Taiwan, to Topeka, Kan., and Boeing estimates that every six seconds there is a 737 taking off somewhere in the world.

The NTSB believes that the two 737 crashes that may have been caused by rudder malfunctions were USAir Flight 427 near Pittsburgh in 1994 and United Airlines Flight 585 near Colorado Springs, Colo., in 1991. The two crashes killed 157 people. There were no survivors in either crash, both of which occurred on landing approach.

A third incident, in 1996, involved an Eastwind Airlines plane on approach to Richmond, Va. With the benefit of new training, the pilots managed to control the plane and no one was injured.

Board members reassured travelers Wednesday that the 737 is generally safe. The question is how to make it fail-safe.

“Whatever this fleeting phenomenon is, it has a chance of happening again,” said member George Black. “It’s the most common type of aircraft, so going to extraordinary lengths might be justified.”

FAA to Consider Recommendations

The 737’s design weakness is in the system that controls the rudder, which is a vertical slab on an airplane’s tail that can turn the craft right or left in flight, the NTSB said.

Investigators said that a valve in the rudder control system can malfunction and make the rudder veer suddenly in the opposite direction pilots intend. They believe that event can cause planes to roll over and spiral out of control.

Because of this, the NTSB said, it is recommending that the FAA require “all existing and future 737s to have a reliably redundant rudder actuation system.” It would be up to Boeing and the FAA to come up with such a system.

The FAA has 90 days to decide whether to accept the board’s recommendations, which could lead to tens of millions of dollars in costs for Boeing and the airlines.

NTSB recommendations carry clout but, though the FAA follows a majority of them, it is not bound to comply. Indeed, in this case, the FAA is skeptical.

The NTSB did not prescribe a new design, but staff members said there are options that could be in place within two to three years without incurring excessive costs.

The board also called on the FAA to require more sophisticated flight recorders on 737s, more realistic training of pilots in how to handle rudder emergencies and other changes to keep chances of rudder malfunctions at a minimum.

The FAA and Boeing both questioned the board’s main recommendation for “reliably redundant” rudder controls. “Reliably redundant” means that the plane must have automatic backup controls that allow it to keep flying and land safely even if the main rudder control fails.

The FAA and Boeing said that fixes the government already has required will prevent 737 rudders from veering in the opposite direction a pilot intends. “It will be impossible for that condition to occur again,” said Tom McSweeney, a senior FAA manager.

Charles Higgins, Boeing’s chief safety officer, concurred. “We have completely eliminated any possibility of a rudder reversal,” he said.

Higgins noted that rudder problems are being blamed for two catastrophic accidents out of an estimated 77 million flights since 737s were introduced in the late 1960s. “Those are pretty small odds,” he said.

But the NTSB said that a better safety margin is needed. The failure of a single valve, it concluded, still could compromise the entire rudder control system. The FAA’s fixes “do not eliminate the possibility of other [rudder failures] that could lead to a loss of control,” the board found.

Addressing Concerns

In a related concern, the pace of the FAA-ordered rudder fixes has been slow.

With an August deadline for the most important of three repairs, only about half the fleet has been fixed. McSweeney said that there are no plans to extend the deadline. “I think the industry is going to make it--if they desire to keep these planes in the air,” he said.

Dramatic testimony during the two-day board meeting ruled out the other leading explanation for the two crashes--advanced by Boeing--that they resulted from pilots overreacting to turbulent air.

Dozens of family members of victims attended the hearing, sitting in a special area at the front of a rented hotel ballroom.

Using computer video recreations, NTSB staff members graphically replayed the last few seconds in the doomed cockpits of USAir Flight 427 and United Airlines Flight 585. The videos, which will be made available on the Internet at www.ntsb.gov, incorporate information from the planes’ data and voice recorders.

In both cases, the pilots had responded calmly and professionally to turbulent air, only to be surprised when the planes suddenly rolled in the opposite direction they intended. They struggled to the last to save their aircraft.

“They couldn’t figure it out,” said Malcolm Brenner, NTSB’s senior human performance investigator. “They didn’t know what they had.”

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Times staff writer Eric Malnic in Los Angeles contributed to this story.

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How 737s Turn

Maneuver in a normal situation

The rudder sweeps the nose of the plane left and right.

Pilots maneuver the rudder in a crosswind or in case of engine failure by pressing down on pedals in the cockpit.

Right pedal turns the plane right and the left pedal turns it left.

In a rudder reversal

The plane moves in the opposite direction the pilot intends.

Right pedal turns the plane left and the left pedal turns it right

Hydraulic valve

A jam of a hydraulic valve of the rudder control system can cause rudder reversal.