Rudder Is Again Suspected as 737 Swerves Suddenly

The National Transportation Safety Board is investigating another possible malfunction of a Boeing 737 rudder system, still considered a prime suspect in at least two major jetliner crashes.

The latest incident occurred Tuesday, when a Metrojet 737 en route from Orlando, Fla., to Hartford, Conn., suddenly swerved to the left while flying at 33,000 feet on autopilot.

The swerve apparently was caused, in large part, by an uncommanded deflection of the jetliner’s rudder--a hinged slab on the vertical tail that helps a plane turn right or left by pushing the tail in the opposite direction.

Officials said the problem seemed to involve the Metrojet 737’s yaw damper, an automated rudder component that is supposed to cancel out the plane’s natural tendency to fishtail slightly.

NTSB investigators said that the yaw damper had to be disconnected--and a standby rudder-control system activated--before the rudder on the Metrojet plane functioned properly again.

The crew declared an emergency and diverted to the nearest airport, Baltimore-Washington International, where the jetliner landed without difficulty. There were no injuries.

The incident put new focus on whether two unsolved Boeing 737 crashes in the 1990s were isolated incidents or were symptomatic of a continuing problem with the world’s most widely used airliner.

The first crash involved United Airlines Flight 585, a 737 that rolled suddenly to one side and plunged nose-first into the ground as it was preparing to land at Colorado Springs, Colo., in 1991. All 25 on board were killed.

The second crash, in 1994, involved USAir Flight 427, a 737 that veered suddenly to the left and slammed into a shallow gully as it was about to land at Pittsburgh International Airport. All 127 on the plane died in the crash.

While no official causes have been determined for either crash, NTSB investigators have said repeatedly that the only flight control system on either plane that could have made them swivel, roll and dive as they did was the rudder.

Documents show that from 1970 to 1995, other Boeing 737s suffered at least 187 “lateral or directional control upsets” apparently caused by rudder control system malfunctions. None of these upsets caused a major emergency or crash, but they did prompt flight crews to make precautionary landings or file incident reports with Boeing Co.

Thus far, no one has been able to identify a particular flaw--or combination of flaws--that might account for the malfunctions on the two planes that crashed. However, concerns over rudder glitches prompted the Federal Aviation Administration to propose in 1996 that Boeing change the design of the rudder control systems on 737s.

Although the FAA never ordered a retrofit on 737s built before 1995, the systems on new 737s have been redesigned. The Metrojet 737 involved in Tuesday’s incident was built before 1995.

Concerns over the rudder glitches are likely to be voiced again next month, when the NTSB will hold new hearings on the Pittsburgh crash. A final report on that crash is expected later this year.

In a related development, NTSB Chairman Jim Hall complained Wednesday that the investigation of the Metrojet incident is being hampered by the fact that the plane was equipped with an outmoded flight data recorder.

The recorder, which logs information on technical aspects of the flight, was equipped to handle only 11 types of data on the plane’s performance, airspeed, attitude and control settings. Newer recorders can log more than 100 types of data.

In addition, the continuous-loop recorder on the plane contains only 30 minutes of data, with older information erased as new information is recorded. As a result, most of the information taken down during the rudder malfunction had been erased before the plane landed.

Hall noted that the NTSB recommended several years ago that older planes be retrofitted with more modern flight data recorders.