September 14, 2000. Copyright 2000. Graphic News. All rights reserved.

BOEING 737 RUDDER SYSTEM AT HEART OF SAFETY REPORT

LONDON, September 14, Graphic News: THE Federal Aviation Administration is to release its final report following a year-long review of the rudder control system of BoeingÕs popular 737 jetliner.

That review by a 22-member independent team of engineers was ordered by the National Transportation Safety Board (NTSB) following its March 1999 report on the crash of USAir Flight 427 near Pittsburgh, Pennsylvania, in 1994. All 132 people aboard died. The rudders were also studied after the 1991 crash of United Flight 585, which killed 25 people in Colorado Springs, Colorado.

It has never been determined with absolute certainty what caused the USAir crash but NTSB investigators believe that the planeÕs rudder turned all the way to one side -- a ÒhardoverÓ -- causing the plane to roll over and plunge to the ground.
The NTSB focused on the airplaneÕs rudder power control unit (PCU) and found more than 45 potential failure modes, some of which could result in uncontrolled movement of the rudder.

Laboratory tests have shown that the PCU can cause rogue movements of the 737 rudder ranging from the rudder ÒcreepingÓ slightly, all the way to a full hardover.
BoeingÕs 737 is designed with a large, single-panel rudder in case one of its two engines suddenly shuts down during takeoff. By moving the powerful rudder all the way to one side, the pilot can avoid asymmetrical thrust of the remaining engine and keep the plane flying straight.

The problem comes if a hardover occurs with both engines operating. The plane will swerve sharply and snap into a roll or, at low altitude, go into a nose dive in a matter of seconds.

Since the 1994 disaster there have been 70 reports of 737 flights thrown off course in a manner that suggest rudder malfunctions.

Captain Paul McCarthy, executive air safety chairman for the Air Line Pilots Association (ALPA) said ÒWe cannot prove in a laboratory what happened to Flight 427,Ó but ALPA experts Òknow exactly what happened.Ó
Until a rudder redesign is available Òwe have a work-around solution that weÕre comfortable with,Ó he said.

The checklist to deal with rudder problems requires the disconnection of the aircraftÕs Òyaw dampnerÓ, a device that reduces side-to-side swing of the tail and makes the flight more comfortable for passengers.

The Federal Aviation Administration anticipates issuing its order in July 2001 once Boeing has completed its rudder redesign. The FAA order would apply only to 1,500 U.S.-registered planes, but other countries usually follow the FAAÕs lead.

BoeingÕs $240 million retrofit programme will apply to more than 3,200 jets currently in service.
/ENDS
Sources: Reuters, Associated Press, Aviation Week and Space Technology, Seattle Times