The US Federal Aviation Administration is to order airlines to retrofit four newly developed rudder-system components in 2,800 Boeing 737s. US Vice President Al Gore revealed the move in a speech on commercial aviation security and safety. The updated components will be incorporated in new-build 737-300, -400 and -500 series from mid-1998 onward, says Boeing, as the components become available. They will be standard on the next generation 737-600/700/800.
The modifications must be completed within three years of the airworthiness directive (AD). The order is compulsory for US-registered aircraft, but Boeing expects most foreign operators to be required to comply because the FAA is the 737's lead certificating authority. Boeing says that worldwide retrofitting will cost $126-$140 million, which it will share with its suppliers including rudder-power control unit (PCU)-maker Parker Bertea. Airlines, which will carry out the work during "overnight maintenance periods", will cover the labour costs.
The changes were prompted by two unexplained 737 crashes: to the USAir 737-300 near Pittsburgh, Pennsylvania, on 8 September, 1994, and to the United Air Lines 737-200 near Colorado Springs on 3 March 1991. In both cases, an uncommanded rudder hard-over is suspected, but not proven, to be a cause. acting FAA administrator Linda Hall Daschle says: "We want to ensure that the rudder moves only when the pilot wants it to."
The changes include:
a rudder-limiter which reduces hydraulic pressure to actuators. This decreases rudder travel by about 33% in the event of a rudder hard-over;
a redesigned rudder power control unit (PCU) with a new servo valve similar to the one proposed by Boeing for the 737-700. This unit, which will undergo extensive testing to evaluate the effect of rudder-control system jamming, is designed to eliminate uncommanded rudder motion, including rudder reversals;
mechanical rate-gyros used in the yaw-damper system will be replaced by dual-configuration solid-state rate-gyros to reduce system faults;
redesigned fasteners on the control rod which links a torque tube to the main rudder PCU input arm to reduce the chance of mechanical failure.
The AD will require that the yaw-damper and rudder-limiter work must be completed within three years.
A two-year deadline has been set for the PCU and rudder-control-rod changes.
Meanwhile, Boeing wants rudder-pedal force to be a flight-data-recorder parameter in all 737s.
Source: Flight International
