Decision prompted by study findings that show increasing incidents are the result of large uncommanded deflections

Following the USA's proposal to make compulsory a complete rudder control system retrofit on the nation's 2,000 Boeing 737s, owners of all 4,500 737s worldwide will be under pressure to do the same.

The move has been prompted by a number of rudder incidents, including the fatal accidents in 1991 of the United Airlines 737-200 at Colorado Springs and the 1994 USAir 737-300 at Pittsburgh, both of which were judged to have been caused by large, uncommanded rudder deflections.

On 13 November, the Federal Aviation Administration (FAA) unveiled a notice of proposed rulemaking (NPRM) requiring that the whole 737 fleet should be fitted with the new rudder control systems which Boeing is currently in the process of designing.

The FAA decision followed "a top-to-bottom review of the 737 rudder system" by the government/industry 737 Flight Control Engineering Test and Evaluation Board. The board's study found that there were other possible failure modes apart from those that were dealt with by the already mandated fitting of a redesigned rudder power control unit and yaw damper system.

The new NPRM is designed to eliminate the danger from "single failure or jam [that could occur] in the linkage aft of the torque tube.

"The new rudder design will allow the flight crew to control the aircraft using normal piloting skills without operational procedures that are unique to this aircraft model," says the NPRM.

Compliance will be required within five years of the issue of the final airworthiness directive, and the FAA estimates the cost at $182,000 per aircraft.

Boeing says six months of laboratory tests of the new rudder control system are due to start in February 2002.

The new system incorporates a completely different rudder power control unit (PCU) design with two independent valves and actuator arms.

A double flight test programme is then scheduled to begin in the third quarter of 2002 involving evaluation of the new rudder control system on both "Classic" and next generation 737s.

Boeing expects certification of the new system around the first quarter of 2003.

The airline says it is "on track" to meet the FAA's compliance target, and estimates retrofits could be completed by late 2006.

Source: Flight International