The US Federal Aviation Administration is to order a fleet-wide replacement of the Boeing 737's rudder system with the rudder redesign made mandatory for future 737s coming off Boeing's production line.

Boeing's new rudder control system concept is designed to overcome a fundamental lack of redundancy in the original rudder system. It will incorporate a completely different rudder power control unit (PCU) design, which features two independent valves driven by two independent actuator arms.

Also, and crucially in the case of an uncommanded full rudder deflection which might jam in the "hard-over" position, the system will include a "pogo" mechanism allowing override.

Boeing plans certification of the new system in the first quarter of 2003. It will then be "cut into" the production line, and by mid-2003 will begin to be retrofitted to the first of around 4,000 737s then expected to be in service. Retrofits, which could be done during heavy maintenance checks or as a stand-alone item, are expected to be completed by late 2006 and are likely to cost an estimated $60,000 per aircraft.

Meanwhile, the FAA plans to simplify the procedure for handling a jammed rudder and mandate maintenance changes to detect potential faults.

The FAA's planned actions follow a year-long evaluation by the 737 Flight Control Engineering Test and Evaluation Board (ETEB), involving 22 experts from government and industry. Formation of the test group was recommended by the US National Transportation Safety Board (NTSB). They studied how the rudder could malfunction and confirmed their analysis through flight testing. The FAA previously ordered retrofit of four newly-developed rudder-system components on 737s, including a redesigned Parker Hannifin rudder power control unit (PCU) which retained the dual concentric servo valve.

The NTSB urged the FAA to mandate a "reliably redundant" 737 rudder system, believing that fatal crashes in 1991 and 1994 were caused by uncommanded rudder hard-overs due to PCU valve jams. The FAA sees no need for immediate action since the rudder can jam only under "extremely improbable conditions". Safety Board Chairman Jim Hall says that the Board is "particularly disappointed" that two ETEB recommendations are not to be adopted - a rudder position indicator for the pilots, and "an independent switch to stop the hydraulic flow to the rudder". Also rejected was an ETEB recommendation for better protection against icing.


Near Term

Revise jammed rudder procedure Provide flight crews with additional training Display rudder position to crew Alert flight and maintenance crews to symptoms of rudder malfunctions Adopt maintenance procedure changes to prevent undetected failures Provide independent shut off of rudder control system Protect rudder control system from effects of icing


Redesign the 737 rudder control system

Source: Flight International