Guy Norris/LOS ANGELES
Design changes to the rudder system of the Boeing 737, plus operational and maintenance procedure changes, may be ordered by the US Federal Aviation Administration in the wake of an Engineering Test and Evaluation Board (ETEB) investigation into potential failure modes that could have contributed to two 737 crashes in 1991 and 1994.
News of the initial recommendations was leaked after completion of the ETEB's draft report, but the committee's full report is not expected to be submitted until June. The ETEB was convened to examine the 737 rudder system. Its work is almost complete, the last phase involving a four-month test flight in a Boeing 737-200 testbed on loan from Purdue University.
The FAA says no discoveries were made that require "any immediate action". It adds, however, that the board's long-term recommendation is for design changes to address "failures that can result in hardovers". In the short term, the ETEB recommendations are believed to include changes to operations and maintenance that will detect any potential control problems before they happen.
The ETEB discovered 30 "Class 1" failure modes and 16 "Class 2" failure modes. Class 1 modes are considered catastrophic failures with little or no chance of recovery, while Class 2 failures are deemed "hazardous" and put a significant burden on the crew. The new failure modes include scenarios for jammed rudder controls and control problems from ice build up.
Boeing, which is independently conducting concept design work on a revised 737 rudder system, says further statistical analysis is required on the ETEB's failure modes to validate the seriousness of the problems. "We are not saying no work needs to be done. We just don't know enough about these findings and how they might apply to aircraft in the real world. We have done some work on what a 'reliably redundant' rudder system might look like. Now we have to see how the ETEB work relates to that," says Boeing. The statistical analysis will determine if the probability of a failure is greater than one in 1,000,000,000. Any mode exceeding this will be re-examined.
The manufacturer warns that any alterations, particularly to the "Classic", will have to be closely studied before implementation to "make sure there are no unintended consequences". It says the introduction of the newer power control unit and digital yaw damper on the Next Generation 737, and the subsequent retrofit of Classics, "will probably get rid of 90% of issues that have been reported, particularly since a high proportion have been yaw damper related".
Source: Flight International
