Boeing's long-running problems with the 737 rudder, thought to have been finally settled six years ago with the redesign of the power control units (PCU), have resurfaced with the discovery of fractures in the input control rods to the units, and a subsequent call for wide-ranging corrective action by the US Federal Aviation Administration.
The original rudder control system redesign was prompted by a number of rudder incidents, and two fatal accidents judged to have been caused by large, uncommanded rudder deflections. The new system had a different rudder PCU design, with two independent valves and actuator arms or rods.
The latest FAA airworthiness directive (AD) follows reports of fractures in the ends of rods connected to both the main and standby PCU. The FAA says the action will "prevent failure of one of the two input control rods of the main rudder PCU, which, under certain conditions, could result in reduced controllability of the airplane and to prevent failure of any combination of two input control rods of the main rudder PCU and/or standby rudder PCU, which could cause an uncommanded rudder hardover event and result in loss of control of the airplane".
Boeing says the fractures in the rods have been caused by a quality control issue that resulted in the units becoming "over-baked and brittle". Although Boeing identified the problem in 2006 and initiated a retrofit programme, the FAA AD and acceleration of the replacement effort was made inevitable by "the subsequent discovery of a second broken rod in December".
Boeing says 880 aircraft have been fitted with the "enhanced" rudder system, of which 681 were discovered to have been delivered with "suspect rods".
Source: Flight International
