David Learmount/LONDON
Rudder procedures on Boeing 737s are being modified again by the US Federal Aviation Administration following the identification of further "rudder hardover" failure modes.
In addition, "flight crews are confused by the [existing] procedure and are not always able to complete it during simulated rudder system malfunctions," according to the 737 Flight Controls Engineering and Test Evaluation Board (ETEB).
As a result, the FAA has issued an airworthiness directive (AD) for implementation by 25 November, revising the pilot drill for all 737 series aircraft in the event of an uncommanded rudder movement. This will remain in effect until the fleet-wide retrofit of a redesigned rudder system is completed in several years.
Tests by the ETEB have established that "failure modes include several single-jam modes that can cause an uncommanded rudder hardover, in addition to several latent failures or jams that, when combined with a second failure or jam, could cause an uncommanded rudder hardover." The FAA adds: "Changes in maintenance procedures will be adopted to enhance the detection of latent failure conditions, reducing the potential for an uncommanded hardover."
The new pilot drill for an uncommanded rudder pedal displacement - "or if the pedal kicks" - consists of these actions:
disengage autopilot; maintain control using all available flight controls; if roll is uncontrollable, reduce the pitch/angle of attack and increase airspeed, sacrificing altitude if necessary; disengage autothrottle; verify that engine thrust issymmetrical; switch off yaw damper; check rudder trim is centred; centre the rudder pedals using maximum force, including the combined effort of both pilots if required; if the rudder pedal position or movement remains abnormal, select the system B flight control switch to "standby rudder".A slight rudder deflection may remain, says the AD, requiring continued rudder pedal pressure. Also, pilots should test rudder effectiveness in the approach configuration "at a safe altitude...using slow rudder inputs".
Source: Flight International
