What happened
While responding to a request for entry into the flight deck, the pilot monitoring (PM) intended to activate the flight and deck door lock switch. During this process, the PM mis-selected the rudder trim switch instead of the intended door switch. This error occurred while the aircraft was approaching cruise altitude and the PM was engaged in conversation with the pilot flying (PF), leading to a period of distraction and diverted attention.
As the rudder trim was applied, the aircraft experienced an immediate yaw and roll response. For the first five seconds, the autopilot successfully compensated for the increasing left yaw by applying opposing roll input. However, once the autopilot reached its maximum roll authority, the aircraft's bank angle to the left began to increase rapidly. The developing upset triggered a bank angle alert, prompting the PF to disengage the autopilot and autothrottle and execute the upset recovery procedure. The aircraft was subsequently returned to straight and level flight.
The investigation
The investigation focused on the sequence of events leading to the mis-selection and the subsequent flight control response. Analysts determined that the error was a human-factors slip, exacerbated by the physical proximity and operational similarities of the switches on the aisle stand panel. Although the crew noticed the need for significant control input to maintain wings-level flight, they were unable to identify the cause of the upset for approximately 70 seconds. The delay in identifying the rudder trim issue was attributed to the minimal movement of the rudder pedals during the trim application and the PF's limited experience with the aircraft type.
Findings
- The pilot monitoring mis-selected the rudder trim switch instead of the flight deck door switch for approximately 8 seconds.
- The autopilot reached its roll authority limit after 5 seconds of continuous trim input, leading to a rapid increase in bank angle.
- The crew failed to promptly identify the left yaw as the cause of the upset, despite the necessity of large aileron inputs to maintain level flight.
- The physical layout of the aisle stand panel, featuring closely located and similar switches, contributed to the error.
- A Boeing Flight Operations Technical Bulletin (FOTB) had been issued two years prior to the event, warning of similar risks regarding rudder trim mis-selection.