Date: 6th September 2023
Location: Enroute from Brisbane to Melbourne, Australia
Aircraft: Boeing 737-800, registration VH-YQR
Operator: Virgin Australia
Flight Number: VA-336
Passengers and Crew: Not disclosed
Injuries: 1 flight attendant sustained minor injuries
Summary of Events
On 6th September 2023, Virgin Australia Flight VA-336 from Brisbane to Melbourne experienced an inflight upset caused by the captain inadvertently activating the rudder trim switch instead of the cockpit door unlock switch. The unintended input resulted in a progressive left rudder trim application that caused significant left yaw and roll, exceeding the autopilot’s roll authority limit.
The aircraft banked to a maximum of 42° left before the first officer identified the rudder trim as the issue and corrected it, returning the aircraft to normal flight. Despite the incident, the flight continued to Melbourne and landed without further incident. One flight attendant sustained minor injuries during the upset.
Aircraft and Crew Information
Aircraft
The Boeing 737-800 is a widely used narrow-body aircraft with advanced flight control systems, including rudder trim adjustments to balance the aircraft during asymmetric thrust or other situations.
Crew
• Captain (Pilot Monitoring): Experienced but inadvertently applied rudder trim due to a misselection.
• First Officer (Pilot Flying): Executed the upset recovery procedure and identified the rudder trim as the cause of the issue.
Sequence of Events
1. Cruise Phase:
• The flight was approaching its cruise altitude when a flight attendant requested access to the cockpit.
• The captain intended to activate the cockpit door unlock switch but mistakenly activated the rudder trim switch.
2. Rudder Trim Activation:
• Left rudder trim was applied for approximately 8 seconds, causing progressive yaw and roll to the left.
• The autopilot compensated for the roll until its authority limit was reached after 5 seconds, at which point the bank angle rapidly increased.
3. Inflight Upset:
• The aircraft banked to a maximum of 42° left, triggering the bank angle alert.
• The first officer disengaged the autopilot and autothrottle, applied right aileron, and initiated upset recovery.
4. Recovery:
• The aircraft was returned to a wings-level attitude, but significant right aileron input (35°) was required to maintain stability.
• The first officer requested the captain check the rudder trim, which was subsequently neutralized, restoring normal flight conditions.
5. Landing:
• The aircraft continued to Melbourne and landed safely.
Initial Investigation and Findings
The Australian Transport Safety Bureau (ATSB) conducted an investigation into the incident, identifying the following probable causes and contributing factors:
Contributing Factors
1. Misselection of Rudder Trim Switch:
• The captain inadvertently activated the rudder trim switch instead of the door unlock switch due to their proximity and similar design.
2. Delayed Recognition of Rudder Trim Application:
• The crew initially failed to identify the rudder trim as the cause of the upset, delaying corrective action.
3. Autopilot Limitations:
• The autopilot could not compensate for the continuous rudder trim input after exceeding its roll authority limit, leading to the upset.
4. Distraction and Attention Diversion:
• The captain’s attention was divided between communicating with the first officer and monitoring the aircraft’s systems, contributing to the error.
5. Crew Experience:
• Limited experience with the specific aircraft systems contributed to the delay in diagnosing and resolving the issue.
Safety Lessons and Industry Impact
The incident underscores the importance of:
1. Switch Identification and Confirmation:
• Crews must positively identify switches before operation to prevent misselection.
2. Human Factors Awareness:
• Operators should emphasize the risks of routine actions and distraction during critical phases of flight.
3. Aircraft Design Improvements:
• Consideration should be given to redesigning or relocating cockpit switches to minimize the risk of misselection.
4. Enhanced Training:
• Simulator training for upset recovery and abnormal rudder trim scenarios should be prioritized.
Recommendations
1. Switch Redesign:
• Relocate or differentiate the rudder trim and door unlock switches to prevent accidental activation.
2. Reinforcement of SOPs:
• Emphasize switch identification protocols during training and recurrent assessments.
3. Enhanced Crew Training:
• Incorporate scenarios involving rudder trim misselection and upset recovery into simulator sessions.
4. Operator Alerts:
• Disseminate findings from this and similar incidents to raise awareness among operators worldwide.
Disclaimer
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Our investigation reports are based on all the evidence and facts we have at the time of writing and posting. We apologise if any details are missed or are not fully accurate.
