Use of Rudder

An A320 aircraft was at 30 000 ft, climbing towards its cruise altitude of 36 000 ft with autothrust and autopilot ON in _MACH I V/S +1000  I NAV_ modes.

When crossing 31 200 ft, the aircraft started to roll to the right and reached 52° in a few seconds. ① The PF reacted with full left and nose-up sidestick input that disengaged the autopilot, and a light left rudder input. These combined control inputs led the aircraft to bank to the left. ② The PF reacted quickly by applying a full right sidestick input and pressing the right rudder pedal to approximately halfway of its travel. This caused the aircraft to bank severely to the right and the STOP RUDDER INPUT warning to trigger.

(fig.1) Event description part 1

③ The PF then applied full left sidestick input and pressed the left rudder pedal to approximately halfway of its travel. The STOP RUDDER INPUT warning triggered again and the aircraft banked left. ④ The PF then applied full right sidestick and pressed the right rudder pedal causing the STOP RUDDER INPUT warning to trigger a third time and the aircraft to bank right.

(fig.2) Event description part 2

⑤ The PF briefly applied full left sidestick input and moved rudder pedals back to neutral position. The STOP RUDDER INPUT warning triggered for the fourth time. ⑥ The PF then managed to stabilize the aircraft using only light sidestick inputs.

(fig.3) Event description part 3

The PF re-engaged the autopilot. The aircraft reached its cruise altitude 7 minutes later and the flight continued to its destination airport.

When the aircraft landed, the flight crew reported heavy turbulence to the maintenance but made no report of the STOP RUDDER INPUT warnings. 

The Maintenance performed the AMM task 05-51-17 “INSPECTION AFTER FLIGHT IN EXCESSIVE TURBULENCE OR IN EXCESS OF VMO/MMO”, found no issues, and released the aircraft back into service.

The event was not immediately reported after occurrence. Almost one year later, during routine data analysis, the unusual flight parameters were detected, prompting communication with the operator. Subsequently, a report was filed, allowing a detailed analysis to be performed.

Analysis of the data confirmed a maximum lateral acceleration of 0.41 g during the event. This acceleration corresponds to a “red” level event according to AMM task 05-51-44 “INSPECTION AFTER FLIGHT WITH HIGH LATERAL LOADS”. However, only the AMM task 05-51-17 “INSPECTION AFTER FLIGHT IN EXCESSIVE TURBULENCE OR IN EXCESS OF VMO/MMO” was performed after the event based on the flight crew’s report after the flight.

The load analysis performed by Airbus using acceleration data concluded that flight loads were in the vicinity of design limits loads but did not exceed them. The correct maintenance tasks were subsequently performed, including AMM task 05-51-17 “INSPECTION AFTER FLIGHT IN EXCESSIVE TURBULENCE OR IN EXCESS OF VMO/MMO” and AMM task 05-51-44 “INSPECTION AFTER FLIGHT WITH HIGH LATERAL LOADS”. With no issues identified, the aircraft was confirmed as safe for return to service.

Two types of rudder are installed on Airbus aircraft: mechanical rudder or electrical rudder.

A mechanical rudder is installed on: 

• A300 aircraft, 
• A310 aircraft,
• A320 family aircraft (except A321 XLR)
• A330-200/300 and A340-200/300 aircraft built before July 2003 (mod 49144 not installed)

The rudder pedals are mechanically linked to the rudder servo controls (fig.4). Flight controls computers add correction inputs to the servo controls ensuring yaw damping, turn coordination, and a rudder travel limitation function to reduce rudder travel amplitude at high speed.

The rudder displacement is proportional to the pedals displacement until the rudder travel limit value is reached. Therefore, a limited rudder input at high speed can lead the rudder to reach its maximum travel limit.

(fig.4) Mechanical rudder

An electrical rudder is installed on: 

• A220 aircraft
• A321 XLR aircraft
• A330-200/300 and A340-200/300 aircraft built after July 2003 (mod 49144 installed)
• A330neo
• A340-500/600 aircraft
• A350 aircraft
• A380 aircraft

The rudder pedals send an electrical signal to the flight control computers (fig.5). There is no mechanical link between the rudder pedals and rudder servo controllers.

On A220, A330neo, A330 & A340 e-rudder and A340-500-600 aircraft, the pedal input is converted into an equivalent rudder deflection command to the servo controls. This command is adjusted to ensure yaw damping, turn coordination and rudder travel limitation.

On A330neo, A330 & A340 e-rudder and A340-500-600 aircraft, the rudder displacement is proportional to the pedals displacement until the rudder travel limit value is reached – the same design principle as aircraft with the mechanical rudder. The pedals can be further pressed but the rudder will remain at its travel limit. At high speed, a very limited rudder pedal input can result in the rudder reaching its maximum travel limit. 

On A220 aircraft, full pedal application needs to be used to reach the rudder travel limit.
A backup control module acts as a backup in the case of an electrical failure or a failure of the flight control computers.

(fig.5) Electrical rudder (A220, A330neo, A330 & A340 e-rudder and A340-500-600 aircraft)

On A350, A380 and A321XLR aircraft, the rudder pedal input is converted into a sideslip angle target that varies depending on the flight phase (e.g. maximum sideslip on A350: 2° of sideslip at VMO and 15° in CONF 3 and FULL at approach speed (Vapp)). The yaw control law then sends a command to the servo controls to adjust the rudder position to achieve this sideslip angle target. 

On A350 and A380, the sideslip target is proportional to the pedals displacement until the maximum sideslip target is reached. The pedals can be further pressed but the sideslip target will remain at the maximum sideslip target.

On A321XLR aircraft, full pedal application needs to be used to reach the maximum sideslip target.

(fig.6) Electrical rudder (A350, A380 and A321XLR aircraft)

Aircraft structures are designed to sustain the loads caused by normal use of the rudder in a wide range of conditions and speeds. However, aggressive, rapid, full or nearly full travel, and rapidly pressing one rudder pedal then the other in opposite succession can lead to rudder inputs that will cause loads higher than the design limit, and can result in structural damage or failure. The rudder travel limit system is not designed to prevent potential structural damage or failure caused by such forceful rudder pedal inputs by the flight crew.
Even though aircraft with electrical rudder systems have flight control laws that may reduce the structural stress caused by forceful and alternating rudder pedal inputs, this should not be considered as protection against structural damage or failure due to such inputs.

On all Airbus aircraft, regardless of the type of rudder installed on the aircraft (mechanical or electrical) yaw damping and turn coordination are automated in normal law. 

The appropriate use of rudder pedals should be limited to the following situations:

• During the takeoff roll or landing roll: to maintain the aircraft trajectory on ground,
• When landing in crosswind conditions: to decrab the aircraft during flare,
• In the event of an engine failure: to counteract the thrust asymmetry until the rudder is trimmed.

The rudder pedals SHOULD NOT BE USED on Airbus aircraft for the following:

• to induce roll,
• to counter roll induced by any type of turbulence, including wake vortices.

Pilots are the primary detectors of high load events, with their awareness and experience identifying potential loads that could lead to structural stress or damage concerns. Every pilot has the responsibility to document these events through appropriate logbook entries.

When reporting a high load event, whether caused by an excessive turbulence encounter or an excessive maneuvre, the flight crew must provide comprehensive information to maintenance personnel. This detailed reporting is essential as it enables maintenance teams to select and perform the appropriate inspections.

If the rudder was used during the maneuvre, it must be specifically documented, particularly if the STOP RUDDER INPUT warning triggered during the flight. This critical information enables maintenance personnel to properly assess lateral loads and conduct necessary inspections, potentially preventing more serious structural issues from developing.

Maintenance personnel should pay particular attention to pilot reports after an excessive turbulence or maneuver event. If these reports lack sufficient information, maintenance personnel should actively seek additional details from the flight crew. This follow-up is crucial to ensure that the appropriate AMM/MP/AMP is applied, unlike in the previously described event where AMM task 05-51-44 “INSPECTION AFTER FLIGHT WITH HIGH LATERAL LOADS” application was missed.

A320 family, A330 and A340 aircraft are equipped with LOAD<15> report capability that triggers in the case of a high load event. However, on A320 family aircraft, not all FDIMU standards are capable of detecting high lateral loads. Therefore on aircraft with FDIMU only capable of detecting vertical loads, a LOAD<15> report may not be triggered when only lateral loads occur. Additionally, even when both vertical and lateral loads are experienced, a generated LOAD<15> report might mention only the vertical loads. This limitation reinforces the critical importance of comprehensive pilot reporting.

On all aircraft types, when there is uncertainty about the presence of lateral loads during an excessive turbulence event reported by flight crew, the maintenance personnel should apply the AMM/MP task 05-51-44 “INSPECTION AFTER FLIGHT WITH HIGH LATERAL LOADS” or the AMP task BD500-A-J05-51-37-01AAA-284A-A “Extreme maneuver/severe turbulence event analysis (Technical data) – Special irregular inspection”. These tasks guide how to assess the recorded lateral load, enabling appropriate maintenance actions to be taken if needed.