Dutch roll is normally associated with swept wing aircraft (whose tendency to roll with yaw is greater than for comparable straight wing aircraft) and high altitude flight, where aerodynamic damping is reduced. While the ATR 42 will not naturally enter a Dutch Roll situation, positive rudder application will initiate it which is why a yaw damper is installed.
The Yaw damper is designed to prevent Dutch Roll or tail oscillations. The RCU is designed to prevent oscillations when the Yaw Damper is not being used which is the case for takeoff and landing. The RCU clutch releases whenever the yaw damper is engaged.
The purpose of the RCU is to improve the aircraft’s directional stability by constraining the movement of the control linkage(a fixed link between the RCU and the rudder)and therefore undesired rudder movement. The RCU is intended to increase the aircraft’s resistance to Dutch roll/oscillations by keeping the rudder centred about its trimmed position until a threshold force is applied by the pilot to move the rudder. It has 2 internal springs to maintain a centering force (approximately 10 kg force at the rudder pedals) towards whatever position the rudder has been trimmed.
When a left or right rudder trim input is made, there is also an electrical input made to an electromagnetic clutch within the RCU causing it to release its linkage to the rudder and then re-engage thus allowing it to centre its 10 kg centering force on the newly trimmed rudder position.
The rudder cam automatically synchronises to present rudder position each time the rudder trim switch is activated. There is no rudder cam position indicator, therefore, before take-off the rudder trim setting must be made with both the rudder trim and the rudder pedals in the neutral position. The rudder trim itself is centered by the pilot(usually on the pre-flight) and then normally just before takeoff, the rudder pedals will be centered by the pilot at which point the rudder trim switch will be moved momentarily. This action disengages and then re-engages the RCU which is then centred about the neutrally trimmed rudder position that had already been set. This is done as a checklist item.
If more than 30 kg force is applied to a rudder pedal, the yaw damper will disengage such as following an engine failure. After re-trimming, the RCU is applying force to the deflected rudder in order to keep it and the associated deflected rudder pedals in their trimmed location.
If the RCU fails, or its centred position differs significantly from the aerodynamically trimmed position, the benefits provided by the RCU in terms of directional stability will be lost. In the latter case, there will be a continuous centering force towards a non-trimmed position as well as a reduced resistance to Dutch roll. Indication of a rudder releasable centering unit failure is a Dutch roll oscillation tendency.
In this case, the situation can only be restored if the pilot exercises positive and continuous control through the rudder pedals to ensure that the rudder does not move from its desired, trimmed position. In other words, lock the (rudder) pedals with your feet to keep them from moving. An operable RCU simulates you holding pressure on both pedals with your feet.
As an example of the result of a faulty RCU, a 1990 event occurred while an ATR was making an approach to land when the yaw damper disengaged and could not be re-engaged until the AFCS computer had been re-set. During this period, the aircraft rolled ± 15°. The subsequent investigation revealed that a link attaching one of the springs within the RCU had failed.
Last edited by JammedStab; 27th Jan 2015 at 00:20.