Boeing 737 Primary Flight Controls
The primary flight controls (figure 1.1) are the flight controls that are used in every phase of flight. The primary flight controls in the Boeing 737 are powered by a Power Control Unit (PCU). The aileron and the flight spoilers are used for rolling the aircraft. The elevator is used for pitching the aircraft and is attached to the horizontal stabilizer, which can also be moved by sys-tems like stabilizer trim and autopilot trim. The rudder is used for yawing the aircraft. The yaw damping system assists in various tasks, like gust damping and turn coordination. Now lets have a look at how these systems operate.

Figure 1.1 - Boeing 737 Primary Flight Controls

Boeing 737 Ailerons
The aileron movement can be accomplished by three different systems. The most important aileron movement is done by the control wheels on the flight deck. These control wheels are connected to each other, by means of cables. This means, that if one control wheel deflects, the other will do exactly the same. Via cables, the control wheels are connected to the aileron Feel and Centring Unit, which is located in the main wheel well. This unit changes the neutral position of the control wheel after trimming and adds ‘feel’ to the control wheel. This is done so that the pilots have a feel of what kind of forces are applied to the ailerons. The Feel and Centring Unit moves a cable to the aileron PCU, which in turn converts ca-ble movements to hydraulic movements. The PCU deflects the ailerons accordingly to the control wheel input. These PCU’s are powered by hydraulic systems A and B (we'll talk about these systems later). The hydraulic systems are powered by bleed air that is generated by the engines.


The ailerons can also be moved by the aileron trim. The aileron trim switches are located on the pedestal, on the aft electronic panel. The switches can be moved left and right and must be moved together in order to work. The aileron trim sends an electrical signal to the aileron Feel and Centring Unit, which moves the cables to the aileron PCU and changes the neutral position of the control wheel. On top of the control wheel is a scale on which the current trim is indicated, accomplished by the control wheel that moves simultaneously with the aileron trim.

Lastly, the aileron can be moved by the autopilot.The autopilot sends a signal to the Feel and Centring Unit. The consequences of this input are the same as with aileron trim.

When the control wheel is deflected more than ten degrees, the flight spoilers will come up on the down going wing, to destroy lift. The spoiler mixer is the system that regulates this move-ment, and is connected to the First Officer’s control wheel.

Boeing 737 Elevators
The elevator can be operated by three systems. The control column is the manual way for the pilots to control the pitch. The control columns of the Captain and the First Officer are, just like the control wheels, linked to each other by means of cables. The cables are routed to the elevator Feel and Centring Unit, which is located in the tail cone and has the same function as the aileron Feel and Centring Unit. From here, the cables are routed to the elevator PCU’s, that move the elevator itself. The elevator PCU’s are powered by hydraulic systems A and B.

The autopilot is the second way by which the elevator is controlled. The autopilot sends a signal to the Feel and Centring Unit. This signal is combined in the Elevator Feel Computer with the position of the stabilizer, pitot-static inputs and hydraulic pressure. The Elevator Feel Computer then sends a signal to the elevator PCU’s to move the elevator and adds feel to the control columns via the Feel and Centring Unit.

The third way by which the elevator is controlled, is the mach trim. This is an automated system that eliminates the effect of the airplane pitching down at speeds approaching the speed of sound. The system operates at speeds above Mach 0.615. The Air Data Inertial Reference Unit (ADIRU) senses the airspeed via the pitot-static tube and sends a signal to the Flight Control Computer (FCC). The FCC sends a command to the Mach trim actuator, which in turn sends a signal to the elevator Feel and Centring Unit. The Feel and Centring Unit moves the elevator via the PCU’s and adds feel to the control column.



Stabilizer
Pitch trim is applied to the stabilizer. The stabilizer can be moved by four systems. The stabilizer trim (figure 1.2) is operated through the wheel on the side of the control stand on the pedestal. A scale indicates how much the stabilizer is trimmed. The green band shows the safe takeoff trim range. On the left side of the scale is the trim wheel itself. The inputs of this wheel are send to the stabilizer trim part of the trim motor in the tail cone. The trim motor then adjusts the position of the stabilizer. This is a manual way of trimming and is normally not used.

The main electric trim is the second way in which the stabilizer is moved. The main electric trim is operated through two switches that can be moved up and down on the control wheel. The trim switches move the trim wheel and uses the same system as the stabilizer trim. The speed of the trimming depends on the speed of the aircraft. If the aircraft flies at a low airspeed, the trimming is faster than when the aircraft flies at higher speeds.

The autopilot trim is the third way the stabilizer is controlled. It sends a direct signal to the auto-pilot part of the trim motor, which moves the stabilizer to the commanded position.

The speed trim is the last system, and it operates at low speeds, aft centre of gravity, low gross weight and high thrust conditions, such as takeoff and go around procedures. It helps the pilot in controlling the aircraft in critical situations.


Figure 1.2 - Stabilizer Trim

Boeing 737 Rudder
The rudder is attached to the vertical stabilizer and is moved by two different systems. The rudder pedals are used for manual input and are mostly used during takeoff and landing. The rudder trim is used for trimming away unwanted forces in the yawing direction. Most frequently, the rudder is operated automatically by the yaw damper*, which helps with preventing Dutch roll and other unwanted movements.



The rudder pedals are operated by the pilot’s feet. The cables from the rudder pedals are routed to the rudder Feel and Centring Unit, located in the tail cone, and then to the rudder PCU. The primary PCU uses hydraulic system A and B, and the standby PCU uses the standby hydraulic system. The rudder trim is operated through a small trim wheel and is located on the pedestal, on the aft electronic panel, next to the aileron trim switches. The trim wheel can be moved left and right for trimming. A scale is added so that the pilot can see how far the aircraft is trimmed. The rudder trim is the input for the Feel and Centring Unit, which controls the PCU and adds feel to the rudder pedals.

The yaw damper is a system that aids the pilot in flying the aircraft. It is powered by hydraulic system B. It prevents Dutch roll, which is a movement that occurs at higher altitudes with less dense air. The aircraft can be yawed by different airflows on the side of the aircraft, which gives the aircraft a tendency to roll left and right. This becomes an oscillation, and is prevented by the yaw damper. The yaw damper also helps in flying coordinated turns and automatically provides gust damping.

Yaw damper operations are controlled by two Stall Management/Yaw Damper (SMYD) computers. One SMYD computer is used for the primary yaw damper, and the other is used for the standby yaw damper. The ADIRU’s send the speed and yaw rate data to the SMYD computer. The aileron input is added to these signals and the SMYD computer tells the yaw damper how much to deflect the rudder via the PCU. The yaw damper can deflect the rudder two degrees from either side of the trimmed position in flaps up flight, and three degrees in flaps down flight.
Yaw damper inputs are not fed back into the rudder pedals via the Feel and Centring Unit, which is why there is a yaw damper indicator installed in the cockpit.
Article Related

Anchors

Articles

Images


Click here for all Boeing photos