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B737 Ng Flight Control System Engineering Essay

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In this chapter the differences of the Airbus A320 and Boeing 737 NG will be discussed. There will be looked at the flight control system of the Boeing 737 NG and the flight control system of the Airbus A320 (2.2) after that a comparison between the conventional system and fly-by-wire system (2.3).

2.1 B737 NG flight control system

This section will discuss the flight controls of the Boeing 737 NG. At first the primary flight controls especially the ailerons (2.1.1) and also the secondary flight controls of the Boeing 737 NG trailing edge devices (2.1.2) will be discussed.

2.1.1 Primary flight controls

In this subsection, the primary flight controls of the Boeing 737NG will be discussed. The ailerons will be investigated as primary flight control in this chapter. The composition of the system (2.1.2a), the input of the system (2.1.2b), the transportation of signals (2.1.2c), the output signals (2.1.2d) and the hydraulic system of the Boeing 737NG will be discussed (2.1.2e)

2.1.1a Composition

The aileron system exist of two ailerons. The Boeing 737 has an aileron on each wing. They are placed on the outer side of the wing to create a greater moment over the airplanes longitudinal axis. Over this axis the airplane will be able to roll. Because the ailerons are mounted on the outer side of the wing the needed forces can be lower than when they are placed on the inner side of the wing. The moment of the aileron is the force multiplied by the arm length.

2.1.1b Input

The ailerons are positioned by the control wheels of the pilots. When the pilot controls his steering wheel a mechanical cardanic movement will make the input signal transferred to the transfer mechanism.The transferm mechanism keep the control wheel movement to a limit

of 107.5 degrees left and right. This mechanism is mounted under the steering column of the pilot. The roll axis force transducer is mounted between the control shaft and aileron drum below the captain's control column. The force transducer provides dual electrically isolated ac output signals that are proportional to the force applied to the control wheel or column. The signals are used by the autopilot computers when the system is engaged in CWS (manual) mode or in CMD mode with no flight mode selected. Both control wheels are cable mounted connected to each other, when the captain steer his control wheel the left aileron control bus drum will rotate and operated the control cable to rotate the right aileron control bus drum. This drum is connected exactly as the captains control wheel and the first officers control wheel will move.

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2.1.1c Transport

By rotating the control bus drum, the control drum also will rotate. The control cable will be operated and will run through the airplane and routed by cable guides and kept on tension with cable tensioners. In the main wheel well the cable will be attached to the aileron body quadrant. On the quadrant the new outgoing cable is mounted. The quadrant is mounted on two aileron power control units (PCU). This PCU's are mounted at the aileron control quadrant. On this quadrant the aileron input shaft is mounted. On this shaft the autopilot control rod, the PCU's four pogo input cranks, the feel and centering unit and the aileron actuator are mounted.

For backup two PCU's are mounted to help the pilot to create a higher force to the ailerons so the pilot don't need all his strength to control the ailerons. Also there are four pogo inputs installed between the body quadrant and the input shaft. These are to control the movement of the cables so that they are on tension at all times and not move when the don't need to. The feel and centering unit is used to position the ailerons to his neutral position. This can be done by movement of both aileron trim switches on the aft electronic panel. The aileron trim actuator can position the aileron input shaft to trim the system. The total trim the pilot have made can be read on the aileron trim indicator on the control wheel. The autopilot aileron actuator is duplicated for safety reasons. The autopilot actuator is with the autopilot input rod mounted to the aileron input shaft. This actuator will be electronic controlled by the flight control computers. All the actuators are hydraulic controlled by system A and B.

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2.1.1d Output

The control cables will led through the wings until they reach the aileron wing quadrant. This quadrant is kept on tension by the aileron cable tension spring. This for that the trenched out cables are kept on tension. The quadrant is connected with and pushrod connection to the aileron. On the aileron an aileron balance tab is mounted and connected with control rods. The aileron is armed with four balance panels to keep the balance in the ailerons. And this panels are equipped with fixed balance weights. The output signal of this system is that the aileron will move up or down after movement that the pilot will make by moving the control wheel.

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2.1.1e Hydraulic system

The ailerons are powered by system A and B. When both of this systems fails, the aileron can still be operated manually by controlling the cables. The left aileron is powerd by system a and the right aileron is powered by system b.

During normal operation of the ailerons, an aileron input goes through the input pogos of each PCU to its input crank. The upper and lower input cranks move, slides in the control valve, and supply hydraulic pressure to the actuator. The lower input crank is connected to the primary slide and the upper input crank is connected to the secondary slide. A torsion spring inside the PCU connects the two input cranks. Movement of the primary slide supplies one-half the total flow rate, and movement of the secondary slide supplies the other half. The primary slide moves to its full effective stroke before the secondary crank starts to move the secondary slide. When the primary and secondary slides move, hydraulic pressure goes through the control valve to one side of the actuator. This moves the actuator housing and the respective aileron body quadrant to the commanded position. The other side of the actuator is connected to the return.

If one PCU can't supply a hydraulic pressure, its bypass valve will be activated and moves to the bypass position. This connects the two sides of the actuator, and prevents a hydraulic lock condition. When the pilot moves the control wheel, the ON side PCU still moves to his commanded position normally. As the ON side PCU moves, it also moves his respective aileron body quadrant and back drives the OFF side PCU actuator housing. When the OFF side housing moves, hydraulic fluid is pushed through the bypass valve. If one PCU input pogo can't move freely, the pilot must supply approximately 20 pounds of additional force to compress or extend the spring inside the pogo. The other PCU input pogo still moves its own input crank and slide to the commanded position. This equalizes pressure on both sides of the actuator and prevents a hydraulic lock condition. Now the other PCU can move the aileron body quadrant assembly normally. As the ON side PCU moves its respective aileron body quadrant it also back drives the OFF side PCU actuator housing. When the OFF side housing moves, hydraulic fluid is pushed through the bypass valve.

During a manual reversion, the bypass valve receives no hydraulic pressure and moves to the bypass position. This connects the two sides of the actuator and prevents a hydraulic lock condition. When the pilot moves the control wheel more than three degrees, the primary and secondary input cranks hit the mechanical stops on the outside of the actuator housing. As the housing moves, hydraulic fluid in the actuator is through the bypass valve. Movement of the housing also moves the aileron body quadrant assembly to the commanded position.


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