1. Flight Controls Purpose of flight controls:

The purpose of a flight control system is transfer motion/force input from a pilot to a flight control surface. In a traditional aircraft, flight control systems are broken down by axis of control: pitch, roll and yaw. Flight control systems can be either reversible or irreversible.
A reversible system is a flight control system where movement applied to the control surface moves the control in the flight compartment. A simple example is shown in Figure 1. Reversible flight control systems are used on smaller aircraft where the hinge moment (surface) loads are small enough that a mechanical linkage system is adequate.

Another reversible flight control system is shown in Figure 2. This is a 2D representation of a system that shows some typical components in a reversible flight control system. The systems shown in Figures 1 and 2 both contain a cable system, however, reversible systems can also be designed using pushrods and bellcranks without cables.

Figure 2 Reversible Flight Control System

An irreversible system is a flight control system that utilizes powered controls so that movement of the surface will not move the control in the flight compartment. An example of an irreversible flight control system is shown in Figure 3. Figure 3 shows a mechanical system connected to a hydraulic actuator. The linkage positions the servo within the actuator that controls which side of the actuator sees high-pressure fluid and which side is ported to return.

Figure 3 Irreversible Flight Control System

Irreversible systems are required when the maximum pilots input force is not sufficient to drive the surface loads. In irreversible systems, the mechanical linkage will drive a hydraulic power control unit (PCU). The linkage controls the spool position in a servo that applies hydraulic pressure to a hydraulic actuator. For traditional aircraft, the transition from reversible to irreversible occurs in the 25,000 – 30,000 gross weight range of the airplane. The rudder surface is typically the first surface to require powered control, where the critical flight condition is an engine out, rejected takeoff occurring just prior to wheels leaving the runway. In some airplanes, a boost system or a hydraulic assist system is used to supplement pilot rudder forces. An assist system is usually less expensive and less weight than a full powered control system. Crosswind landings also result in high rudder hinge moments and must also be considered.
Generally speaking, flight control systems are simply mechanisms that can be broken down into basic components (see Mechanisms modules for the basic components) plus four bar linkages and cables. For example, the system shown in Figure 3 is consists of four bar linkages connected in series until they reach the flight control hydraulic servoactuator. The reversible system shown in Figure 1 consists of 2 four bar linkages separated by a cable system. Therefore, any flight control system can be evaluated and analyzed as discussed in the Mechanism and Four Bar Linkage modules. In fact, the basic principles of a four bar linkage apply to any basic mechanism and are key to understanding general mechanism characteristics.
Beyond the mechanism fundamentals, flight control systems can also include hydraulic system and hydraulic actuators, electromechanical actuators (including power screws), cable stretch, structural stiffness, springs, load variation with flight condition and surface deflection, and dynamic effects.
As stated above, the purpose of a flight control system is to transfer motion and force from the flight compartment to the surface or actuator. To design a flight control system, the required movement of the surface or actuator servo plus the required load must be known. Input motion is limited by human factor considerations and maximum human force capability. 2. Primary and secondary flight controls :
2.1 Primary controls:
Generally, the primary cockpit flight controls are arranged as follows. * a control yoke (also known as a control column), center stick or side-stick (the latter two also colloquially known as a control or joystick), governs the aircraft's roll and pitch by moving the ailerons (or activating wing warping on some very early aircraft designs) when turned or deflected left and right, and moves the elevators when moved backwards or forwards * Rudder pedals, or the earlier, pre-1919 "rudder bar", to control yaw, which move the rudder; left foot forward will move the rudder left for instance. * Throttle controls to control engine speed or thrust for powered aircraft.
The control yokes also vary greatly amongst aircraft. There are yokes where roll is controlled by rotating the yoke clockwise/counterclockwise (like steering a car) and pitch is controlled by tilting the control column towards you or away from you, but in others the pitch is controlled by sliding the yoke into and out of the instrument panel (like most Cessna, such as the 152 and 172), and in some the roll is controlled by sliding the whole yoke to the left and right (like the Cessna 162). Centre sticks also vary between aircraft. Some are directly connected to the control surfaces using cable, others (fly-by-wire airplanes) have a computer in between which then controls the electrical actuators.
Even when an aircraft uses variant flight control surfaces such as a V-tail ruddervator, flaperons, or elevons, to avoid pilot confusion the aircraft's flight control system will still be designed so that the stick or yoke controls pitch and roll conventionally, as will the rudder pedals for yaw. The basic pattern for modern flight controls was pioneered by French aviation figure Robert Esnault-Pelterie, with fellow French aviator Louis Blériot popularizing Esnault-Pelterie's control format initially on Louis' Blériot VIII monoplane in April 1908, and standardizing the format on the July 1909 Channel-crossing Blériot XI. Flight control has long been taught in such fashion for many decades, as popularized in ab initio instructional books such as the 1944 work Stick and Rudder.
In some aircraft, the control surfaces are not manipulated with a linkage. In ultra-light aircraft and motorized hang gliders, for example, there is no mechanism at all. Instead, the pilot just grabs the lifting surface by hand (using a rigid frame that hangs from its underside) and moves it.
2.2 Secondary controls
In addition to the primary flight controls for roll, pitch, and yaw, there are often secondary controls available to give the pilot finer control over flight or to ease the workload. The most commonly available control is a wheel or other device to control elevator trim, so that the pilot does not have to maintain constant backward or forward pressure to hold a specific pitch attitude .(other types of trim, for rudder and ailerons, are common on larger aircraft but may also appear on smaller ones). Many aircraft have wing flaps, controlled by a switch or a mechanical lever or in some cases are fully automatic by computer control, which alter the shape of the wing for improved control at the slower speeds used for takeoff and landing. Other secondary flight control systems may be available, including slats, spoilers, air brakes and variable-sweep wings.

Roll control schematic

ELEVATOR CONTROL SCHEMATIC

STABILIZER CONTROL SCHEMATIC

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YAW CONTROL SCHEMATIC (B737 CLASSICS)

SPEED BRAKES SCHEMATIC (B737 CLASSICS)

TRAILING EDGE SCHEMATIC

LEADING EDGE DEVICES SCHEMATIC (B737 CLASSICS)

Ground Operation * All flight and ground spoilers automatically rise to full extend on landing, if the Speed Brake Lever is in the ARMED position and both Thrust Levers are in IDLE. When spin-up occurs on any two main wheels, the Speed Brake Lever moves to the UP position, and the flight spoilers extend. When the right main landing gear shock strut is compressed, a mechanical linkage opens a hydraulic valve to extend the ground spoilers. * If a wheel spin-up signal is not detected, the Speed Brake Lever moves to the UP position, and all spoiler panels deploy automatically after the ground safety sensor engages in the ground mode. * After touchdown, all spoiler panels retract automatically if either Thrust Lever is advanced. The Speed Brake Lever will move to the DOWN detent. * All spoiler panels will extend automatically if take-off is rejected and either Reverse Thrust Lever is positioned for reverse thrust. Wheel speed must be above 60 kt in order for the automatic extension to take place. * A failure in the automatic functions of the speed brakes is indicated by the illumination of the SPEED BRAKE DO NOT ARM Light. In the event the automatic system is inoperative, the Speed Brake Lever must be selected manually to the UP position after landing.
HIGH LIFT DEVICES * High lift leading edge devices are used in combination with the trailing edge flaps to increase lift during take-off and landing and decrease stall speeds. The trailing edge flaps and leading edge devices, when extended, increase the wing area and the effective wing camber. * Trailing edge flap position 0-15 provide increased lift; positions 15-40 provide increased lift and drag to permit slower approach speeds and maneuvering capability. Flap position 15, 30 and 40 are certified landing flap positions.
Trailing Edge Flaps * The Flap Lever positions a flap control valve that directs hydraulic pressure to actuate the flap drive unit to position the flaps. The drive unit also controls the leading edge control valve, so that the leading edge devices operate in conjunction with the trailing edge flaps. * If an asymmetrical condition develops between the right and left wing trailing edge flaps, hydraulic power is automatically removed from the flap drive unit. * In the event of hydraulic system B failure, the trailing edge flaps can be alternate operated (electrically). In this case, control of the flaps is from the flight control panel. * The guarded Alternate Flaps Master Switch actuates a flap bypass valve to prevent hydraulic lock of the flap drive unit and arms the Alternate Flaps Position Switch. This switch controls an electric motor that operates the drive unit to extend or retract the trailing edge flaps. No asymmetry protection is provided through the alternate flap drive system. Alternate Trailing Edge Flap extension 0 to 15 takes about 2 minutes.
Flap Load Relief B737 Classics * A flap load limiter is installed in the trailing edge flap drive system, which is operative at the flaps 40 position. The Flap Lever does not move. * When the flaps are set at 40, the TE-flaps- retract to 30 if airspeed exceeds 158 knots (737-300 & 500) or 162 knots (737-400). - extend to 40 when airspeed is reduced below 153 knots (737-300 & 500) or 157 knots (737-400).
Flap Load Relief B737 NG * The Flaps/Slat Electronics Unit (FSEU) provides a TE flap load relief function which is operative at the flap 30 and flaps 40 positions. The Flap Lever does not move. * When the flaps are set at 40, the TE flaps: retract to 30 if airspeed exceeds 163 knots.
- extend to 40 when airspeed is reduced below 158 knots. * When the flaps are set at 30, TE flaps:
- retract to 25 if the airspeed exceeds 176 knots.
- extend to 30 when airspeed is reduced below 171 knots.
Leading Edge Devices * The leading edge devices consist of four flaps and six (737-300/-400) or eight (737-800) slats :
- two flaps inboard of each engine,
- three (ex. 737-300/-400) or four( ex. 737-800) slats outboard of each engine. * Flaps are hinged surfaces that extend by rotating downward from the lower surface of the wing leading edge. Slats are sections of the wing leading edge that extend forward to form a sealed or slotted leading edge depending on the trailing edge flap setting.
Leading edge devices are normally extended and retracted by hydraulic power from system B. The leading edge control valve is controlled by the trailing edge drive unit so that the leading edge devices operate in conjunction with the trailing edge flaps. When the trailing edge flaps leave the UP position, the leading edge flaps extend fully, and the leading edge slats extend to an intermediate Position. As the trailing edge flaps extend past the 5 position, the leading edge slats move to FULL EXTEND. When the flaps are retracted the sequence is reversed. * In the event of hydraulic system B failure the leading edge flaps and slats can be driven to FULL EXTEND position using power from the standby hydraulic system. In this case the Alternate Flaps Master Switch energizes the standby pump, and the Alternate Flaps Position Switch, held in the down position momentarily, extends the leading edge devices.
NOTE. The leading edge devices cannot be retracted by the standby hydraulic system. * Indicator lights on the center instrument panel provide overall leading edge devices position status. The Leading Edge Device Annunciator on the aft overhead panel indicates the positions of the individual flaps and slats.
Auto Slat Operation * A dual channel auto slat system provides improved handling qualities at high angles of attack during take-off or approach to landing. Works only with flaps 1, 2 or 5 (With Flaps 10° do not forget that slats are fully extended !) * When TE flaps 1 through 5 are selected, the leading edge slats are in the EXTEND position. As the aircraft approaches the stall angle, the slats automatically drive to the FULL EXTEND position, prior to stick shaker activation. The slats return to the EXTEND position when the pitch angle is sufficiently reduced below the stall critical attitude.

Power Transfer Unit * Slat operation is normally powered by system B hydraulics. An alternate source of power is provided by system A through a Power Transfer Unit (PTU) if a loss of pressure from the system B engine driven pump is sensed. The PTU provides system A pressure to power a hydraulic motorized pump, pressurizing system B fluid to provide power for slat and auto slat operation. * A single channel failure causes the AUTO SLAT FAIL Light to illuminate upon Master Caution recall and extinguish after reset. * A dual channel failure would result in failure of the autoslat system indicated by illumination of the AUTO SLAT FAIL Light.

3. CONTROLS & INDICATORS
FLIGHT CONTROL SWITCH

STBY RUD : * Activates the standby pump & opens the standby rudder shutoff valve to pressurize the standby rudder power control unit (indicated by the flight control LOW PRESSURE light extinguished). * Closes Flight Control Shutoff valve, isolating ailerons, elevators & rudder from corresponding hydraulic system pressure.

* B737 NG: Yaw Damper switch can be turned back on, activating standby Yaw Damper, if both FLT CONTROL switches are OFF. * Corresponding hydraulic system pressure is isolated from ailerons, elevators & rudder. ON (Guarded position)Normal operating position.

ALTERNATE FLAPS MASTER SWITCH

OFF (Guarded position) * Normal operating position.
ARM:
- 4 subsequent actions:
1) Closes TE Flap Bypass Valve
2) Activates standby pump
3) Arms Standby hydraulic LOW PRESSURE light
4) Arms the Alternate Flaps Position Switch

ALTERNATE FLAPS POSITION SWITCH
(Spring-loaded from DOWN to OFF)
Functions only when the Alternate Flaps Master Switch is in ARM
DOWN:
Momentarily selecting DOWN fully extends LE devices using standby hydraulic pressure
Holding the Switch DOWN electrically extends TE Flaps. (Caution: No asymmetry protection !)
- (Plan a flaps15 landing & remember that it takes approx. 2 min. from 0° to 15°)
UP:
- Electrically retracts Flaps
- LE devices remain extended & cannot be retracted by the alternate flap system.

FLIGHT SPOILER SWITCH * (Used for maintenance purposes only) OFF: * Closes the respective flight spoilers shutoff valve.
YAW DAMPER SWITCH
OFF:
- Disengages yaw damper
- Switch drops to OFF automatically when the system B FLT CONTROL switch is positioned to OFF or STBY RUD or when power interruption to n°1 Transfer Bus for 2 sec..
ON :
- Engages yaw damper to rudder power control unit (B737 NG : does not require hydraulic pressure to be held in the ON position.

YAW DAMPER INDICATOR * Indicates yaw damper inputs to the rudder * B737 NG : Operation of standby yaw damper inputs are not indicated.

STABILIZER TRIM WHEEL * Provides for manual operation of stabilizer * Overrides any other stabilizer trim inputs * Handle should be folded inside Stab Trim Wheel for normal operation. * Rotates when stabilizer is in motion.

STABILIZER TRIM AUTOPILOT CUCUTOUT SWITCH * CUT OUT
Removes power from stabilizer autopilot trim.

STABILIZER TRIM MAIN ELECTRIC CUTOUT SWITCH
CUT OUT
- Removes power from stabilizer main electric trim.
STABILIZER TRIM OVERRIDE SWITCH
OVERRIDE
- Bypasses the control column actuated stabilizer trim cutout switches to restore power to the main electric trim.

SPEED BRAKE LEVER
DOWN (detent)
- All flight & ground spoiler panels in faired position.
ARMED
- Automatic speed brake system armed
- Upon touchdown, the speed brake handle moves to the UP position, and all flight & ground spoilers Extend.

FLIGHT DETENT
- All flight spoilers are extended to their maximum position for inflight use.
UP
- All flight spoilers & ground spoilers are extended to their maximum position for ground use.
FLAP LEVER
- Selects position of the flap control valve directing hydraulic pressure for the flap driver unit.
- Position of the LE devices is determined bi TE flap position.
- B737 Classics: At position 40, arms the flap load relief system, which automatically causes flap
Retraction to Flaps 30 in the event of excess airspeed.
- B737 NG : At position 30 or 40, arms the flap load relief system, which automatically causes flap
Retraction to Flaps 25 or 30 in the event of excess airspeed.

FLAP GATES
Prevents inadvertent flap lever movement beyond :
- Position 1 : to check flap position for 1 engine inoperative G/A
- Position 15 : to check flap position for normal go-around.

FLAPS POSITION INDICATOR
- Indicates position of LEFT & RIGHT TE flaps
- Provides TE flaps asymmetry protection circuit.