Boeing 737 Secondary Flight Controls
The secondary flight controls (figure 1.1) consists of three devices which add lift or drag and help in flight path adjustment. Leading edge (LE) devices add lift for slower approach and landing speeds. The trailing edge (TE) flaps also add lift together with drag to lower stall speeds during various phases of flight. Spoilers or speed-brakes help roll the airplane, add drag for deceleration and in-crease the rate of descent.

Figure 1.1 - Boeing 737 Secondary Flight Controls

Boeing 737 Leading Edge Devices
The LE devices (figure 1.2) on the Boeing 737 consist of the leading edge flaps and slats and are both located on the leading side of the wing and are hydraulically powered by hydraulic system B. The commands for the LE flaps and slats come from the TE flap system.

Figure 1.2 - Leading edge flaps and slats

Although the LE flaps have the same function as the LE slats they operate in a different way. The LE flaps are located near the wing root, where more powerful flow control is required, and consist of 2 Krueger flaps (figure 1.3). LE flaps have 2 positions; up and extended. The LE Slats (figure 1.4) are portions of the LE which are moved forward and down to create a path for air similar to a slot. The slats have 3 positions, up, extended and full extended. Both the LE devices and TE flaps are hydraulically powered by hydraulic system B. Extension of the LE devices occurs in se-quence with the movement of the TE flaps. When the TE flap system is operated, it sends inputs to the LE flap and slat control valve. The flap lever moves a cable system that supplies a mechanical input to the TE flap system. As the TE flaps move, feedback from the TE flap system moves the LE flap and slat control unit. The control unit sends hydraulic power through the autoslat control valve to the LE flap and slat actuators. Eventually the actuators move the LE flaps ands slats. The position of LE devices is observed by lights presented on the overhead panel.

Figure 1.3 - LE Flaps


Figure 1.4 - LE Slats

To prevent the aircraft from going into a stalled position, the autoslat system increases wing lift by extending the LE devices automatically. When the aircraft is attempting take-off or is in approach for landing and the TE flaps are set at 1, 2 or 5, the autoslat system will move the LE slats from extended to the full extended position before a stick shaker occurs. The LE slats will retract to the extended position when the pitch angle is below the stall attitude.

Boeing 737 Spoilers
The TE flaps on the Boeing 737 consist of a double slotted flap mechanism, which produces an increase in both lift and drag by extending the wing chord line. Just like the LE slats the TE flaps allow high-pressure air to flow from beneath the wing to the upper surface through slots. Powered by hydraulic system B, movement of the flaps is initiated by moving the flap lever (figure 1.5) located on the centre pedestal in the cockpit. When the flap lever moves, the TE flap control and LE control operate. Hydraulic pressure goes to the TE drive unit and TE flaps/slats. Eventually the flaps and slats move to their desired position. The position and movement of the flaps is shown on the flap position indicator (figure 1.6).

The flap setting is restricted by speed. The maximum speed for each flap position is presented on a “flaps limit placard” (figure 1.7) in the cockpit. In case the airplane has a higher than normal airspeed with flaps 30 or 40 selected, the Flap/Slat Electronics Unit (FSEU) supplies flap load relief. This means that when the airspeed reaches a speed, which is higher than the maximum speed for that particular flap setting, the FSEU tells the flaps to retract to a higher flap setting. The movement of the flaps is then shown on the flap position indicator while the flap lever stays in position.

Figure 1.5 - Flap lever

Figure 1.6 Flap position indicator Figure 1.7 - Flap limit speeds

Boeing 737 Rudder
The Boeing 737 is fitted with six hydraulically powered spoilers on both wings. These are divided into two ground spoilers and four flight spoilers. The ground spoilers only operate while the aircraft is on the ground while the flight spoilers operate during different phases of flight and can have multiple functions.

The ground spoilers are located on both sides of the flight spoilers and operate in conjunction with them when the aircraft is on the ground. The ground spoilers do not operate under lateral control but can be used as speed brakes when initiated by the pilot. This can be done by positioning the speedbrake lever in manual mode control.

The flight spoilers consist of four panels located on the aft upper wing. There are two ways in which the flight spoilers are used and controlled during flight operations, namely:
- manual mode control
- automatic mode control
In manual mode the spoilers are operated by pilot inputs and help decelerate the aircraft or increase the rate of descent. In automatic mode the spoilers respond to control wheel movement and are used to roll the aircraft when the outer ailerons are locked-out. When the flight spoiler panels are raised into the air stream they spoil or reduce lift and apply drag on the wing. The combination of these two effects (lift reduction and drag) is utilized into providing lateral control of the airplane. Both the flight spoilers and ground spoilers are powered by hydraulic system A and B.

Figure 1.8 - Flight and ground spoilers Figure 1.9 - Speed brake

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