Speed brakes are hinged, movable secondary control surfaces used for slowing down the speed of the aircraft by increasing the profile drag. These surfaces are also called "dive brakes" or "dive flaps." On some aircraft, they are hinged to and faired with the side or bottom of the fuselage. On others, they are attached to the wings. Regardless of their location, their purpose is the same.

The fuselage speed brake system is normally electrically controlled and hydraulically operated. See figure 9-40. In an emergency, it can be controlled manually. 

The brake surfaces are installed on the sides of the aft portion of the fuselage below and forward of the horizontal stabilizer. They hinge at their forward end. When in the closed position, they fit flush with fuselage skin. An elevator speed brake interconnect provides a connection between the left-hand speed brake and the aircraft nose down elevator control cable. When the speed brakes open, the cable pulls and provides a nose down action to counteract the tendency of the aircraft to assume a nose up condition. The speed brakes may be actuated by the two-position, spring-loaded-to-neutral control switch on the throttle lever or by the manual override control handle. When operating the switch to open the speed

Figure 9-40.Speed brake control system.

 brakes, the control circuit energizes to operate the opening solenoid of the control valve. Pressure is sent to the actuating cylinders, extending the speed brakes. To close, the opposite solenoid energizes, repositioning the control valve and directing pressure to the retract side of the actuating cylinders, closing the speed brakes.

When you depress or pull the manual override handle to operate the speed brakes, a plunger manually positions the control valve to direct pressure to the actuating cylinders. The spring bungee connected to the manual control lever returns the manual override handle assembly to the neutral position when the handle is released. If electrical power is applied while the manual override handle is actuated, the system will remain in the position selected by the handle. If the handle is released, the system actuates to the position selected by the control switch on the throttle lever. The speed brakes cannot be stopped at intermediate positions between fully closed and fully open. The restrictor in the open line restricts return fluid flow from the actuating cylinders when the speed brakes are being closed.

If the hydraulic system fails, the check valve in the pressure line traps pressure between the control valve and the actuating cylinders. If the speed brakes are open, this pressure will hold them open. If the speed brakes are actuated to the closed position, the pressure in the system will shift the primary slide in the control valve. This movement will relieve the trapped pressure and allow the speed brakes to close from the air load against them.

A blowback relief valve, installed in the hydraulic return line, allows for automatic retraction of the speed brakes under high air loads. When the speed brakes are open, the force of the airstream against the surfaces tends to force them closed. The force builds up the hydraulic pressure in the speed brake system. When the pressure reaches a maximum of 3,650 psi, it relieves through the blowback relief valve.