DS 2
1
DN180

1 AIRPLANES WITH SPEED BRAKE TEST SWITCHES
GND
R283 GND SENSING M980 AUTOSPEED BRAKE CONTROL ACCESSORY UNIT (E3-2)RLY (E11)
Speed Brake Control System Schematic
Figure 7

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737-300/400/500MAINTENANCE MANUAL
 D.  The speed brakes are activated automatically by the speed brake lever actuator to aid braking during landing roll. When the speed brake lever actuator drives the speed brake control lever, the flight and ground spoiler motion is the same as when the lever is manually moved. The system is armed prior to touchdown. Arming is accomplished by raising the speed brake control lever to release the lever latch and moving the lever aft into the ARMED detent. The speed brake lever actuator will drive the control lever full aft to the UP position when the correct combination of main landing gear wheels rotate at a speed greater than 60 knots or when the right main gear strut is compressed and air sensing switch and ground sensing switch are both in ground mode (Fig. 7). Rotation of at least the inboard wheels on both main gears, the outboard wheels on both main gears, or both wheels on either main gear will provide the wheel rotation signal. The electrical switching for the system is accomplished by an arming switch on the speed brake forward drum and relays in the M980 autospeed brake control accessory unit. Activation of relays RI, RO, LI, and LO is accomplished by solid state relay control circuits in the antiskid control unit. Additional control inputs are provided to the automatic speed brake lever actuator by the main gear ground and air sensors. These inputs are separate and parallel to wheel speed control inputs, either of which can deploy the speed brakes. The ground and air sensor inputs act through air and ground sensing, wheel speed, and squat relays in the landing gear logic shelf. Prior to touchdown, with the air and ground sensors on the right main gear strut in the air mode, the air coils of the associated latching relays are energized to arm the circuit for speed brake deployment. On touchdown, when the right main gear shock strut is adequately compressed, the air and ground sensing relays move to the ground mode. This energizes the associated wheel speed and squat relays. If the air and ground sensing relays are both in ground mode, (right main gear strut compressed), the circuit is completed through the go-around relay to actuate the speed brake lever actuator. A time delay relay is incorporated in the ground coil of the air and ground sensing relays to delay pull-in for approximately 4 seconds. If the air and ground sensors move to ground mode within 4 seconds of each other, the circuit will be completed to deploy the speed brakes. If not, the ground coil of the first latching relay to sense ground mode will energize and open the circuit. The circuit cannot be reset except by go-around, since the air and ground sensors on the main gear must move to the air mode to reset the air coils of the air and ground sensing relays in the landing gear logic shelf. However, if the landing were to be completed, the wheel speed relays would automatically deploy the speed brakes on wheel spin-up. The necessary sequence for proper automatic speed brake operation is as follows:
 (1) Airplane in air.
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 (2)  
Speed brake control lever placed in ARMED position.

 (3)  
Airplane landed with speed brake control lever in ARMED position. If, with the airplane in air, the speed brake control lever is placed in the ARMED position and then returned to the DOWN position, and the airplane is then landed in this condition (without speed brakes, such as the airplane being used for pilot training and doing touch and go landings), the spoilers will deploy upon the next arming on the ground. Also if thrust reverse is used the spoilers will drive to the up position.
　
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