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 I.  The MASTER CAUTION annunciator is illuminated through the master caution system if any annunciation on the overhead panel is illuminated. Pressing the MASTER CAUTION annunciator extinguishes both the MASTER CAUTION and the system annunciator light.
 J.  The FLT CONTROL annunciator is illuminated when any annunciator on the flight control module is on. The MASTER CAUTION annunciator is pressed to extinguish the annunciator. Pressing the system annunciator assembly recalls the faulty system if it still exists.
 14. Angle of Airflow (ALPHA) Sensor (Ref 27-32-00)
_______________________________
 A.  The angle-of-airflow sensor (also termed angle-of-attack) (AOA) provides the FCC's with a measurement of airflow angle, relative to the mean chord line of the wing, called the alpha angle. The signal from the sensor is used as a reference input to insure that speed and attitude are maintained to prevent reaching stall conditions. Two sensors are mounted on the airplane. The sensor mounted on the left side of the airplane, near the nose, provides the alpha signal to FCC A. The sensor on the right provides the alpha signal to FCC B. The sensor consists of a synchro transmitter, mounted inside of the airplane connected to the airflow vane, external to the airplane. The airflow vane contains an electrical heater, used during flight to prevent icing.
 15. Flap Position Transmitters (AMM 27-51-00/001)
 A.  The flap position transmitters provide DFCS inputs indicating actual flap position. The two flap position transmitters are located on the outboard flap torque tubes. The units are mechanically driven by the flap torque tubes. Electrical angular rotation of the synchro from 0 degrees to 270 degrees represents flap positions from 0 degrees to 40 degrees. This information is used in the pitch channel to generate program commands for use in the takeoff and go-around modes.
 16. Flap Limit Switch (AMM 27-51-00/001)
 A.  The flaps up limit switches provide the FCC and the stabilizer trim servo motor with flaps up or down logic voltage. There are two flaps up limit switches: S245 Flap Switch No. 1, also known as Flaps Up Switch (AMM 27-51-87/401) and S290 Flap Switch No. 2, also known as Mach Trim Flap Switch (AMM 22-21-31/401). The flaps up limit switches are located on the flap control unit in the aft right corner of the main wheel well. The switches are mechanical cam operated with 28v dc supplied to the common contact. The normally closed (flaps up) contact provides voltage to the FCC and the normally open (flaps down) contact provides outputs to the FCC and stabilizer trim servomotor.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
 17. DFCS Power Distribution (Fig. 4, 5, 6)
_______________________
 A.  DFCS power is derived from two ac and three dc power buses. The buses involved are: number 1 and 2, 115 volt ac electronic buses; number 1 and and 2, 28 volt dc electronic buses and 28 volt dc switched hot battery bus.
 B.  During normal operation, each FCC receives power from a separate 115 VAC bus. The two 28 VDC power buses supplying the FCCs are connected together through a crossfeed diode and contacts of disconnect relay TR3. Souce switching allows both FCCs to receive power from the same power bus if a bus to one of the FCCs fails. Source switching logic inhibits dual channel autopilot operation if both FCCs are using the same power bus.
 C.  The two 28 VDC power buses that supply power to the two FCCs are connected together through contacts of disconnect relay TR3. It is required that these two 28 VDC power buses be isolated from each other during flight director approach operations. This precludes a failure in one 28 VDC bus causing the other bus to fail in a way that could result in erroneous flight director displays. The two 28 VDC buses to the FCCs are isolated by de-energizing disconnect relay TR3 when either flight director or autopilot glideslope is engaged.
 D.  115 Volt AC Electronic Bus 1 provides power to:
 (1)  
C849 STABILIZER TRIM (P18-0), 3-phase, 115 volts ac to the stabilizer trim servo.

 (2)  
C723 AIL FORCE LIMITER (P18-1), 115 volts ac to the AFC ACC unit for the aileron force limiter.

 (3)  
C1041 SNSR EXC A (P18-1), 115 volts ac to the AFC ACC unit. An autotransformer in the AFC ACC unit provides 26 volts ac to A FCC and some of the sensors for the A channel.

　
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