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 (6)  
The underwater locator beacon (ULB) aids in location of a submerged DFDR by sending out an acoustic signal when the unit is submerged in water.

 (7)  
The ULB is contained in a cylindrical watertight case designed to withstand high impact shock and deep water immersion. It is mounted on the front panel of the DFDR.

 (8)  
The ULB is battery operated and is designed to operate for at least 30 days when immersed. It will withstand water depths of 20,000 feet and has a maximum detection range of 2,000 to 4,000 yards.

 3. Operation
_________
 A. Functional Description (Fig. 2)
 (1)  Power Distribution and Control
 (a)  
During normal operation, ac power is supplied to the flight recorder system through the left and right ENG START-TO-STOP relays. The relays are wired in parallel so that any one relay can provide power to the system.

 (b)  
The left and right ENG START-TO-STOP relays are actuated when the corresponding engine is first started. The engine ignition switches control the operation. At engine start, when an engine ignition switch is first turned on, the corresponding ENG START-TO-STOP relay (left or right) is energized. After the fuel control switch is set to RUN, it will cause the flight recorder control relay to remain energized after the ignition switch is selected to OFF. The relays remain energized until the fuel control switches are set to CUTOFF.

 (c)  
The system No. 2 AIR/GND relay is also wired in parallel with the ENG START-TO-STOP relays to provide power to the flight recorder system. This relay de-energizes when the airplane becomes airborne, resulting in a contact closure that provides an additional means to maintain power to the system during flight.

 (d)  
Manual power switching is accomplished with the ON-NORM-TEST switch on the FRCP (or FDEP). During normal operation, the switch is set to NORM and power is controlled by the automatic switching network. When the switch is set to either ON or to the momentary TEST position, power is supplied to the system.

 
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本文鏈接地址：757 AMM 飛機維護手冊 儀表 CHAPTER 31 - INSTRUMENTS 1(40)