General
The towing power system uses the main battery (hot battery bus) as its power source. The battery supplies dc power to the tow dc bus. The towing static inverter supplies ac power to the tow ac bus.
Static Inverter
The static inverter gets 28v dc power from the hot battery bus. The static inverter has a converter that increases the dc voltage, and an inverter that changes the dc power into single phase, 115v ac power.
Control
The towing power system operates when all of these conditions are true:
*
Battery switch is off

*
Towing power switch is on

*
There is no ground handling dc power.

These conditions cause the main tow bus relay to energize. The hot battery bus then supplies dc power to the tow dc bus and the towing static inverter. When the main tow bus relay energizes, it also causes the two towing relays to energize. These relays switch the necessary loads to towing power.
Loads
Each load that connects to a tow bus has its own circuit breaker. These are the tow ac bus loads:
*
Right position lights

*
Left position lights

*
Captain’s panels flood lights

*
Accumulator brake pressure indicator back-lighting.

These are the tow dc bus loads:
*
Audio management unit (captain’s card)

*
Audio management unit (connector board)

*
Captain’s audio control panel

*
Captain’s speaker

*
Accumulator brake pressure indicator

*
Brake source light

*
Towing power light.

See the communications chapter for more information about the communications components (CHAPTER 23).
See the landing gear chapter for more information about the accumulator brake pressure indicator (CHAPTER 32).
See the lights chapter for more information about the lights (CHAPTER 33).
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TOWING POWER -FUNCTIONAL DESCRIPTION

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DC GENERATION -INTRODUCTION

General
The dc generation system changes ac power from the generators into dc power. It supplies power for dc loads and to charge the batteries. The batteries are an alternative power source for some loads when the generators do not operate.
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DC GENERATION -INTRODUCTION

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DC GENERATION -SUBSYSTEMS INTRODUCTION

General
The dc power system has these subsystems:
*
Transformer rectifier units (TRUs)

*
Batteries

*
Standby power

*
Towing power

*
Flight controls dc (FCDC) power.

Transformer Rectifier Units (TRUs)
TRUs change ac power to dc power. There are four TRUs in the dc power system:
*
Left

*
Center 1 (C1)

*
Center 2 (C2)

*
Right.

Batteries
Batteries supply dc power to loads that require power when no ac power source is available.
Standby Power
Standby power supplies ac power and dc power to important systems when all normal ac power is lost. The RAT generator and the main battery are the sources of standby power.
TRUs change the RAT ac power to dc power. The static inverter changes dc power to ac power.
Towing Power
Towing power supplies ac power and dc power to components that are necessary to safely tow the airplane. The main battery is the source of towing power. A static inverter changes dc power to ac power.
Flight Controls DC (FCDC) Power
Power supply assemblies (PSAs) supply power to the flight controls system. The related FCDC battery protects the PSA from power interruptions during power transfers.
Training Information Point
The external power system also has a TRU. The ground handling TRU supplies power to the ground handling bus. See the external power system section for more information (SECTION 24-40).
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