Divided A/C Systems

Purpose of A/C Systems Knowledge

Foundation for Safe, Efficient, Confident Aircraft Operation supporting Decision-Making and Regulatory Compliance

Professional Pilot Systems Objective

Anticipate system behavior in Normal and Abnormal Conditions, recognize malfunctions, and apply appropriate procedures

Main Interdependent Aircraft Systems

Integrated network where a fault in one system may affect several others

Aircraft Systems Classification

Air, Fuel, Hydraulic, Electrical, and Safety/Warning systems

Air Systems (ECS)

Environmental Control System including Pneumatic, Air Conditioning, and Pressurization subsystems

Fuel System

Provides continuous supply of Clean Fuel while maintaining Structural Balance and Center of Gravity

Hydraulic System

Uses Pressurized Incompressible Fluid to operate heavy-duty aircraft systems

Electrical System

Provides Power Generation, Transformation, and Distribution throughout the aircraft

Safety and Warning Systems

Systems designed to prevent accidents including GPWS/EGPWS and TCAS

Environmental Control System (ECS)

System providing Ventilation, Cooling, and Cabin Pressure

Pneumatic System (Bleed Air)

Supplies Compressed Air for packs, engine start, and anti-ice systems

Air Conditioning System

Uses Air Cycle Machine (ACM) to cool and condition bleed air for cockpit and cabin

Pressurization System

Maintains Cabin Altitude typically between 6,000 and 8,000 feet

Cabin Altitude Control

Regulated by controlling airflow through the Outflow Valve

Bleed Air System

System providing compressed air as an energy source to operate other aircraft systems

Primary Bleed Air Source

Engines provide compressed air extracted from compressor stages

Low-Pressure (LP) Bleed Port

Used during High Thrust conditions

High-Pressure (HP) Bleed Port

Used during Low Thrust or High Altitude conditions

APU (Auxiliary Power Unit)

Small turbine engine providing Ground Air Conditioning, Engine Start, and Emergency Backup

Ground Pneumatic Source

External high-pressure air unit used for Fuel Saving and Emissions Reduction

Bleed Air Valve

Regulates bleed air Pressure and Temperature and closes during over-pressure, leak, or fire

Bleed Trip Sensor

Boeing 737 device that automatically closes bleed valve during over-temperature

Pre-Cooler

Heat exchanger using Fan Air to cool bleed air

Starter Valve

Controls flow of high-pressure air to start the engine and closes when Self-Sustaining Speed is reached

Air Conditioning Function

Regulates Temperature, Humidity, and Air Quality in the aircraft cabin

Pack System

Air conditioning unit processing bleed air through cooling and expansion stages

Primary Heat Exchanger

Uses Ram Air to initially cool bleed air

ACM Compressor

Increases Pressure and Temperature of air

Secondary Heat Exchanger

Further cools compressed air

Expansion Turbine

Reduces temperature Below Ambient during air expansion

Water Separator

Removes moisture using Centrifugal Force

Mixing Chamber

Blends conditioned air with Trim Air for temperature control

Trim Air System

Injects Hot Air to fine-tune cabin zone temperature

Ram Air System

Provides Unpressurized Ventilation and cooling if packs are unavailable

Air Conditioning Redundancy

Typically Two Independent Packs operating in parallel

Pressurization System Purpose

Prevent Hypoxia and maintain Structural Integrity of the aircraft

Pressurization Principle

Continuous air inflow with controlled outflow through the Outflow Valve

Outflow Valve Closed Position

Results in Lower Cabin Altitude

Outflow Valve Open Position

Results in Higher Cabin Altitude

Differential Pressure

Difference between cabin and outside pressure measured in psi

Positive Pressure Relief Valve

Prevents Over-Pressurization of the cabin

Negative Pressure Relief Valve

Prevents external pressure from exceeding cabin pressure during descent

Pressurization Automatic Mode

Primary mode using Dual-Channel Controllers for redundancy

Pressurization Standby Mode

Alternate mode requiring Manual Landing Field Elevation Input

Pressurization Manual Mode

Pilot directly controls Outflow Valve as last line of redundancy

Fuel System Purpose

Ensure continuous, reliable fuel supply to Engines and APU

Fuel System Balance Function

Maintains aircraft Center of Gravity and Structural Balance

Main Fuel Tanks

Wing tanks supplying fuel directly to engines

Center Fuel Tank

Tank holding additional fuel used with Priority Feeding logic

Priority Fuel Feeding

Center tank fuel used first to Minimize Wing Bending Moment

Surge or Vent Tank

Safety tank preventing fuel overflow to atmosphere

APU Fuel Supply

Typically supplied from the Left Main Tank

AC Electric Fuel Pump

Primary source of fuel system pressure

Crossfeed Valve

Allows any tank to supply Any Engine to correct fuel imbalance

Scavenge Pump

Transfers residual fuel from low points to main fuel sumps

Fuel Shutoff Valve

Isolates fuel flow during Fire or Engine Shutdown

Automatic Fueling Mode

System automatically stops fueling when preset quantity is reached

Manual Fueling Mode

Fuel valves operated manually by ground personnel

Over-Wing Refueling

Method allowing fuel loading directly into Wing Tanks Only

Hydraulic System Purpose

Provide high force using Pressurized Hydraulic Fluid

Hydraulic Operating Pressure

Typically 3,000 to 5,000 psi

Hydraulic Reservoir

Stores hydraulic fluid for system operation

PTU (Power Transfer Unit)

Transfers hydraulic power mechanically without mixing fluids

PTU Activation Condition

Operates when One Engine-Driven Pump Fails

Accumulator

Stores pressurized hydraulic fluid for Emergency Demand

RAT (Ram Air Turbine)

Emergency device providing hydraulic and electrical power during total power loss

Engine-Driven Pump (EDP)

Primary source of hydraulic pressure

Electric Motor-Driven Pump (EMDP)

Backup or support hydraulic pump

Electrical System Purpose

Provide electrical power for aircraft systems and avionics

AC Power System

Standard aircraft power supply of 115 Volts, 400 Hz, 3-Phase

DC Power System

Low-voltage electrical system operating at 28 Volts

IDG (Integrated Drive Generator)

Device converting variable engine speed into constant 400 Hz AC Power

APU Generator

Provides electrical power on ground and backup power in flight

TRU (Transformer Rectifier Unit)

Converts AC Power to DC Power

Static Inverter

Converts DC Power to AC Power for standby systems

Electrical Distribution System

Network of AC and DC Buses distributing power throughout aircraft

Essential Bus

Electrical bus supplying power to critical flight systems

Emergency Bus

Bus supplying power during electrical failures

Battery Bus

Electrical bus powered directly by battery and always available

Total AC Power Loss

Aircraft powered by 28 Volt Battery supplying DC and standby AC via inverter

GPWS (Ground Proximity Warning System)

Reactive system using Radar Altimeter to detect terrain proximity

EGPWS (Enhanced GPWS)

Predictive system using GPS and Terrain Database for forward-looking terrain warning

CFIT Prevention

Primary purpose of GPWS and EGPWS is to prevent Controlled Flight Into Terrain

GPWS Mode 1

Excessive Descent Rate warning ("Sink Rate")

GPWS Mode 2

Excessive Terrain Closure Rate warning ("Terrain")

GPWS Mode 3

Altitude loss after Takeoff or Go-Around warning ("Don't Sink")

GPWS Mode 4

Unsafe terrain clearance warning with gear or flaps not configured

GPWS Mode 5

Glideslope deviation warning

GPWS Mode 6

Advisory callouts and Bank Angle warning

GPWS Mode 7

Windshear warning

TCAS (Traffic Alert and Collision Avoidance System)

Airborne system preventing mid-air collisions using transponder signals

TA (Traffic Advisory)

Warning of potential traffic conflict in approximately 40 seconds

RA (Resolution Advisory)

Instruction to climb or descend to avoid collision in approximately 25 seconds

RA Vertical Rate Requirement

Typically 1,500 to 2,000 feet per minute

Increase RA Vertical Rate

Typically 2,500 to 3,000 feet per minute

TCAS Pilot Rule

Pilot must follow Resolution Advisory immediately, even if conflicting with ATC instructions

TCAS Limitation

Cannot detect aircraft without functioning transponders

Proximate Traffic Definition

Aircraft within 6 NM horizontally and 1,200 feet vertically for situational awareness only