Boeing 737-800 System Description

1. Primary Flight Controls 8. Electrical
2. Secondary Flight Controls 9. Fuel
3. Engines 10. Hydraulics
4. APU 11. Landing Gear and Brakes
5. Bleed Air 12. Flight Management System
6. Pressurization 13. TCAS
7. Air Conditioning  

•Please watch the following systems videos online. They give a general introduction and description of the systems of the Boeing 737 Aircraft. You may need to watch each video more than a few times to get familiar with it.

•Once you are familiar with the video, read the related system description on this document and relate to the video.

•Below are a brief textual description of each system. You will need to memorize this information as you will be asked to describe each system in your oral exam of your Aircraft Dispatcher Certification.

•Please complete memorizing the systems before commencing your classroom portion of the Aircraft Dispatcher Course.


B737-800 has:

  • 2 elevators
  • 2 ailerons
  • 1 rudder
  • 8 flight spoilers

Pilot control inputs are sent to Power Control Units (PCUs). PCU is the interface between pilot’s mechanical input and a series of hydraulic actuators acting on each control surfaces.

LEFT PCU is powered by HYDRAULIC A


Manual Reversion = When pilots control flight control surfaces via cables and linkages that allow for manual control in event of complete hydraulic systems failure.

Elevator feel and centering unit is a computer, provides forces to control column. This unit receives input from:

  • Pitot Tubes
  • Hydraulic System A and B
  • Horizontal Stabilizer
  • Elevator Feel Shift Module

B737-800 is trimmable in all 3 primary flight controls.

Mach Trim provides stability at Mach number above .615. Elevators are adjusted relative to stabilizer position.

 Mach number is obtained from ADIRU (Air Data Inertial Reference Unit)

Ailerons are coupled. When one moves down, another moves up. Ailerons are also operated via PCU. Each hydraulic system controls a single aileron.


This include the Flaps, Slats and Spoilers

Flaps: Trailing edge device that change the area and camber of the wing for increased lift and drag

Slats: Leading edge device that smoothen airflow on the top of the wing and provide extra lift

Spoilers: Panels on the upper surface of the wing. Device to destroy airflow, or lift generation on top of the wing.

Spoilers can be used for:

  • Aiding roll rate control inflight
  • Allow increased rate of descent without increasing airspeed
  • On the ground, spoilers are deployed to disrupt lift so that aircraft is not going to become airborne again, therefore providing better friction and braking performance, reducing runway length required to stop.

Boeing 737-800 has double-slotted flaps. Flap extends from the wing, and from that flap, extends another flap.

Flaps Position available: 1, 2, 5, 10, 15, 25, 30, and 40 (about 38 seconds from fully retracted to fully extend)

Leading and Trailing edge flaps and slats may be controlled by one of the two ways:

  • Hydraulically (Normal Operation, operated by Hydraulic system B)
  • Electric motor (alternate operation in case of hydraulic B failure)

PDU (Power Drive Unit) unit that converts hydraulic pressure into mechanical force that operates trailing edge flaps.

Leading edge slats and flaps CANNOT be retracted inflight after alternate operations.


B737-800 is powered by two CFM56-7b engines

Engines are used to power:

  • Propulsion / Thrust
  • Electric System (with engine driven generator)
  • Pneumatic system (bleed air)
  • Hydraulic system

Components of CFM56-7b:

  • Fan (24 fan blades)
  • Booster / Low pressure compressor (3 stages)
  • Hi Pressure Compressor (9 stages)
  • Combustion Chamber – 450 Celsius to 1700 Celsius
  • High Pressure Turbine (1 stage)
  • Low Pressure Turbine (4 stages)
  • Exhaust

A splitter split air into Primary Airflow (around 20%) and Secondary Airflow (bypass air). Ratio between primary airflow and secondary airflow is known as “Bypass Ratio”. CFM56-7b is a high bypass ratio engine. The higher the bypass ratio, better fuel efficiency and lower engine noise.

Engine indicating systems inside the cockpit:

  • Primary engine Indication
  • Low-Pressure Shaft Speed (N1)
  • Exhaust Gas Temperature (EGT)
  • Fuel Flow, Fuel Quantity per tank
  • Total Air Temperature
  • Active thrust mode

Secondary engine indications

  • High-Pressure Shaft Speed (N2)
  • Oil Pressure, Oil Temperature
  • Oil Quantity
  • Engine vibration level

Fuel: JET A / JET A1

Electronic Engine Control (EEC) monitors and controls fuel flow to the engine.

  • APU

APU (Auxiliary Power Unit) is a miniature jet engine that provides electrical power & pneumatic services (bleed air) on the ground and in the air. It allows the aircraft to be self-sufficient on the ground to be without the need for ground power.

Type of APU used in B737-800 is Honeywell 131-9B APU that is certified up to 41,000 feet.

APU inlet door is located on the right side of the aft fuselage and is automatically controlled.

After combustion, APU exhaust blows through the muffler then the tail cone. High flow of APU exhaust jet create low pressure area inside the APU compartment which pull outside air in through a second hole in the tail cone which is called the “Adductor Inlet”, drawing outside air into APU compartment and helps cool the APU.

ECU (Electronic Control Unit) continually monitors and controls the APU from start to shut down. ECU also provides shutdown protection if any parameters goes out of limit.

APU Primary Engine Components:

  • Power Section
  • Load Compressor
  • Accessory Gearbox

Purpose of pneumatic compressor in the APU is supplying bleed air for:

  • Air-conditioning
  • Pressurization
  • Ice protection
  • Engine start

Fuel supply to the APU is regulated by APU’s FCU (Fuel Control Unit). APU draws fuel from the same line as the main engines. Normal operations take fuel from the left side of the fuel system.


Bleed air is compressed high pressure and high temperature air extracted from the engine or APU. The average pressure and temperature are 40 PSI and 250 degrees Celsius.

Used for:

  • Engine Starting
  • Air Conditioning and cabin pressurization
  • Engine cowl and Wing Anti Ice
  • Water Tank Pressurization
  • Hydraulic reservoir Pressure
  • TAT Probe

Bleed air is produced by:

  • Number 1 and 2 Engine, 5th and 9th stage of high pressure compressor section of engine
  • APU – Auxiliary Power Unit
  • GPU – Ground Power Unit

Bleed air from these sources is ducted into a left and right Pneumatic Manifold systems and supplies bleed air to aircraft systems.

It is important to remember Ground source connected into the right part, APU connected to left part.


Pressurization uses bleed air which is filtered and cooled, providing the flight deck and passenger cabin with fresh air.

Pressurization is achieved by sealing the aircraft cabin climbs to higher altitude.  Air is modulated and disposed as fresh new air enters it in order to maintain a constant cabin pressure.

The pressurization pressurize cabin altitude to about 8000’.

The bleed air and PACKs (Pressurization and Air Conditioning Kits) introduce air into the cabin.

Cabin Pressure Control (CPC) system is in charge of controlling the rate which air flows out of the cabin, by closing or opening valve in the aft fuselage, known as “Outflow Valve”.

Cabin pressurization also has a relief system that is provided as a protection mechanism if pressurization control system were to fail.

Pressurization can be controlled automatically or manually.


Air Conditioning System Provides:

  • Fresh conditioned air for air pressurization and ventilation
  • Control cockpit and passenger cabin temperatures
  • Recirculate around half of the cabin air for ventilation purposes
  • Remove unpleasant air from lavatories and galleys
  • Avionics and system cooling

Air-conditioning wise, airplane is divided into 3 zones:

  • Cockpit / Flight Deck
  • Forward Passenger Compartment
  • Rear Passenger Compartment

Source of conditioned air:

  • Ground supplied pre conditioned air
  • Recirculation system
  • Air Conditioning PACKs (Pressurization and Air Conditioning Kits) (2 available on-board)

All these are supplied into the MIX MANIFOLD, a giant pool for conditioned air distribution.

PACKs supply air in accordance to the coldest of 3 cabin zones.

The PACKs are supplied by bleed air, cooled, filtered, and delivered to 3 zones.

Cockpit always receive fresh air, while forward and rear passenger receive both fresh and circulated air.

Air Temperature Selectors in the cockpit allow temperature control.

PACK – Pressurization and Air Conditioning Kit, they are two small air compressors that take bleed air from the engines, after which the air is cooled, filtered and delivered to the flight deck and passenger cabins.

PACKS do the following:

  • Regulates Bleed Air
  • Removes excessive heat
  • Controls the temperature and humidity

Systems that rely on electric power:

  • Avionics
  • Landing Gear Motor
  • Leading Edge and trailing edge devices
  • Lighting systems, etc.

Both AC and DC power are available. AC power from IDG = 115/200v 400 Hz. DC power from batteries and Transformer Rectifier Unit (TRU) = 28v

AC power sources:

  • 2 Engine Driven Generators
  • APU Generators
  • External Power

AC Power (115/200v 400 Hz) GND and IDG rated at 90 kVA. DC to AC conversion is achieved through an Inverter.

Bus is a main platform where electricity is supplied to and drawn from. The 2 Primary Buses AC Buses are:

  • AC Transfer Buses 1 = powers DC bus 1, through TRU 1
  • AC Transfer Buses 2 = powers DC bus 2, through TRU 2

DC Power (28v power) Sources:

  • 28v Battery & Charger
  • 3 Transformer Rectifier Units (TRUs) that convert 115v AC to 28v DC

DC System has 2 batteries, Main Battery, and Auxiliary Battery (both 46Amp 24 volt DC) used for starting APU and providing backup electrical source.

Batteries does not charge if:

  • Refuel is in Progress
  • APU is Starting
  • STBY power in use
  • Battery overheating

These Batteries guarantee AC/DC power for 60 minutes.

  • FUEL

B737-800 Fuel Capacity: 46,063 lbs.

Fuel System purpose:

  • Containing and distributing fuel
  • Cooling of hydraulic fluids, each tank contains heat exchanger

Engine fuel feed system features heat exchangers to cool oil from the Integrated Drive Generator (IDG), these heat exchangers intentionally heat fuel.

Fuel systems consist of 5 tanks, 3 main tanks, and 2 surge tanks. These tanks are integral to the airframe and are made of the same aluminum alloy as parts of the aircraft surrounding it.

The 5 tanks are:

  • Main Tank 1 (Left Wing) cap. 8630 lbs. – fill first
  • Main Tank 2 (Right Wing) cap. 8630 lbs. – fill first
  • Centre Tank; cap 28,800 lbs. – Burn first
  • Left Surge Tank (Left Wing outboard) Cap. 235 lbs.
  • Right Surge Tank (Right Wing outboard) Cap. 235 lbs.

Other Fuel System elements:

  • Fuel Pumps
  • Fuel Valves
  • Quantity Indicators

Fuel Quantity Indication System (FQIS) measures the mass of fuel in each tanks in either lbs. or kg and relay the information which displayed in the flight deck’s Display Units (DUs).

FMC also receives this fuel information directly via the Fuel Quantity Processor Unit (FQPU)

Common Display System (CDS) has the fuel information which are needed by flight crew most often

Fuel Types: JET A (Freeze -40 C, Flash 51 C), JET A1 (Freeze -47 C, Flash 42 C)

Don’t allow fuel temperature to fall below the following figures:

-43C for JET A1, or -37 C for JET A.

737-800 is fitted with 6 primary electric fuel pump.


737-800 has 3 hydraulics systems:

A, B, and standby. They can be interconnected.

Hydraulics powers:

  • Flight Control surfaces (A,B)
  • Leading edge flaps and slats (B)
  • Trailing edge flaps (B)
  • Spoilers (A,B)
  • Landing Gears (A,B)
  • Wheel brakes (A alternate, B main)
  • Nose wheel steering (A main, B alternate)
  • Thrust Reversers (A, B)
  • 2 Autopilots (A, B)
  • Power Transfer Unit (A motor, B pump)

Hydraulics fluid used BMS (Boeing Material Specification) 3-11 Type 4/5.

Hydraulics has 2 system reservoirs, A (smaller, 6, 8 gal cap) and B (larger, 10, 7 gal cap).

Hydraulics reservoirs are pressurized by Bleed Air.

Each reservoir has 2 pumps, engine driven pumps, and AC Electric pump.

Hydraulic fluids are used to help heat up fuel, and fuel are used to help cool hydraulic fluids. 


The Landing Gear consist of:

  • 2 main landing gears (2 wheels each) in the center fuselage
  • 1 nose landing gears (2 wheels) in the forward fuselage

Each landing gears has a series of shock absorber that compresses when there’s a weight over them. MAX landing weight is over 65 tons.

Landing Gear is extended and retracted by the landing gear lever. It has 3 positions:

  • UP – Landing gear is fully retracted and secured
  • OFF – Landing gear free from hydraulic pressure
  • DOWN – Landing gear commanded to extend

There are three landing gear indicator lights positioned above the landing gear lever. There is also an additional set of three lights for each gear down and locked indication in the overhead panel.

(Landing Gear deployment and retraction is powered by Hydraulic A, however during retraction B may also provide pressure).

In case of hydraulic failure, landing gear can be extended manually by 3 Gear Release Handles (one for each gear). After deployed manually, it can’t be retracted with the gear release handles.

There are two ways to steer the nose wheel:

  • With Rudder Pedal (max 7 degrees deflection, used in take-off)
  • With nose wheel steering wheel, or tiller bar (78 degrees deflection, used in taxi)

(Nose wheel steering normally powered by Hydraulic A, alternate steering powered by hydraulic B)

  • FMC / FMS

Flight Management Computer / Flight Management System

Flight Management Computer is the pilot’s mean of controlling the optimum flight profile for the aircraft.

2 FMCs are available. FMCs take information for flight that the pilots have entered and create the optimum flight path. FMC then take that flightpath and through the use of Auto flight system component, will control the aircraft with utmost precision.

2 modes directly impacted by FMC are the:

  • LNAV (Lateral Navigation, or Horizontal Navigation)
  • VNAV (Vertical Navigation)

FMC is a computer in the background and pilots and flight crew will most likely never see the FMC.

CDU (Control Display Unit) however, is often mistaken as a FMC. CDU is the interface with screen and keyboard with which data from the FMC is displayed in.

2 CDUs are available, one in the captain seat, one in the FO’s seat.

  • TCAS

Traffic Alert and Collision Avoidance System

Is a system designed to identify and reduce the risk of mid-air collision between aircraft by “interrogating” transponder-equipped nearby aircraft.

TCAS will display cockpit indications, alerts, and conflict resolutions for the crew of nearby potential conflict.

TCAS I   : Traffic display and advisory only – designed primarily for general aviation

TCAS II  : In addition to traffic advisory, provides vertical conflict resolution, will requires both aircraft to be Mode S Transponder equipped. – Used mainly on commercial airliners.

TCAS II is mandatory for aircraft with 30+ seats or weighing more than 15,000 kg.

Transponder is a device that automatically transmits a coded signal when interrogated by ATC ground radar or TCAS.

Transponder is short for “Transmitter Responder”

Types of Transponder:

  • Mode A : Squawk Code Only
  • Mode C :  Includes altitude info
  • Mode S : Mode A+C, improved accuracy, improved interrogation, and communication datalink

This datalink is used by TCAS II to coordinate resolution advisory between two aircraft. Therefore, a Mode S Transponder is a requirement for TCAS II.

Transponder Mode A will be invisible to TCAS.