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Aircraft Pneumatic System Overview CBT

Oct 2, 2025,

Overview

This lecture explains the aircraft pneumatic system, including its functions, main components, sources, controls, valve operations, leak detection, start-up procedures, and ECAM indications.

Pneumatic System Overview

  • The pneumatic system provides high-pressure air for:
    • Air conditioning
    • Engine starting
    • Wing anti-icing
    • Water system pressurization
    • Hydraulic reservoir pressurization
    • Aft and forward cargo heating (if installed)
  • Main sources of pneumatic air:
    • Engine bleed air
    • APU (Auxiliary Power Unit) bleed air
    • High-pressure ground connection
  • The system is controlled via the overhead air conditioning panel and monitored by the ECAM (Electronic Centralized Aircraft Monitor).
  • The cross bleed duct connects the engine bleed systems and can receive air from the APU or ground source.

Main Components and Controls

  • Key switches on the overhead air conditioning panel:
    • Engine 1 bleed push button switch
    • Engine 2 bleed push button switch
    • APU bleed push button switch
    • Cross bleed selector switch
  • The cross bleed duct allows interconnection between engine, APU, and ground pneumatic sources.
  • The ECAM bleed page displays:
    • Bleed air valve positions (open/closed)
    • Bleed air temperature and pressure
    • Source of bleed air and which systems are supplied

Engine Bleed System

  • Each engine has a dedicated bleed system that:
    • Selects the appropriate compressor stage for bleed air
    • Regulates bleed air temperature and pressure
  • Engine bleed valves:
    • Open pneumatically when bleed pressure exceeds 8 psi and the APU bleed valve is closed
    • Act as shutoff and pressure regulating valves
  • High pressure (HP) valve:
    • Opens if intermediate stage pressure is low, maintaining 32–40 psi
  • Pre-cooler:
    • Uses engine fan air to cool bleed air, limiting temperature to 200°C
    • Fan air valve is pneumatically operated
  • The two engine bleed systems can be isolated or interconnected via the cross bleed valve and duct.
  • Engine bleed push button switches:
    • Normally in the ON (lights out) position
    • Bleed valve opens if pressure >8 psi, APU bleed valve is closed, no leaks or overpressure/overheat, engine fire push button is in, and engine start valve is closed
    • Selecting OFF closes the engine bleed valve and HP valve if open
    • Fault light and ECAM warning appear for overpressure, overheat, leaks, or valve disagreement
  • ECAM bleed page:
    • Shows valve positions (green for normal, amber for disagreement)
    • HP valve status (green for commanded, amber for disagreement)
    • Pre-cooler inlet pressure (green for normal, amber for abnormal)
    • Bleed air temperature at pre-cooler outlet (green for normal, amber for low or overheat)

APU Bleed System

  • The APU supplies bleed air through its load compressor; the APU bleed valve acts as a shutoff.
  • The APU bleed valve opens if:
    • APU speed is above 95%
    • APU bleed push button switch is ON
  • With the switch OFF, the APU bleed valve is closed.
  • Fault light indicates an APU bleed leak; ECAM caution is displayed.
  • On the ECAM bleed page:
    • APU bleed valve is green when open
    • Crossline symbol means closed or not fully open; inline means fully open
    • Crossline amber indicates the valve is fully closed with the master switch ON and the bleed switch ON for more than 10 seconds
  • Conditions for APU bleed valve to open:
    • APU at normal speed
    • No leak detected
    • APU bleed push button ON

Bleed Monitoring Computers (BMC)

  • BMC1 and BMC2 each control and monitor one side of the bleed air system (left and right, respectively).
  • If one BMC fails, the other takes over most monitoring functions.
  • BMCs receive data on:
    • System bleed pressure and temperature
    • Valve positions
  • BMCs are connected to other systems needing bleed system information and to each other.
  • Both BMCs provide information and warnings to the ECAM.

Cross Bleed Valve Operation

  • The cross bleed valve allows the two sides of the pneumatic system to be isolated or interconnected via the cross bleed duct.
  • Normally closed when engines are running, keeping the two sides isolated.
  • Cross bleed selector switch (on overhead panel) has three positions:
    • AUTO: With APU running and APU bleed ON, both APU bleed and cross bleed valves open, engine bleed valves close. If APU bleed is OFF, APU bleed and cross bleed valves close, engine bleed valves open if engines are running and bleed switches are ON.
    • SHUT: Closes the cross bleed valve.
    • OPEN: Opens the cross bleed valve.
  • ECAM bleed page:
    • Cross bleed valve shown in green when in commanded position (open or closed)
    • Amber if in transit or not in commanded position
    • Cross bleed duct not displayed when valve is closed; displayed in green when open

Leak Detection System

  • Leak detection loops are installed in:
    • Fuselage
    • Engine pylons
    • Wings
    • Along the APU duct
  • Single detection loops in engine pylons and APU duct; double loops in each wing.
  • Leak detection:
    • Two wing loops must detect a leak, or one loop detects a leak and the other is inoperative, to trigger a wing leak signal.
    • Pylon leak: Illuminates engine bleed push button fault light, closes associated engine bleed and cross bleed valves automatically (cross bleed valve remains open during engine start).
    • Wing leak: Illuminates engine bleed push button fault light, closes associated engine bleed and cross bleed valves, and closes APU bleed valve if leak is in left wing (except during engine start).
    • APU duct leak: Illuminates APU bleed push button fault light, closes APU bleed and cross bleed valves if open (except during engine start).

Start Procedures

  • Cockpit check before engine start:
    • Ensure engine 1 and engine 2 bleed push button lights are extinguished (switches ON)
    • APU bleed push button light illuminated if APU is supplying pneumatics
    • Cross bleed selector switch in AUTO
  • APU is the normal pneumatic source for engine starting:
    • Select APU bleed ON to open APU bleed valve and supply pneumatic pressure to engines
    • With APU running and APU bleed ON, blue ON light illuminates if no leaks detected
    • APU bleed valve opens (shown on ECAM), cross bleed valve opens, pneumatic pressure indicated at engines with engine bleed valves closed
  • If APU is unserviceable:
    • Start engine 2 first using external ground pneumatic source (external air connector is on left side, near engine 1 danger area)
    • Green ground air symbol always displayed on ECAM bleed page
    • With external air connected and supplying pressure, ECAM shows left cross bleed duct and pressure at engine 1; engine bleed switches OFF
    • Open cross bleed valve to supply pneumatic pressure to engine 2 for starting
  • After engine 2 is started with ground pneumatics:
    • Start engine 1 using cross bleed start (after ground equipment is removed)
    • Engine 2 bleed ON to pressurize system, engine 1 bleed OFF to prevent reverse flow
    • Open cross bleed valve to supply pneumatic pressure to engine 1
  • With both engines running, engine bleed switches ON, cross bleed selector in AUTO, and APU shut down, the system returns to normal engine supply configuration.

ECAM Messages & Actions

  • Engine 1 bleed fault:
    • Triggered if bleed pressure or temperature exceeds limits
    • Single chime, master caution, ECAM bleed page, and engine bleed fault light displayed
    • Engine bleed valve and HP valve close automatically
    • ECAM action: Open cross bleed selector switch to allow both packs to run from one engine bleed
    • If wing anti-ice is required, affected pack must be switched off
  • APU bleed fault:
    • Displayed if APU bleed valve is not in commanded position with APU running and APU bleed ON
    • Crew awareness only; no ECAM action required
  • Left wing leak:
    • Detected by left wing detection loop
    • Related engine bleed valve, APU bleed valve, and cross bleed valve close automatically (except during engine start)
    • If wing anti-ice is ON, it must be turned off and icing conditions avoided
  • APU bleed leak:
    • Detected by APU leak detection loop
    • APU bleed and cross bleed valves close automatically (except during engine start)
    • ECAM displays "air APU bleed leak"; no crew action required unless detected during engine start, in which case ECAM will request APU bleed OFF

Key Terms & Definitions

  • Pneumatic system: Supplies high-pressure air for various aircraft systems.
  • Bleed air: High-pressure air taken from engine or APU compressor stages.
  • BMC (Bleed Monitoring Computer): Monitors and controls bleed air systems.
  • ECAM: Electronic system for monitoring and displaying aircraft alerts and system status.
  • Pre-cooler: Device that cools engine bleed air before distribution.
  • Cross bleed valve: Valve that connects or isolates left and right pneumatic systems.

Action Items / Next Steps

  • Review ECAM bleed pages and practice identifying valve positions, system status, and warnings.
  • Study engine start procedures using both APU and external air sources.
  • Read supplementary procedures for external air and cross bleed engine starts.
  • Familiarize yourself with ECAM messages and required crew actions for pneumatic system faults and leaks.

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