Airbus A320: Auxiliary Power Unit (APU)

Airbus A320 - Auxiliary Power Unit (APU)

• To initiate the APU, both BAT 1 and 2 push buttons must be selected to ON or selecting both BAT 1 and 2 is not required if external power is available.
• The Electronic Control Box (ECB) acts as a full authority digital electronic controller executing the APU system logic across all operational modes, including:
  • Start sequence and monitoring
  • Speed and temperature monitoring
  • Bleed air monitoring and shutdown procedures
• Normal APU shutdown, upon releasing the APU Master Switch happens with a delay if bleed air was being used; otherwise, it occurs without delay in all cases.
• Activating the APU START push button will open the air intake flap and supply fuel pressure and also starts the APU if the APU MASTER SW was previously selected ON.
• The APU possesses a dedicated, integral lubrication system for lubrication and cooling.
• The ECB (Electronic Control Box) manages fuel flow.
• When the APU MASTER SW is selected ON, the APU computer runs a self-test, the air intake flap opens, and fuel pressure is supplied to prepare the APU for starting.
• The APU MASTER SW FAULT light indicates:
  • Low oil pressure
  • Automatic shutdown
  • APU overheat or fire
  • Batteries selected OFF while the APU operates
• The APU can be started at:
  • Any altitude when using normal aircraft electrical power
  • Up to 39,000 feet using only the aircraft batteries
• The APU system page is displayed on the ECAM either when selected by the crew or automatically upon APU start and AC power availability.
• After a manual shutdown following APU bleed usage, the APU continues running for 60 seconds.
• With engines off, if the AVAIL light on both APU START and EXT PWR push buttons is lit, electrical power for aircraft busses is from external power.
• BAT 1 and 2 push buttons might not need to be selected ON if external power is available and are normally left in the lights out position and the APU generator is de-energized when selected OFF.
• The APU has a maximum start altitude of 39,000 ft when using only batteries.
• The APU fuel pump operates continuously while the APU is running.
• The APU BLEED valve automatically closes above 29,000 feet during descent, controlled by the ECB.
• In an APU FIRE situation with no cockpit occupants, an APU EMER shutdown occurs on the ground, and the fire bottle discharges automatically and also in the air, an APU EMER shut down occurs and the APU fire bottle is discharged automatically.
• The APU consists of a single shaft gas turbine that provides power to drive the accessory gearbox (electrical generator) and produces bleed air for engine starting and pneumatic supply.
• An APU MASTER SW pushbutton FAULT light comes on during a fire on the ground or in flight.
• With battery power, an APU fire test shows the APU FIRE pushbutton illuminated along with the SQUIB and DISCH lights.
• The starter engages if the air intake is closed and both the MASTER SW and START pushbuttons are ON.
• An amber FLAP OPEN indication appears when APU air inlet flaps are fully open.
• APU may obtain starting power from:
  • Ground services
  • Normal aircraft supply
  • Aircraft batteries, alone or with external power
• An APU SHUT OFF pushbutton on the External Power Panel shuts down the APU from outside the aircraft.
• Causes for an APU automatic shutdown, especially on the ground, may include:
  • Fire
  • EGT overtemperature
  • Underspeed/overspeed
  • Reverse flow
  • Low oil pressure
  • DC power loss
  • No acceleration
  • High oil temperature
  • ECB failure
  • Slow start
  • Loss of overspeed protection
• The maximum altitude for APU bleed operation is 15,000 ft.
• The APU FIRE test will not cause automatic shutdown of the APU when performed on the ground by flight crew.
• The APU is supplied fuel from either the left or right fuel feed line
• When on the ground, the No Break Power Transfer function becomes inhibited if an APU shutdown is initiated by one of the following:
  • Automatic APU shutdown triggered by the ECB;
  • APU shutdown from the REFUEL/DEFUEL panel as well as from the APU FIRE pushbutton
  • APU shutdown from emergency control at the external power panel
• The APU can be used with wing anti-ice only for electrical power; avoid opening the APU bleed valve during wing anti-ice use.
• The APU may be started on ground using only own aircraft batteries.
• When starting the APU, a FUEL LO PR indication appears amber if low APU fuel pressure is detected.
• The LOW OIL LEVEL advisory pulses amber if the APU oil quantity nears its minimum.
• In an A319/320/321 with APU supplying bleed air and the PACK FLOW selector set to LOW:
  • A319/320 will have HIGH pack flow
  • A321 will have NORMAL pack flow
• The Auxiliary Power Unit (APU) has a maximum operating altitude of 39,000 feet.
• Aircraft are equipped with a single AC operated and one DC operated APU fire extinguisher bottle.
• The APU bleed valve closes automatically during climb but does not reopen during descent.
• With battery power only, an APU fire test will illuminate the APU FIRE pb and SQUIB + DISCH lights

Airbus A320 - Air Conditioning/Pressurization/Ventilation

• Conditioned air is supplied to the cockpit, avionics bay, and cabin.
• The Hot AIR fault light on the air conditioning panel results in the hot air pressure regulating valve closing, and trim air valves opening. While the hot air press. reg. valve closes and the trim air valves open
• Trim air supplies warm air to the air conditioning system.
• In case of zone controller primary and secondary channel failure, Pack 1 maintains 24 deg.C, and Pack 2 maintains 15 deg.C.
• In normal flight with a closed circuit configuration, the avionics ventilation system maintains the cooling air temperature by passing air through a skin heat exchanger.
• To allow ram air into the mixture unit, use the Ram air switch at any time
• With primary channel failure, the secondary computer takes over but regulation isn't optimized.
• No effect results in loss of the pack regulation backup mode for a secondary channel failure.
• For primary and secondary channel failure, the anti-ice valve modulates pack outlet temperature between 5-30 deg.C.
• If the hot air pressure regulating valve fails open, temperature stays at the selected value with no optimized regulation.
• Engaging the APU bleed valve causes a normal pack flow selection.
• The trim air valve optimizes temperature by adding hot air.
• The hot air pressure regulating valve regulates the pressure of hot air tapped upstream of the packs.
• The pack flow control valve is pneumatically operated and electrically controlled
• If engine flow demand cannot meet heating or cooling needs in one zone, the APU must supply additional air.
• The normal maximum cabin altitude reaches 8,000 ft.
• The maximum negative differential pressure for the cabin is 0 psi.
• Packs and the Lp ground unit may be used simultaneously during long stops in hot airfields if airflow from the ground cart is less than 1.2 kg/s.
• Air conditioning and pressurization can utilize engine bleed air/recirculated air and if selected APU bleed air with recirculated air
• The avionics ventilation system INLET and EXTRACT valves are expected to be in the Open position during exterior preflight on warm days.
• Pack flow control valves automatically close during engine start.
• A trim air valve optimizes temperature in each aircraft zone.
• Normal pressurization occurs after rotation.
• Normal depressurization occurs on landing touchdown.
• The pack flow controller should be on LO with a low passenger load, reducing bleed air demand for fuel efficiency.
• The amber AFT ISOL VALVE fault light means that either the inlet or outlet isolation valve(s) disagrees in position.
• Pressurization controllers get inputs from LGCIU, ADIRU, FMGS, and EIU.
• Three zone controllers send information and instructions to two pack controllers for three zones to control the cabin and cockpit conditioned air.
• Selecting RAM AIR ON is advised for dual pack failure or smoke removal.
• Adjusting a temperature selector rotary knob sends a signal to the zone controller, requesting a different temperature.
• With both APU BLEED and engine BLEED switches ON with engines running, the engine bleed valves are closed.
• Pressurization is normally automatic but manual interference is possible using the CABIN PRESS MODE SEL to OVERRIDE and MAN V/S CTL toggle switch, can also manually set landing elevation using the LND ELEV AUTO selector.
• Setting the pack flow controller to HI is utilized in cold conditions to achieve a higher cabin temperature range.
• The ram air valve is used for emergency smoke removal, ventilation after dual pack failure, and cargo & avionics cooling.
• Activating the EXTRACT push button de-energizes the extract fan.
• The Ram air valve should be opened for increased ventilation on the ground.
• There is a limit to not open the ram air valve if cabin pressure is greater than 1 psi.
• When the ditching push button is selected on in the automatic pressurization system mode, all valves below the water line close.
• The maximum negative differential pressure for the cabin is 0 psi.
• The HOT AIR valve push button controls the trim air valve.
• Pressurization auto controllers are set by the active flight plan loaded in the MCDU and the QNH entry on the MCDU Approach Performance page refines the depressurization schedule for the landing.
• You can find aft cargo indications on the ECAM cruise page and the COND page.
• You can find avionics ventilation system indications on the in-flight ECAM cruise page and the CAB PRESS pages.
• With APU supplying the packs, the pack controller sends a signal to the Pack Outflow Control Valve to increase airflow when a zone temperature cannot be satisfied.
• In case of zone controller primary and secondary channel failure, Pack 1 maintains 24 deg.C and Pack 2 maintains 15 deg.c
• With the PACK FLOW controller set to NORM and ECAM showing PACK FLOW as high, HI flow is automatically selected regardless of PACK FLOW selector position because air is being supplied by the APU.
• Select PACK FLOW to HI for smoke removal and in hot/humid conditions.
• Recirculated air, conditioned air and hot trim air are mixed before distribution to each zone.
• The A320 has three rotary temperature selectors and three temperature zones for the:
  • Cockpit zone
  • Forward cabin zone
  • Aft cabin zone
• The temperature selectors are located in the cockpit and the cabin.
• During normal flight, the avionics ventilation system maintains the cooling air temperature by passing air through a skin heat exchanger.
• Placing the avionics ventilation system in the smoke configuration de-energizes the blower fan, extract fan runs, and opens the air conditioning extract valves.
• Cycle the CABIN PRESS MODE SEL pushbutton to the MAN position then back to AUTO, to change controllers during flight.
• If the hot air “FAULT” light illuminates, it indicates that the hot air pressure regulating valve closes and the trim air valves open.
• There are three trim air valves for the:
  • Cockpit zone
  • Forward cabin zone
  • Aft cabin zone
• In the event of a loss of the bleed system pressure, the pack valves remain in their last position.
• The RAM AIR switch should be used only when differential pressure is less than 1 psi.
• The Pack Flow selector allows the pilot to increase pack flow but will not allow a manual decrease in flow if needed by the aircraft demands and also allows high flow regardless of switch position when the APU is used for air conditioning.
• Avionics ventilation system is in the closed configuration while airborne under normal operations.
• The maximum altitude associated with the pressurization system is 39,100 feet
• Trim air valves are controlled by the zone controller.
• Only the aft cargo compartment is heated and ventilated that are mixed with hot trim air, and the air is drawn through isolation valves by extraction fans
• The APU BLEED FAULT can also mean that An APU leak is detected, Malfunction or overheat
• Aft cargo ventilation is controlled by the cargo ventilation controller along with the aft cargo rotary selector knob.
• The vent fan runs with normal ships power and open isolation valves.
• The cabin zone temperature sensors are ventilated by the air extracted by the lavatory and galley fans.
• Placing the BLOWER push button to override insures the blower fan will continue to run.
• Pressurization indications are on the CAB PRESS & BLEED pages.
• Two outflow valves exist having one main and one back-up with only 2 doors visible.
• An ECAM warning triggers upon reaching a cabin altitude of 9,550 feet.
• Positioning the pack flow control knob to HI causes 120% normal air flow.
• Pressing the PRESS page reflects the landing elevation value.
• The ENG BLEED fault light illuminates due to:
  • Overpressure (downstream of bleed valve)
  • Bleed overheat
  • Wing or engine leak on particular side
• A duct overheat is detected if the CARGO HEAT HOT AIR FAULT light indicates.
• During Ground function operation, the outflow valve is fully open.
• The outflow valve is powered by one of two electric motors.
• The purpose of the safety valve is to avoid excessive positive and negative pressure differential.
• When landing elevation is set to AUTO, landing elevation is sent from the FMGS to the controller.
• Packs, cabin air, emergency ram air inlet and Lp ground connector - all connected to the mixing unit.
• Once set to “ON” the air conditioning packs operate automatically and independently.
• Emergency ram air inlet, When set to “ON” the ram air valve will open and supply airflow provided ditching is not selected.
• Cond. Zone regulator fault (primary channel failed) secondary channel operates as backup normal
• Temperature control is automatic and is regulated by the zone controllers, pack 1 and 2 controllers
• If a pack controller fails (primary and secondary channel failure), the pack outlet air temperature is controlled by the Anti-ice valve
• The zone controller optimizes temperature by action on the Trim air valve
• When using APU bleed to supply the packs, with the pack flow selector at LO, the pack airflow is 120% of normal
• The pack flow control valve closes automatically in case of Pack overheat, engine starting, or operation of the fire or ditching push button
• In case of trim air system fault (zone controller primary channel failure), the secondary channel of the zone controller regulated the zone temperature at: 15 deg.C
• In case of total zone controller failure, Hot air and trim air valves close and packs deliver air at a fixed temperature (20 deg.C pack 1 and 10 deg.pack2)
• During landing run, Ram Air Inlet flaps open when speed is less than 77 kts (after 30 seconds delay
• The ditching switch when selected sends a closure signal to all of the following:
  • Outflow valve
  • Ram air inlet and ventilation extract valves
  • The pack flow control valves
• In flight with pressure controller 1 in use, if it fails: Transfers automatically to controller 2
• In normal operation, pressurization is fully automatic
• The safety valves are operated Electrically