Aircraft Electrical System

Description

• The basic element of the APU is a single shaft gas turbine, which delivers power for driving the accessory gearbox (electrical generator) and bleed air (engine starting and pneumatic supply).
• APU has an integral independent lubrication system for lubrication and cooling
• An Electronic Control Box (ECB) is a full authority digital electronic controller that performs APU system logic for all modes of operation, such as sequence/monitoring of start, speed/temperature monitoring, and bleed air monitoring/shutdown.
• ECB also controls fuel flow

APU Start

• To start the APU, BAT 1 and 2 push buttons must be selected to ON, unless external power is available.
• Alternatively, if external power is available, BAT 1 and 2 push buttons do not need to be selected ON, and should remain in the lights out position
• With battery power only, an APU fire test will have the APU FIRE pushbutton, SQUIB, and DISCH lights illuminated
• The APU can obtain power for starting from ground services, normal aircraft supply, aircraft batteries, or a combination with external power
• Before start, APU MASTER SW selection initiates an automatic computer self-test, air intake flap opening, and fuel pressure supply.
• Starter engages if the air intake is closed and the MASTER SW and START pushbuttons are ON
• Maximum altitude for APU start using batteries only is 25,000 ft
• Maximum altitude for normal APU operation is 39,000 feet
• If external power and the APU START pushbutton AVAIL lights are illuminated with engines off, external power is the source for aircraft buses

Indications

• ECAM displays the APU system page when selected by the crew or automatically when the APU is started and AC power is available.
• An amber FUEL LO PR indication appears during APU start if low fuel pressure is detected.
• A FLAP OPEN indication displays in amber when the APU air inlet flaps are fully open; the indication is normally displayed in green

APU Shutdown

• Normal APU shutdown after the APU Master Switch is released occurs without delay in all cases
• A delay occurs if bleed air was in use
• If APU bleed was used, after a manual shut down sequence, the APU keeps running for 60 seconds
• On the ground, the No Break Power Transfer function is inhibited if APU shutdown is triggered by the ECB, the REFUEL/DEFUEL panel, the APU FIRE pushbutton, or emergency control on the external power panel
• APU cannot be shut down from outside the aircraft via the APU SHUT OFF pushbutton on External Power Panel

APU Bleed

• APU bleed valve closes at 29,000 feet descending, automatically by the ECB
• Maximum Altitude for APU bleed is 22,000 ft
• APU can be used with wing anti-ice on but only for electrical power; avoid opening the APU bleed valve while using wing anti-ice
• It is possible to use simultaneously packs and Lp ground unit during long stops in a hot airfield if the external temperature is greater than 50 deg.C.

APU Fire Protection

• APU MASTER SW pushbutton FAULT light comes on for fire on ground or in flight
• If there is an APU FIRE and nobody is in the cockpit, an automatic APU EMER (emergency) shutdown occurs and the fire bottle discharges automatically on the ground, but in the air, an APU EMER shut down will occur and the APU fire bottle will be discharged automatically.
• Fire protection is available if APU start is initiated on battery power only
• During an APU fire test, the automatic shutdown of the APU on the ground will not occur while the flight crew is performing this test

APU Fuel

• APU is supplied from the left or right fuel feed line
• fuel pump runs all the time when the APU is running

Bleed Air and Packs when APU is running

• In A319/320/321 when APU is supplying Bleed air, with the pack flow selector to low, the packs are high and A321 is Normal.
• HI flow is automatically selected regardless of PACK FLOW selector position because air is only being supplied by the APU.

Air Conditioning/Pressurization/Ventilation

• Conditioned air is distributed to the cockpit, avionics bay, and cabin as well as holds 1 and 2 only.
• The crew should select RAM AIR ON for dual pack failure or smoke removal
• Normal maximum cabin altitude is 9,550 ft +/- 350 ft
• Maximum negative differential pressure for the cabin is 1 psi
• The different sources of air for air conditioning and pressurization are engine bleed air and recirculated air, or if selected, APU bleed air and recirculated air
• During exterior preflight on a warm day, expect to find the avionics ventilation system INLET and EXTRACT valves open
• During engine start, pack flow control valves automatically close
• Each aircraft zone's temperature is optimized by a TRIM AIR valve
• Normal pressurization occurs after rotation
• Normal depressurization occurs on landing touchdown
• Pack flow controller should be set to LO with a low passenger load to reduce bleed air demand and improve fuel efficiency
• Pressurization controllers receive inputs from the LGCIU, ADIRU, FMGS, and EIU
• In flight below 12,500ft, the ram air valve can provide an emergency source of conditioned air during non-pressurized flight.

Controls and Indications

• Bleed air supplied from the APU, pack flow is automatically selected to normal flow

Overheat

• Hot air "FAULT" light illuminates on the air conditioning panel if the hot air press reg. valve closes and the trim air valves open or in closed position.
• Placing the avionics ventilation system in the smoke configuration de-energizes the blower fan, allows the extract fan to run, and opens the air conditioning extract valves
• Cabin temperature will be controlled at the upper limit of 30 deg.C
• The outflow valve is fully released on the ground
• 120 % of normal air during engine starting
• When APU is supplying the packs, the pack controller sends a demand signal to increase airflow on the flow in Pack control valve.
• During landing is 70 kts (after 20 seconds delay) by hot