A320 APU Explained | Auxiliary Power Unit Start, Bleed & Operations

Q: How do you start the A320 APU?

APU start on A320: APU MASTER switch ON (starts sequence), APU START pushbutton ON (initiates start). ECAM displays APU page automatically. Monitor N% (APU speed), APU AVAIL light illuminates when APU reaches ~95% N and stabilizes (typically 60-90 seconds). APU bleed automatically available. Can use APU on ground and in flight up to FL 250 (start) or FL 410 (already running).

Q: What are APU limitations on A320?

Key APU limitations: Maximum altitude for APU start is FL 250. APU can run up to FL 410 if already started. APU bleed air available on ground and flight. Electrical load shedding required if APU supplying both electrical and bleed simultaneously on ground. Do not use APU bleed during takeoff or landing. APU automatic shutdown if fire detected or critical parameters exceeded.

The APU (Auxiliary Power Unit) is a small gas turbine engine located in the tail cone of the A320. It provides electrical power and pneumatic bleed air independently of the main engines. The APU enables ground operations without external power, engine starts, in-flight electrical backup, and emergency bleed air supply. Understanding APU start sequence, limitations, and ECAM indications is essential for A320 type rating.

APU Overview

Location

Tail cone (aft fuselage), accessible via external service panel. Exhaust exits at aircraft tail.

Power Output

Electrical: 90 kVA (115V 400Hz AC). Bleed Air: Sufficient for air conditioning and one engine start.

Fuel Source

Feeds from left wing tank via dedicated APU fuel pump. Independent from engine fuel system.

Operation Range

Ground and flight up to FL 410 (if already running). Start possible up to FL 250.

APU Functions

Electrical Generation: Powers aircraft electrical systems when engines off or as backup in flight
Bleed Air Supply: Provides pneumatic air for air conditioning, pressurization, and engine start
Independence: Operates independently from main engines, external power, or ground equipment
Emergency Backup: Can provide electrical power and bleed air in flight if needed

APU Start Sequence

APU start is automatic after pilot initiates the sequence. Typical start time is 60-90 seconds.

Start Procedure

APU MASTER switch ON: Arms APU system, opens fuel valve, energizes APU ECU (Electronic Control Unit)
APU START pb ON: Initiates start sequence (automatically springs back to OFF position)
APU page displays: ECAM automatically shows APU page with N%, EGT, APU AVAIL
Starter engages: Electric starter motor spins APU
Ignition and fuel: At preset N%, ignition activates and fuel introduced automatically
Light-up and acceleration: APU ignites, EGT rises, N% increases
Starter cutout: Starter automatically disengages around 55% N
Self-sustaining operation: APU continues to accelerate to 100% N
APU AVAIL: Green light illuminates when APU reaches ~95% N and stabilizes (approximately 60-90 seconds after start initiation)

✅ APU AVAIL Light

APU AVAIL green light indicates APU is ready to supply electrical power and bleed air:
• N% stabilized around 100%
• Parameters within limits (EGT, oil pressure, etc.)
• APU generator available for electrical bus connection
• APU bleed automatically available

APU AVAIL is the "ready to use" indication - without it, APU cannot supply aircraft systems even if running.

ECAM APU Page Parameters

Parameter	Description	Normal Range
N%	APU speed (rotational speed)	0% (off) to 100% (running), stabilizes ~100% when AVAIL
EGT	Exhaust Gas Temperature	Rises during start (up to ~650°C), stabilizes lower when running
FLAP OPEN	APU air intake flap status	Opens when APU MASTER ON, closes when APU shutdown complete
APU BLEED	Bleed air valve status	Automatically opens when APU AVAIL (can be closed manually)

APU Electrical Supply

APU generator provides 115V 400Hz AC power, rated at 90 kVA.

APU Generator Connection

Automatic Connection (on ground): If APU AVAIL and no other power source, APU generator automatically powers electrical buses
Manual Connection: APU GEN pushbutton controls APU generator connection to buses
Priority: External power has priority over APU power when both available
In-Flight Backup: APU can be started in flight to provide electrical backup if generator(s) fail

⚠️ APU Electrical Load Limitations

On ground, if APU supplies BOTH electrical power AND bleed air simultaneously, electrical load shedding occurs automatically:
• Some galley electrical loads shed
• Some non-essential electrical systems temporarily unavailable
• Normal - APU has limited capacity, prioritizes bleed for engine start and A/C

Load shedding does NOT occur in flight (bleed typically not used in flight).

APU Bleed Air Supply

APU provides pneumatic bleed air for air conditioning, pressurization, and engine start.

APU Bleed Operation

Automatic Opening: APU BLEED valve automatically opens when APU AVAIL
Manual Control: APU BLEED pushbutton on overhead panel allows manual close if needed
Engine Start: APU bleed commonly used for engine start on ground (one engine at a time)
Air Conditioning: APU bleed supplies pneumatic air to A/C packs when engines off
In-Flight Use: APU bleed available in flight as emergency pneumatic source (e.g., after dual engine bleed failure)

APU Bleed vs Engine Bleed Priority

Phase	Primary Bleed Source	APU Bleed Status
Ground, engines off	APU (or external air)	ON for A/C and engine start
Ground, engines running	Engine bleed	Can be OFF (APU may continue running for electrical only)
Takeoff/Landing	Engine bleed	OFF (APU bleed not used during takeoff/landing)
Flight (normal)	Engine bleed	OFF (APU typically not running in flight)
Flight (emergency bleed)	APU bleed if available	ON if needed for emergency pneumatic supply

🚨 APU Bleed During Takeoff/Landing

APU bleed must be OFF during takeoff and landing:
• Risk of APU failure during critical phase
• APU fire during takeoff/landing extremely hazardous
• Engine bleed provides adequate pneumatic power
• Company procedures typically require APU off (master OFF) before takeoff

Some airlines allow APU running for electrical backup during takeoff, but APU BLEED must be OFF.

APU Shutdown

Normal APU shutdown requires only APU MASTER switch OFF.

Normal Shutdown Procedure

APU MASTER switch OFF: Initiates automatic shutdown sequence
APU AVAIL extinguishes: APU no longer available for electrical or bleed
Cooldown period: APU continues running for cooldown (typically 60-120 seconds)
Fuel shutoff: APU fuel valve closes automatically
N% decreases to zero: APU spools down and stops
APU FLAP closes: Air intake flap closes when APU completely stopped

❄️ APU Cooldown

APU automatically runs for cooldown after MASTER switch OFF to prevent thermal shock and ensure proper bearing lubrication. Cooldown time varies based on APU operating duration and EGT at shutdown. Do not attempt to interrupt cooldown - allow APU to shut down automatically. Flap closing indicates shutdown complete.

APU Limitations

Altitude Limitations

Operation	Maximum Altitude	Notes
APU Start	FL 250	Cannot start APU above FL 250
APU Operation	FL 410	APU can run up to FL 410 if already running before climb
APU Bleed	FL 200	APU bleed air available up to FL 200 only (electrical up to FL 410)

Operational Limitations

Bleed during takeoff/landing: APU BLEED must be OFF
Single engine start: APU bleed sufficient for ONE engine start at a time only
Electrical load shedding: Occurs on ground when both electrical and bleed loads high
Maximum EGT: Automatic protection limits, varies by APU model (typically ~680°C)
Fire detection: APU fire triggers automatic shutdown

APU Fire Protection

APU has dedicated fire detection and extinguishing system.

APU Fire Detection

APU fire detector loop in APU compartment
If fire detected: APU FIRE warning, ECAM alerts, master warning
Automatic APU Shutdown: APU automatically shuts down if fire detected ON GROUND
Manual Shutdown Required: In flight, pilot must manually shut down APU via ECAM procedure

APU Fire Extinguishing

Fire Warning: APU FIRE light on overhead, ECAM alert
APU MASTER OFF: Immediately turn APU MASTER switch OFF (if not already automatic on ground)
APU FIRE pushbutton: Push APU FIRE pb (red, guarded) - isolates APU, arms fire extinguisher
Agent discharge: Press AGENT discharge button to extinguish fire (one shot only)
Monitor: Confirm fire extinguished, if fire persists no additional agent available

🔥 APU Fire On Ground vs In Flight

On Ground: APU automatically shuts down if fire detected. This protects aircraft and allows immediate evacuation if needed. Crew confirms APU shutdown and uses fire extinguisher if fire persists.

In Flight: APU does NOT automatically shut down (in case crew needs APU electrical power in emergency). Crew must manually shut down APU via ECAM procedure. APU FIRE pushbutton and agent available for fire suppression.

APU In-Flight Start

APU can be started in flight for electrical backup or emergency pneumatic supply.

In-Flight Start Considerations

Maximum start altitude: FL 250
Electrical backup: If generator(s) fail, APU provides backup electrical power
Pneumatic backup: If engine bleed fails, APU bleed available up to FL 200
Start time: Same as ground (60-90 seconds to APU AVAIL)
Battery power required: Sufficient battery power needed to start APU if all generators failed

Frequently Asked Questions

What is the APU on A320?

The APU (Auxiliary Power Unit) is a small gas turbine engine located in the tail cone of the A320. It provides electrical power (90 kVA, 115V 400Hz AC) and pneumatic bleed air when main engines are not running. APU operates independently, powered by fuel from left wing tank via dedicated APU fuel pump. Used on ground for engine starts, air conditioning, and electrical power without external equipment. Can also operate in flight up to FL 410 (start up to FL 250) as electrical or pneumatic backup. APU enables aircraft independence from ground support equipment.

How do you start the A320 APU?

APU start procedure: 1) APU MASTER switch ON (arms system, opens fuel valve), 2) APU START pushbutton ON (initiates automatic start sequence, springs back to OFF). ECAM displays APU page showing N% and EGT. Starter engages, ignition and fuel introduced automatically, APU lights up and accelerates. Starter cuts out around 55% N, APU continues to 100% N. APU AVAIL green light illuminates when APU stabilizes around 95-100% N (typically 60-90 seconds after start). APU then ready to supply electrical power and bleed air. APU bleed automatically available when AVAIL light on.

What are APU limitations on A320?

Key APU limitations: Maximum altitude for APU START is FL 250 (cannot start above). APU can RUN up to FL 410 if already started before climb. APU BLEED air available only up to FL 200 (electrical to FL 410). Do NOT use APU bleed during takeoff or landing (fire risk in critical phase). APU bleed sufficient for ONE engine start at a time, not simultaneous starts. On ground with both electrical and bleed loads, automatic load shedding occurs (galley power reduced). APU fire on ground triggers automatic shutdown. In flight APU fire requires manual shutdown. Maximum continuous EGT ~680°C (automatic protection).

Can you use APU bleed for both engines simultaneously?

No, APU bleed capacity is sufficient for ONE engine start at a time only. Attempting simultaneous engine starts using APU bleed may result in insufficient air pressure to either engine, potentially causing hung start or hot start. Correct procedure: Start engine 1 using APU bleed, wait for engine 1 to stabilize at idle, then start engine 2 using either APU bleed or engine 1 cross-bleed. Once engine 1 running, its bleed can assist (cross-bleed start) which is often preferred. For air conditioning with engines off, APU bleed can supply both packs simultaneously (different from engine start requirement).

What happens if APU fails to start?

If APU fails to start (no N% increase, no EGT rise, or abort during start): 1) APU MASTER switch OFF, wait for cooldown completion (APU flap closes). 2) Identify cause: check fuel quantity in left tank, check for ECAM messages, check APU fault history. 3) If safe to retry, attempt second start after cooldown. 4) If second start fails, APU unavailable - use external power and external pneumatic for engine start (ground cart or air start unit). 5) Report to maintenance. Common causes: low battery voltage, APU fuel pump failure, APU starter motor failure, APU fuel valve stuck, APU fire detection fault. In flight, failed APU start means no APU backup available.

Why does APU have automatic cooldown after shutdown?

APU automatic cooldown after MASTER switch OFF prevents thermal shock and ensures proper bearing lubrication. When APU shuts down, internal temperatures very high. Immediate fuel cutoff without cooldown can cause: thermal stress on hot components (turbine blades, bearings), inadequate bearing lubrication during coast-down, potential for warping or cracking. Cooldown allows APU to run at reduced power briefly (60-120 seconds) to stabilize temperatures, maintain oil flow to bearings, allow gradual component cooling. Cooldown time varies based on how long APU ran and EGT at shutdown. APU flap closes when cooldown complete, indicating safe shutdown. Never interrupt cooldown by forcing APU MASTER back ON during cooldown.

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A320 APU System (DSC-49)