The IDG combines a Constant Speed Drive (CSD) and a generator in a single unit. The CSD uses a hydraulic transmission to maintain constant generator speed (6000-8000 RPM) regardless of engine speed variations.

Key Features:

• Outputs 90 kVA or 120 kVA (typical)
• Maintains ±0.5 Hz frequency stability
• Oil-cooled system with separate reservoir
• Cannot be reset in flight once disconnected

The GCU monitors and protects the generator system:

• Voltage Regulation: Maintains 115V ±3V
• Frequency Control: Keeps output at 400 Hz ±5 Hz
• Load Sharing: Balances load between generators (within 10%)
• Overload Protection: Trips generator if load exceeds 150% rated capacity
• Differential Protection: Detects internal generator faults

DC power at 28V is supplied by:

• Transformer Rectifier Units (TRUs): Convert 115V AC to 28V DC
• Batteries: Backup power and APU start capability
• External Power: Ground operations

Maintains battery charge and provides 28V DC to essential buses. Typical charging rate is C/10 (10-hour charge rate). Overcharge protection prevents exceeding 30.8V for NiCd batteries.

BTCs allow connection between buses for load sharing or backup power. Logic prevents paralleling of out-of-phase generators. Automatic load shedding occurs if single generator powers entire system.

RAT Performance:

• Powers essential AC and DC buses
• Provides hydraulic pressure (some aircraft)
• Output increases with airspeed
• Cannot be retracted once deployed
• Minimum operating speed: 130 knots (typical)

Converts 28V DC to 115V AC at 400 Hz. Used to power essential AC bus when generators fail. Capacity typically 1 kVA. Powers standby instruments and essential avionics.

External Power Unit (EPU) Specifications:

• Voltage: 115/200V AC at 400 Hz
• Also provides 28V DC
• Interlocked with generators - cannot parallel
• Voltage must be within ±5% of nominal
• Frequency tolerance: ±5 Hz

Skydrol (Phosphate-Ester Base):

• Most common in modern aircraft
• Fire resistant
• Operating range: -40°C to +135°C
• Purple/red color
• Attacks paint, plastics, human skin
• Specific gravity: 1.04-1.06

Operating Pressure: Typical systems operate at 3000 PSI (207 bar). Some modern aircraft use 5000 PSI systems for reduced weight.

• Type: Variable displacement axial piston pump
• Driven from engine accessory gearbox
• Output: 20-30 GPM at 3000 PSI
• Swashplate angle varies to maintain constant pressure
• Automatic compensation for load changes
• Case drain returns leakage fluid to reservoir

Provide backup and ground operation capability:

• Power: 115V AC 400 Hz
• Flow rate: 8-15 GPM typical
• Used for: Engine start, ground ops, backup in flight
• Lower capacity than EDPs
• Thermal protection prevents overheating

Gas-charged accumulators serve multiple functions:

• Pressure Storage: Provides instant pressure for system needs
• Shock Absorption: Dampens pressure surges
• Thermal Expansion: Compensates for fluid expansion
• Emergency Power: Multiple brake applications without pump

Precharged with nitrogen to 1000-1500 PSI. System pressure compresses gas to 3000 PSI, storing energy.

Power Transfer Unit (PTU):

The PTU is a hydraulic motor-pump combination that allows one system to power another. Typical configuration: System A can power System B loads through PTU.

• Automatically activates on pressure differential > 500 PSI
• Characteristic barking sound during operation
• Used primarily for landing gear operation if one system fails
• Does not transfer fluid - only mechanical energy

Normal operating pressure: 3000 PSI

Low pressure warning: 2200-2400 PSI

Overpressure relief: 3600-3800 PSI

Reservoir quantity displayed as percentage or level. Low level warnings typically at:

• Caution: 40% remaining
• Warning: 20% remaining
• Minimum for operation: 10-15%

Operation:

• Spring-loaded piston mechanism
• Activates on excessive flow rate (indicating rupture)
• Closes within 0.1 seconds
• Limits fluid loss to 1-2 quarts
• Located at actuator inlets
• Cannot be reset in flight

Pressure Regulating Valve (PRV):

• Reduces bleed air to 35-45 PSI
• Maintains constant pressure regardless of engine power
• Protects downstream components from over-pressure
• Pneumatically operated with electronic control

Precooler (Heat Exchanger):

• Cools bleed air from 250°C to 150-200°C
• Uses fan air for cooling
• Essential for downstream component protection
• Over-temp protection closes bleed valve at 290°C

Central distribution system connecting all bleed sources. Includes crossbleed capability allowing:

• Single engine operation of all systems
• APU bleed to supply both sides
• Ground cart connection
• Engine cross-start capability

• Relief valve opens at 75-80 PSI
• Prevents downstream component damage
• Vents overboard through dedicated duct
• Automatic reset when pressure normalizes

Ground Start (APU Bleed):

1. APU started and supplying bleed air
2. Start valve opened - air to starter
3. Engine accelerates to 20% N2
4. Fuel introduced and ignition activated
5. Light-off at 20-25% N2
6. Self-sustaining at 50-56% N2
7. Start valve closes automatically
8. Idle speed: 60-65% N2

Starting second engine using first engine's bleed air:

• Engine 1 at stable idle
• Crossbleed valve opened
• Engine 2 start initiated
• Engine 1 automatically increases to 70% N1
• Provides adequate bleed air pressure
• Start time slightly longer than APU start

If engine shutdown in flight, restart procedure:

• Minimum windmill speed: 15% N2
• Optimum altitude: FL200-FL250
• Maximum altitude: FL300-FL350 (aircraft dependent)
• Windmilling provides sufficient N2 for fuel ignition
• No starter required if adequate windmill speed
• APU provides bleed if windmill insufficient

Primary fuel storage in integral wing structure:

• Left and right main tanks
• Surge tanks at wingtips (expansion space)
• Vent system prevents pressure buildup
• Sealed with sealant, not separate bladders
• Structure serves as tank walls

Located in fuselage center section:

• Used for additional capacity on longer flights
• Fuel used first to unload wing structure
• Contains 2 boost pumps
• Automatic pump shutoff when tank empty
• May have fuel temperature limitations (heated area)

AC Electric Boost Pumps:

• Submerged in fuel tank
• Pressure: 30-50 PSI
• Flow rate: 700-900 GPH each
• Provide positive pressure to engine fuel pump
• Prevent vapor lock at altitude
• Cooled by surrounding fuel

Engine-Driven Fuel Pump:

• Gear-type positive displacement pump
• Mounted on engine accessory gearbox
• Final pressure boost to 300-700 PSI
• Supplies fuel to FCU (Fuel Control Unit)
• Can operate with failed electric pumps
• Suction feed capability up to FL250-FL300

Fuel/oil heat exchangers prevent fuel freeze:

• Hot engine oil heats cold fuel
• Also cools engine oil
• Maintains fuel above -40°C (Jet A)
• No pilot control - automatic operation
• Critical for preventing fuel system icing

Allows any engine to receive fuel from any tank:

• Cross-feed valve connects fuel manifolds
• Used for: Engine failure, fuel imbalance, single engine taxi
• Can feed both engines from one tank
• Normal position: CLOSED (each engine feeds from its tank)

Capacitance-Type Probes:

• Multiple probes in each tank
• Measure fuel dielectric constant
• Compensate for fuel density variations
• Display in pounds or kilograms
• Accuracy: ±1% typical

Totalizer: Calculates total fuel remaining by summing all tank quantities. Includes:

• Fuel used indication
• Fuel flow rate per engine
• Range remaining calculation
• Predicted fuel at destination

• Removes particles down to 10-25 microns
• Bypass valve opens if filter clogs
• Impending bypass indication alerts crew
• Water separator included in filter assembly
• Filters must be drained regularly

Maintains atmospheric pressure in tanks:

• Prevents tank collapse as fuel consumed
• Allows thermal expansion
• Surge tanks capture overflow fuel
• NACA vents prevent icing
• Flame arrestors prevent external ignition

Standard method for large aircraft:

• Single point connection under fuselage
• Refueling rate: 600-1000 GPM
• Pressure: 50-55 PSI
• Automated fuel distribution to tanks
• Preset quantity shutoff
• Full fuel load in 15-30 minutes

Gravity feed through tank filler caps:

• Backup method or small aircraft
• Much slower than pressure refuel
• Manual monitoring required
• Risk of overfill if not monitored

Removing fuel from aircraft tanks:

• Uses boost pumps in reverse
• Or dedicated defuel pump
• Rate: 300-500 GPM
• Required for heavy maintenance or CG issues
• Same safety precautions as refueling

Two-Chamber Design:

• Upper Chamber: Compressed nitrogen gas (1800-2200 PSI)
• Lower Chamber: Hydraulic fluid
• Compression forces fluid through orifice
• Energy dissipated as heat
• Nitrogen spring returns strut to extension
• Proper servicing: 2-4 inches exposed piston typical

Hydraulic System Operation:

• Gear selector lever commands extension/retraction
• Hydraulic pressure moves actuators
• Sequence valves ensure proper timing
• Doors open before gear moves
• Gear locks in position with mechanical over-center locks
• Extension time: 15-30 seconds
• Retraction time: 10-20 seconds

Protect gear wells and reduce drag:

• Open before gear extends/retracts
• Close after gear fully extended/retracted
• Some aircraft leave MLG doors open with gear down
• Hydraulically actuated
• Blown open by gear during emergency extension

Three Green Lights: Gear down and locked

No Lights: Gear up and locked

Red Light: Gear in transit or disagree

Position sensors (typically 3 per gear leg):

• Proximity switches detect lock position
• Redundant sensing for reliability
• All sensors must agree for green light

Most Common Method:

• Emergency gear handle pulled
• Releases hydraulic pressure from up side
• Mechanical uplock releases
• Gear falls under own weight
• Airflow and gravity assist extension
• Extension time: 30-90 seconds
• Airspeed required: 140-180 KIAS minimum

Some aircraft use stored hydraulic pressure:

• Accumulator charged to 3000 PSI
• Sufficient for 1-3 gear extensions
• More reliable than gravity alone
• Provides positive down-lock
• Independent of main hydraulic systems

Squat Switches (Weight-On-Wheels):

• Located on main gear struts
• Detect aircraft on ground vs. in air
• Control multiple systems:
  • Prevents gear retraction on ground
  • Enables ground spoiler deployment
  • Activates autobrakes
  • Inhibits TCAS/GPWS on ground
  • Changes flight control logic
• Redundant switches for reliability

Nose gear steering automatically disconnects:

• During takeoff at 60-80 knots
• Prevents excessive nose gear loads
• Rudder becomes primary directional control
• Reconnects during landing rollout
• Manual override available for ground ops

Prevents wheel lock-up during braking:

• Wheel speed sensors on each main wheel
• Compare wheel speeds continuously
• Reduce brake pressure if wheel decelerating too rapidly
• Optimizes braking efficiency
• Prevents tire blow-out from flat spots
• Inoperative below 20 knots

Hydraulic Multi-Disc Brakes:

• Multiple steel discs (rotors and stators)
• Hydraulic pressure forces discs together
• Typical pressure: 3000 PSI
• Heat generated: up to 500°C during rejected takeoff
• Brake temperature monitoring critical
• Cooling time required after heavy use

Backup system using different hydraulic source:

• Separate hydraulic system
• May have reduced effectiveness
• Anti-skid typically still available
• Autobrakes may be unavailable

Mechanically traps hydraulic pressure:

• Lever/button sets parking brake
• Isolates brake hydraulics
• Pressure maintained even with pumps off
• Will slowly release as fluid leaks internally
• Chocks required for long-term parking

Automatic brake application during landing or rejected takeoff:

Landing Settings:

• LOW: Gentle deceleration, long runways
• MEDIUM: Normal operations
• MAX: Short runway, contaminated surface
• Activated at main gear touchdown
• Deactivated by pilot brake application

Rejected Takeoff (RTO):

• Maximum brake pressure immediately
• Activates above 85 knots
• Requires throttle reduction to idle
• Cannot be overridden

Aircraft Tire Characteristics:

• Pressure: 180-220 PSI (much higher than car tires)
• Rated for specific loads and speeds
• Speed rating: up to 225 mph
• Tread depth minimum: 1/32 inch in grooves
• Inflation with nitrogen (not air)
• Life: 200-300 landings typical

Convert pilot input into hydraulic surface movement:

• Servo valve directs hydraulic pressure
• Main actuator moves control surface
• Feedback mechanism ensures precise control
• Multiple hydraulic systems for redundancy
• Force: 20,000-50,000 lbs per actuator

Each primary control surface typically powered by:

• System A hydraulic
• System B hydraulic
• System C hydraulic (backup)
• Loss of any two systems still allows control
• Degraded handling with system failures

Multiple Functions:

• Flight Spoilers: Assist ailerons in roll control
• Speed Brakes: Increase drag for descent/deceleration
• Ground Spoilers: Dump lift on landing, increase braking
• Typically 5-8 panels per wing
• Hydraulically actuated
• Deflection: up to 60°

Ground Spoiler Logic:

• Armed before landing
• Auto-deploy when:
  • Main wheels spin up (wheel speed sensors)
  • AND throttles at idle
  • OR reverse thrust selected
• Immediate lift dump on touchdown
• Transfers weight to wheels for braking
• Improves stopping distance by 30-40%

Adjusts entire horizontal stabilizer angle:

• Electric motor drives jackscrew
• Backup: alternate trim system
• Range: 0° to 13° nose-up typical
• Trim speed: 0.2°/second
• Autopilot controls for hands-off flight
• Position indicator in cockpit

Components:

• Sidestick or control column (with sensors)
• Multiple redundant flight control computers (3-7 computers)
• Electro-hydraulic actuators at control surfaces
• Multiple data buses for communication
• Voting logic resolves computer disagreement

Angle of Attack Protection:

• Maximum AoA: α-max
• Computer prevents exceeding limit
• Full aft stick commands α-max, not stall
• Automatic nose-down if approaching stall

Load Factor Protection:

• Positive limit: +2.5g (clean), +2.0g (config)
• Negative limit: -1.0g (clean), 0g (config)
• Prevents structural overstress

High Speed Protection:

• VMO/MMO protection
• Automatic nose-up at VMO+6 / MMO+0.01
• Cannot be overridden by pilot

Maximum Differential Pressure: 8.1-9.1 PSI

This is the difference between cabin pressure and outside pressure. Limits:

• Normal operation: up to 8.1 PSI
• Relief valve opens: 8.6-9.1 PSI
• Structure designed for: ~10 PSI (safety margin)

Cabin Pressure Controller:

• Automatically controls outflow valve position
• Inputs: destination field elevation, cruise altitude
• Controls climb/descent rates
• Typical rate: 300-500 ft/min cabin altitude change
• Provides smooth pressure changes for passenger comfort

Control cabin pressure by regulating air exhaust:

• Located in aft fuselage (typically 2 valves)
• Motor-driven for position control
• Automatic and manual modes
• Fully open: no pressurization
• Fully closed: maximum pressurization (limited by relief)

Manual valve to depressurize cabin:

• Used for: smoke/fumes elimination, emergency
• Opens outflow valves fully
• Cabin altitude equalizes with aircraft altitude
• Oxygen masks required above 10,000 ft

Three-Wheel Boot Cycle:

1. Compressor: Further compress bleed air
2. Heat Exchanger: Cool air using ram air
3. Turbine: Expand air (temperature drops significantly)
4. Water Separator: Remove condensed moisture

Output air: 5-15°C, mixed with hot bleed air for final temperature control

Typical transport aircraft: 2 packs (left and right)

• Each pack can condition entire cabin (reduced capacity)
• Pack flow: AUTO, HIGH, LOW
• AUTO: Varies based on demand
• HIGH: Maximum cooling/heating
• LOW: Reduced flow (engine performance)

Zone temperature control:

• Cockpit zone: independent control
• Forward cabin zone
• Aft cabin zone
• Cargo compartments (if heated)
• Trim air valves add hot bleed air to reach desired temp
• Range: 18-30°C typical

Cabin Airflow Pattern:

• Fresh air enters from overhead
• Flows downward along cabin walls
• Exits at floor level
• Minimizes mixing between rows
• Improves air quality and comfort

Air Composition:

• 50% fresh air from packs
• 50% recirculated cabin air
• Recirculated air filtered (HEPA filters typical)
• Filters remove 99.97% of particles
• Complete air change every 2-3 minutes

Gasper Outlets:

• Individual passenger air nozzles
• Supply from recirculation system
• Temperature not individually controlled

Equipment Cooling:

• Avionics cooling fan
• Draws cabin air through electronic bays
• Exhausts overboard
• Smoke detection in avionics bay

Cargo compartments:

• Forward: heated and ventilated (pets, perishables)
• Aft: may be heated and ventilated
• Bulk: typically not heated
• Temperature: 7-24°C
• Smoke detectors in all compartments
• Fire suppression in Class C/E holds

System Operation:

• Hot bleed air (200-250°C) directed to inlet cowl
• Heats leading edge structure
• Each engine independently controlled
• Thrust loss: 3-5% when operating
• Mandatory in icing conditions

Takeoff Considerations:

• Engine anti-ice: ON for takeoff in icing conditions
• Performance penalty considered in calculations
• Improved takeoff safety outweighs thrust loss
• N1 indication may be slightly lower with anti-ice on

Hot Bleed Air System:

• Piccolo tubes distribute air inside wing leading edge
• Heats first 3-8% of chord
• Covers 3-4 inboard slats typically
• Outboard wing not protected (no ice in these areas)
• Temperature: 150-200°C

Ice Detection Methods:

• Visual: Accumulation on wipers, mirrors, wings
• Ice Detector Probe: Senses ice accretion, alerts crew
• Advisory: Amber light when ice detected
• Some aircraft have automatic anti-ice activation

Airframe Icing Effects:

• Increased drag: up to 50%
• Reduced lift: up to 30%
• Increased weight
• Stall speed increase: 10-20 knots
• Stick shaker/pusher activation speed increases
• Reduced control effectiveness

Electrically Heated Probes:

• Pitot tubes heated to prevent ice blockage
• Static ports heated
• AOA vanes heated
• TAT probes heated
• Power: 115V AC
• Heating element in probe structure
• Auto-activation with engine start (some aircraft)

Electrical Heating:

• Conductive film in windshield glass
• Power: 115V AC
• Each windshield panel independently heated
• Temperature: 40-50°C typical
• Prevents ice/fog formation
• Also prevents bird strike shattering

Rain Removal:

• Electric or hydraulic motor drive
• Multiple speed settings
• Intermittent mode available
• Park position when off
• Rain repellent application option

Rain Repellent:

• Chemical applied to windshield
• Causes water to bead and blow off
• Effective above 150 knots
• Lasts 15-30 minutes
• Reduces need for wipers in heavy rain
• Improves visibility significantly

Continuous Loop Detection:

• Pneumatic or electronic sensing elements
• Dual loop system (Loop A & Loop B)
• Detects overheat or fire
• Fire warning if either loop detects fire
• Fault warning if one loop fails
• Detection temperature: 200-260°C

Smoke Detectors:

• Optical sensors detect smoke particles
• Multiple detectors per compartment
• Test button verifies system
• Immediate crew alert on detection
• Class B/E compartments have fire suppression

• Required in all lavatories
• Aural warning in lavatory and cockpit
• Automatic fire extinguisher in waste bin
• Halon cartridge activates automatically
• No crew action required for bin extinguisher

Halon 1301 System:

• Each engine: 2 fire bottles
• APU: 1 fire bottle
• Bottles pressurized with nitrogen
• Discharge pressure: 600 PSI
• Agent distributed via piping in fire zone
• Discharge time: 1-2 seconds
• Effective immediately

Automatic System:

• Fire detected: APU auto-shuts down
• Fire bottle auto-discharges (on ground only)
• In flight: manual discharge required
• APU automatically shuts down 10 seconds after fire detect
• Bottle should extinguish fire before significant damage

Requirements:

• Smoke detection system
• Built-in fire suppression
• Controlled ventilation
• Class B: accessible to crew
• Class E: not accessible but suppression available

Two-Shot System:

• First bottle: Immediate discharge (knockdown)
• Second bottle: Metered discharge over 60-90 minutes
• Maintains suppressive concentration
• Prevents re-ignition
• Ventilation system shut off to maintain concentration

Halon BCF (Halon 1211):

• Most common type in aircraft
• Effective on Class A, B, C fires
• Non-conductive (safe for electrical fires)
• Capacity: 2-5 lbs
• Discharge time: 8-15 seconds
• Use in confined space: caution (displaces oxygen)

• Used for Class A fires (paper, wood, cloth)
• More effective for deep-seated fires
• Never use on electrical fires
• Never use on lithium fires (makes worse)

Minimum extinguishers required:

• 1 in cockpit (readily accessible)
• 1 per galley
• 1 per cabin section (large aircraft)
• Additional in cargo compartments (Class B only)
• Mounted in brackets or compartments
• Regular inspection required
• Pressure check before each flight

High-Pressure Gaseous System:

• Storage: 1800-2200 PSI
• Cylinder capacity: 77-115 cubic feet
• Duration: 2+ hours for 2 crew
• Pressure regulator reduces to 70-85 PSI
• Quick-don masks at each crew station
• 100% oxygen on demand
• Built-in microphone and headset connections

Features:

• Donning time: <5 seconds
• One-hand operation
• Automatic seal inflation
• Smoke goggle provision
• Selector: NORMAL, 100%, EMERGENCY
• EMERGENCY: positive pressure, prevents smoke inhalation

Supplemental portable oxygen for:

• Flight attendants during emergency
• Medical emergencies (passenger use)
• Crew moving in cabin during depressurization
• Capacity: 15-30 minute duration
• Pressure: 1800 PSI
• Regulators provide continuous flow or demand

System Type: Most aircraft use chemical oxygen generators rather than stored gas.

How They Work:

• Sodium chlorate canister
• Mechanical pull-pin activation
• Exothermic chemical reaction produces oxygen
• Generates heat: 200-260°C
• Duration: 12-22 minutes depending on cabin altitude
• Cannot be shut off once started
• One generator per group of 2-4 seats

Deployment Logic:

• Automatic at 14,000 ft cabin altitude
• All overhead panels drop simultaneously
• Oxygen flows only when mask pulled and generator pin extracted
• Manual deployment available to crew
• Cockpit switch overrides automatic system

Duration varies with cabin altitude:

• At 25,000 ft cabin: 22 minutes
• At 30,000 ft cabin: 18 minutes
• At 35,000 ft cabin: 15 minutes
• At 40,000 ft cabin: 12 minutes

Sufficient time to descend to 10,000 ft at maximum descent rate.

For passenger medical use:

• Separate system from emergency oxygen
• Stored gas cylinders (not chemical)
• Continuous low-flow delivery
• Typically 2-4 liters/minute
• Must be airline-supplied (personal oxygen prohibited)
• Medical clearance may be required

• Pressure check before each flight
• Minimum pressure: 1600 PSI typical
• Service to 1800-1850 PSI
• Contamination check periodically
• Mask inspection and cleaning
• Generator replacement per schedule
• Regulator functional test

• Gas turbine engine: Single-shaft design
• Generator: 90 kVA typical (115V AC 400Hz)
• Bleed air system: Supplies air conditioning/pressurization
• Fuel system: Uses main aircraft fuel
• Oil system: Self-contained lubrication
• Fire detection/suppression: Dedicated system

Normal Start:

1. Master switch: ON
2. Start button: Press
3. Starter motor accelerates APU
4. Fuel introduced at 10% RPM
5. Ignition activated
6. Self-sustaining at 50-60% RPM
7. Idle speed: 95-102% RPM
8. Generator available at 95% RPM
9. Bleed air available at 95% RPM

Total start time: 60-90 seconds typical

Power Source for Start:

• Battery (always available)
• External power (if connected)
• Main aircraft generators (in flight)

Run-Time Limitations:

• Continuous operation: Unlimited on ground (with cooling)
• High ambient temperature may require reduced bleed load
• Maximum ambient: 45-50°C for start
• APU can run during refueling (check company policy)

• Can be started and operated at any altitude within limits
• Provides backup electrical power
• Can assist with engine restart
• Bleed air available below FL200-FL250
• Fuel penalty: 1-2% of total fuel burn

Normal Operating Values:

• RPM: 95-102%
• EGT: 600-650°C normal, 750°C max
• Oil Temperature: 80-100°C normal, 155°C max
• Oil Pressure: 25-60 PSI
• Generator Load: 90 kVA maximum

Fire Detection:

• Dual-loop system in APU compartment
• Detects fire or overheat
• Triggers automatic shutdown on ground
• Triggers automatic fire bottle discharge on ground
• In flight: manual shutdown and bottle discharge required

Fire Suppression:

• Single Halon 1301 bottle
• On ground: auto-discharge after shutdown
• In flight: manual discharge via APU fire button
• Bottle pressure monitored
• Low pressure indicates discharge or leak

Fuel Consumption:

• Ground operation: 100-200 lbs/hour
• In-flight operation: 200-300 lbs/hour
• Trade-off vs. ground power unit
• Some airports require GPU use (environmental)

• Oil service: every 200-300 hours
• Inspection: part of regular aircraft checks
• Inlet door check (prevents FOD)
• Fire bottle pressure check
• Generator and bleed valve functional test
• APU hours tracked separately from engine hours