W8: APU PT. 1

Auxiliary Power Unit (APU)

Considered as the critical system of modern gas turbine aircrafts

35 to 350 shaft horsepower (SHP)

It is a self contained turboshaft engine that produces between ______, with its main role of supplying pneumatic and electrical power to the aircraft’s system

Main Engine Starting

Provides high torque and electrical energy for main engine starter motors

Ground Power

Operates the aircraft’s electrical systems and air conditioning when GPU are unavailable

In-Flight Emergencies

Acts as a backup for main engine generators to provide electrical and pneumatic redundancy

Bleed Air De-loading

Supplies pneumatic air to airframe systems so the main engines can focus more energy on thrust

• Air Inlet Door

• Inlet Duct

• APU Controller

(APU Main Components)

Inlet and Control Section

Air Inlet Door

An externally mounted door that opens via electric actuator

Inlet Duct

The passage that directs incoming air from the door to the compressor section

APU Controller

The “brain” that monitors the start sequence, fuel flow management and the transitions from battery power to self-sustaining operation

• Electric Starter (DC Motor)

• Ratchet & Pawl Clutch Mechanism

(APU Main Components)

Starting System (Electrical)

Electric Starter (DC Motor)

Provides initial rotation to the accessory gearbox

Ratchet & Pawl Clutch Mechanism

Transmits torque from the starter to the drive gear which disengages when engine speed reaches 50% RPM

• Drive Gear

• Fuel Cluster

(APU Main Components)

Accessory Gearbox (AGB)

Drive Gear

The mechanical interface that connects the starter, the engine shaft and other accessories

Fuel Cluster

Contains the mechanical fuel, pump, valves, filters and distribution manifold

• Impellers

• Diffusers

(APU Main Components)

Compressor Section (Two-Stage Centrifugal)

Impellers

Rotating components that accelerate airflow radially to build velocity

Diffusers

Stationary components that convert high-velocity air into high-pressure air before entering the combustion chamber

• Combustion Chamber

• Fuel Nozzles

• Swirlers

• Igniter Plugs

(APU Main Components)

Combustion Section (Reverse-Flow Annular)

Combustion Chamber

Component where compressed air enters, mixes with fuel and ignites

Fuel Nozzles

Responsible for the atomization of fuel

Swirlers

Components that twist the incoming air to ensure better fuel-air mixture

Igniter Plugs

Provides high-voltage sparks necessary to initiate combustion

• Three-Stage Axial Turbine

• Exhaust Duct

(APU Main Components)

Turbine and Exhaust Section

Three-Stage Axial Turbine

Consists of stators and rotors. This section drives the main engine shaft to keep the compressor and gearbox spinning

Exhaust Duct

Directs hot combustion gases out of the aircraft’s tail cone

Brayton Cycle

APU operates on the _____, a continuous process involving four stages

• Intake

• Compression

• Combustion

• Exhaust

Brayton Cycle: Four Stages

Intake and Compression

Air enters through an intake door on the aircraft fuselage. Most APUs uses a centrifugal compressor rather than axial flow compressor because it offer higher pressure rise per stage in a smaller footprint and it is more resistant to compressor surge/stalling

Combustion and Power Generator

▪APUs typically use can-type combustion chambers, in which the high-energy gases expand through the turbine stages

▪Approximately 2/3 of the energy produced is used just to drive the engine’s own compressor. The remaining “free” energy is converted into shaft power to drive the gearbox or provide bleed air.

• Intake/Compression Section

• Combustion/Turbine Section

• Gearbox

Three Main Sections of APU

1. Electric Starter

2. Accessory Gearbox

3. Engine Shaft

4. Inlet Door

5. Compressors

6. Nozzles

7. Igniter Plugs

Identification: Summary of Operation

The APU starts using DC battery power to spin the 1. ____. This rotates the 2. ____ and the 3. ____, drawing air through the 4. ____. Once the 5. ____ reach 7% RPM, fuel is injected through the 6. ____ and ignited by the 7. ____. By 50% RPM, the starter disengages, and the engine becomes self-sustaining, eventually reaching 100% RPM to provide pneumatic and electrical power to the aircraft

Load Control Valve

an electrically operated butterfly valve that manages how much compressed air is delivered to the pneumatic manifold

Lubrication System

wet-sump lubrication system

APUs typically use a ____ where the oil reservoir is an integral part of the engine rather than an external tank

Sight gauges

For monitoring of oil level

Magnetic Drain Plugs

To trap metal/FODs

Ignition and Starting

▪APUs are equipped with capacitor discharge ignition system which provides high-intensity spark even for low temperatures and varying pressures of high-altitude flight.

▪It also features dual exciter units and igniter plugs for redundancy.

• Surge/Stall Prevention

• Fire Protection

• Drain Valves

Protective Systems

Surge/Stall Prevention

managed via Inlet Guide Vanes (IGV) or surge valves

Fire Protection

APUs are installed with fire detection and extinguishing systems

Drain Valves

Drains unburned fuel in the combustion chamber after shutdown, preventing “hot starts” or afterfires

Borescope Inspections

▪It is done to inspect that interior without disassembling the engine

▪Turbine blades or the nozzle vanes are inspected for cracks or dents

Lubrication Servicing

Mechanics regularly check oil levels through the use of sight glasses and monitor magnetic drain plugs for metal particles

Filter Maintenance

Filters are serviced to ensure continuous flow. Both fuel and oil systems utilize metal mesh filters

7-10

Filter Maintenance typically has ____ micron range

Pneumatic and Electrical Interfaces

Includes the inspection of the load control valve which ensures the gearbox and generators are functioning properly

Fire Protection

At the start of operations, fire test for the APU is usually conducted to test fire protection systems