Aoa 737ngx Groundwork Apu Handout

9

      n       o         i         t       c       e       s

Auxiliary Power Unit

The material covered in this document is based off information obtained from the original manufacturers’ Pilot and Maintenance manuals. It is to be used for simulation purposes only.

Copyright © 2011 by Angle of Attack Productions, LLC All rights reserved

Table of Contents

Table of Illustrations

APU Overview

3

Figure 9-1. Auxiliary Power Unit Diagram

6

APU Engine Primary Components

4

Figure 9-2. Fuel Supply Diagram

8

APU Fuel Supply

7

Figure 9-3. Inlet Guide Vanes

APU Start

9

APU Operational Modes

11

APU Altitude Operational Limits

14

APU Shutdown

15

APU Normal Shutdown

15

APU Protective Shutdown

15

APU Automatic Load Shedding

17

13

APU Overview The Auxiliary Power Unit, or APU, is a gas turbine engine capable of providing electrical and pneumatic services on the ground and in the air. It allows the aircraft to be selfsufcient on the ground without the need for ground power. The 737NG uses the AlliedSignal, now Honeywell, 131-9B APU. The 131-9B is able to start and operate up to the aircraft’s maximum certied altitude of 41,000 feet. The APU is installed within a reproof compartment in the tail of the aircraft.

A rewall isolates the APU compartment from the aircraft fuselage and the horizontal stabilizer assembly. The APU air inlet door is located on the right side of the aft fuselage and is automatically controlled. This is a NACA type inlet, a concept originally developed by the US National Advisory Committee for Aeronautics in 1945.

It is a low drag inlet, designed to allow air to ow into the duct in ight. There is an inlet ow deector that modies the airow into the intake to ensure that it is laminar and appropriate for ingestion into the APU.

After combustion, the APU exhausts gases through a mufer and out of the tailcone. The high speed ow of the APU exhaust forms a low pressure area inside the APU compartment which pulls outside air in through a second hole in the tailcone. This is called the eductor inlet, and draws outside air into the APU compartment, cooling the APU oil. This is an efcient means of cooling and removes the need for a separate cooling fan, eliminating another moving part. An Electronic Control Unit, or ECU, continuously monitors and controls the APU from start to shutdown. It also provides shutdown protection in the event that any one of several parameters goes out of limits. Shutdown protection is discussued in more detail later.

APU Engine Primary Components The APU is ver y different to the two CFM56 engines on the 737. It has three primar y engine components (see fgure 8.1) : ●

The power section.

●

The load compressor.

●

The accessory gearbox.

The power section drives the load compressor and the accessory gearbox. The power section consists of: ●

A single stage centrifugal compressor.

●

A combustion chamber.

●

A two stage axial ow turbine.

Air enters the APU through the air inlet, and is directed into the centrifugal compressor which throws it outwards, compressing it. The compressed air is directed into the combustion chamber where it is mixed with fuel and ignited. Ignition and expansion of the gas in the combustion chamber forces it through the turbines, spinning them. The turbines are connected to a single shaft, which in turn is connected to the centrifugal compressor. Also attached to this same shaft are a starter-generator, gearbox, and the

pneumatic load compressor. The purpose of the pneumatic load compressor is to supply bleed air to aircraft systems that require it, such as air conditioning, pressurization, ice protection, and for engine start. The key difference here is that the two main engines supply bleed air from the power section, while the APU has a dedicated compressor for the job. Because the pneumatic load compressor is attached to the same shaft as the engine compressor, they both spin at the same RPM. In order to vary the amount of bleed air taken from the APU, the ECU opens and closes Inlet Guide Vanes in the load compressor inlet. These control the amount of air that enters the load compressor, and consequently the amount of air taken from the APU for aircraft systems.

The Inlet Guide Vanes move from 15 degrees to 110 degrees as bleed air demand changes. The accessory gearbox is also mounted to the APU shaft.

APU Eng.Prim. Components (Cont.) This reduces the high rotational speed of the shaft to a lower speed for the accessories mounted on the gearbox. The gearbox turns the APU starter-generator, and other components. The starter-generator is used when starting the APU and generates electrical power once it is running.

Notes

COMPRESSOR AIR INLET

APU BLEED AIR

SURGE CONTROL

VALVE

VALVE

EDUCTOR INLET

STARTER GENERATOR

FROM FUEL

FCU

SYSTEM FROM EDUCTOR INLET

OIL COOLER

Figure 9-1. Auxiliary Power Unit Diagram 

APU Fuel Supply Fuel supply to the APU is controlled by the APU Fuel Control Unit. The Fuel Control Unit regulates the fuel supply for different running conditions, and uses a motor driven by the accessory gearbox. The APU uses the same fuel supply as the two main engines. Fuel piping is arranged such that the APU normally takes fuel from the left side of the fuel system. (see fgure 8.2)  The APU is capable of drawing fuel without positive pressure from the fuel pumps. When no fuel pumps are operational, fuel is suction fed from Main Tank 1 using the Fuel Control Unit’s own motor.

Under normal conditions once the APU is running, an AC Fuel Pump is used to pressurize the system. There are two AC Pumps for each fuel tank. The fuel system features a Crossfeed Valve that effectively isolates each side of the fuel system from the other. With the Crossfeed Valve closed, any of the three AC pumps on the left side of the system can supply the APU. This includes the two Main Tank 1 pumps and the left Center Tank pump. The Main Tank 1 Aft Fuel Pump is normally used to feed the APU on the ground. If the APU will be run for an extended period, the left Center Tank pump may be used to prevent a fuel imbalance.

Operating without the assistance of a fuel pump can reduce the service life of the Fuel Control Unit motor however. To address this, an automatically operated DC Fuel Boost Pump is installed. This pump draws fuel from Main Tank 1 when the APU Fuel Control Unit senses low fuel pressure. This provides positive pressure and preserves the service life of the Fuel Control Unit.

With the Crossfeed Valve open, fuel may also be fed from Main Tank 2. Operation of the DC Fuel Boost Pump is automatic, but the AC Pumps must be manually selected ON or OFF on the Forward Overhead Panel. When an AC pump is used and pressurizes the system, the DC pump automatically shuts off.

The DC Fuel Boost Pump is usually used during APU startup when no AC power source is available to power the AC Pumps.

APU fuel consumption is roughly 225 pounds per hour running both packs. This is ver y much a ballpark gure – fuel consumption varies depending on a variety of conditions.

 TO APU

Figure 9-2. Fuel Supply Diagram 

APU Start APU start is controlled by the ECU, Electronic Control Unit. It is a fully automatic start sequence. The Battery Switch   on the Forward Overhead Panel must be set ON before the APU can be started. The APU Fire Switch   on the Aft Electronics Panel must also be IN , and the APU Fire Control Handle  in the main landing gear wheel well must be in the UP position . Controls and indications for the APU are located on the Forward Overhead Panel. The start sequence is commenced by holding the APU switch  momentarily to START . The switch is spring loaded back to the ON position , and will return there when released. When the switch is selected to START , the Electronic Control Unit opens the APU Fuel Shut-off Valve and the APU Air Inlet Door.

Either 28v DC power from the battery or 115v AC power from AC Transfer Bus #1 may be used to start the APU. This passes through the Start Power Unit which converts it to 270v DC power.

The Start Power Unit forwards this to the Start Converter Unit which converts it to AC power for the starter-generator on the APU gearbox. As the name implies, the starter-generator performs two main functions: ●

It supplies the initial rotation of the APU during the start cycle.

●

And provides a source of electrical power for aircraft systems once the APU is running.

If starting on the battery, there will be a signicant amperage draw indicated on the AC/DC Metering Panel when the starter-generator kicks in. This is usually in the region of a 400 amps draw – it takes a lot of power to get that APU turning. Additionally to the negative amps indication, the BAT DISCHARGE light   will illuminate. The APU draws power from the Main Batter y for startup, so the Auxiliary Batter y is automatically isolated during APU start. The LOW OIL PRESS light  will illuminate during the start

APU Start (Cont.) process, and will extinguish once APU oil pressure reaches normal levels. The APU’s Exhaust Gas Temperature indication may uctuate throughout its entire range during start prior to normal EGT rise. This is normal, and has no adverse effect on starting the APU. Note that there are no limitation indications on the EGT gauge – EGT is monitored automatically by the ECU, and the APU will be shut down automatically if it exceeds limits. It is therefore not necessary to monitor EGT during APU start. The ECU commands ignition and fuel injection during startup automatically as the APU reaches the appropriate speeds. The start cycle will terminate automatically after 120 seconds if the APU has not yet reached the required RPM to disengage the starter. The start cycle may therefore take as long as 120 seconds, and the APU should be run for a further minute after start before it is used as a bleed air source.

This minute of idle running is intended as a stabilization period to extend the service life of the APU. Although the start cycle itself takes a minute or so, if powered from the battery at this point it uses the equivalent of approximately 7 minutes of battery life. Once the start cycle is complete, and the APU has reached 95% speed, the ECU gives a ‘Ready to Load’ signal to other aircraft systems. This signals that the APU is ready to accept pneumatic and electrical loads. The electrical system indicates this to the crew by way of the APU GEN OFF BUS light, which illuminates blue when the APU is capable of powering an AC bus but is not yet doing so. There is no direct equivalent indication for the air system.

APU Operational Modes We have already stated that the APU should be run for at least one minute after start before it is used as a bleed air source. Taking bleed air from the APU places a considerable load on it, far more demanding than taking electrical power from the starter-generator.

the APU power section components.

The ECU selects from four bleed air modes depending on demand from aircraft systems:

The ECU sets ‘Duct pressurization mode’ when the APU Bleed Air Valve is open, but there is no actual demand from the air system. In this case, the Inlet Guide Vanes open further to allow the load compressor to pressurize the pneumatic system air ducts.

●

No bleed mode

●

Duct pressurization mode

●

Main engine start mode

●

Air conditioning system mode

The ‘no bleed mode’ is set when there is no bleed air demand from the pneumatic system and the APU Bleed Air Valve is closed. When the pilot selects the APU Bleed Air switch OFF  on the Forward Overhead Panel, the APU Bleed Air Valve closes.

The ECU closes the Inlet Guide Vanes to 15 degrees. Even without any bleed air demand, the load compressor will still be spinning as it is attached to the shaft along with

To keep the load compressor cool, the Inlet Guide Vanes do not close fully, even in ‘no bleed mode’ with no bleed air demand. They close only as far as 15 degrees.

‘Main engine start mode’ opens the Inlet Guide Vanes as needed to meet the high air ow requirement of main engine start. Air conditioning system, or ACS mode sets the Inlet Guide Vane position as necessary to supply air to the air conditioning system. The air conditioning system itself has four modes of operation: ●

One pack inight.

●

One pack ground.

APU Operational Modes (Cont.) ●

Two packs, normal.

●

Two packs, high.

The ECU opens the Inlet Guide Vanes to the appropriate position for each of these modes so as to supply the required airow. APU fuel consumption is considerably greater when operating a single pack than when operating both. A single pack must work much harder than two packs to cool the cabin to a given temperature. The APU must therefore supply higher pressure bleed air to allow the single pack to function. To supply higher pressure bleed air, the APU Inlet Guide Vanes must open further than they would otherwise have to to supply both packs. The further open the Inlet Guide Vanes are, the greater the torque required to keep the APU rotating at a constant speed (see fgure 8.3) . This requires the Fuel Control Unit to inject more fuel, increasing fuel consumption. Additionally, although a single pack requires greater

pressure than two packs would, it requires less actual quantity of airow. There is therefore a considerable excess of bleed air produced that is not required. This excess bleed air is exhausted through a Surge Control Valve, which ducts it through the APU exhaust. This increases exhaust gas temperatures, and the additional airow through the exhaust can increase the noise signature of the APU by approximately 2 decibels. Running both packs on the ground therefore reduces noise, reduces fuel consumption and extends the life of the APU hot section. This is the recommended practice.