Introduction to Aviation

Resource personAli Khalid

Department of Aviation Management & Technology Superior University, Lahore.

Learning Objectives

By the end of this session , you will be able to:

Understand the basics of pneumatic system on an aircraft.

Understand the concept of bleed air with its various applications in aircraft systems.

Introduction

Bleed air in gas turbine engines is compressed air taken from within the engine, after the compressor stages and before the fuel is injected in the burners.

The modern turbofan engine is a very effective gas generator and this has led to the use of engine bleed air for a number of aircraft system

either for reasons of heating or as a source of air for cabin conditioning and pressurization systems.

intro

Bleed air is extracted from the engine compressor and after cooling and pressure reduction/regulation it is used for a variety of functions.

On the engine, high-pressure bleed air is used as the motive power.

Medium-pressure bleed air is used to start the engine in many cases, either using air from a ground power unit, APU or cross-bled from another engine on the aircraft which is already running.

intro

On the aircraft, bleed air tapped from the engine is used to provide air to pressurize the cabin.

A proportion of bleed air is fed into air-conditioning packs which cool the air, this cool air is mixed with the remaining warm air in the cabin such that the passengers are kept in a comfortable environment.

Uses

Bleed air system users

Bleed air is used as the primary source of air into the cabin and fulfils the following functions:

Cabin environmental control – cooling and heating.

Cabin pressurization. Cargo bay heating. Fuel system pressurization.

Other sub systems

However there are other subsystems where the use of engine bleed air is key. These subsystems are:

Wing and engine anti-ice protection Engine start Thrust reverser actuation Hydraulic system

Wing and engine anti-ice

The protection of the aircraft from the effects of aircraft icing represents one of the greatest and flight-critical challenges which confront the aircraft.

Wing leading edges and engine intake cowlings need to be kept free of ice accumulation at all times

contd

In the case of the wings, the gathering of ice can degrade the aerodynamic performance of the wing, leading to an increased stalling speed which increases the possibility of losing the aircraft control.

Ice that accumulates on the engine intake and then breaks free entering the engine can cause substantial engine damage with similar catastrophic results.

contd

Aircraft windscreens are kept clear of ice by the use of window heating so that the flight crew has an unobstructed view ahead.

Finally, the aircraft air data sensors are heated to ensure that they do not ice up and result in a total loss of air data information that could cause a hazardous situation or the aircraft to crash.

Engine start

The availability of high-pressure air throughout the bleed air system can be used to start other engines of the aircraft.

A start valve is incorporated which can be activated to supply bleed air to the engine starter.

contd

On the ground the engines may be started in a number of ways:

By use of a ground air supply cart

By using air from the APU – probably the preferred means

By using air from another engine which is

already running

Thrust reversers

Engine thrust reversers are commonly used to deflect engine thrust forward during the landing roll-out to slow the aircraft and preserve the brakes.

Thrust reversers one on each side of the engine deploy, which are pneumatically operated by means of air turbine motor actuators to deflect the fan flow forward thereby achieving the necessary braking effect.

Hydraulic system

Pneumatic pressure is commonly used to pressurize the aircraft hydraulic reservoirs.

The hydraulic reservoir is pressurized using regulated bleed air from the pneumatic/bleed air system.