Grnd-57 Theory of Flight 9 Bleed Air Systems

Bleed Air Systems

Incorporated into most turbojet, turbofan, and turboprop aircraft

Network of ducts, valves, and regulators to extract medium to high pressure air from the compressor section of the engines or APU to various locations in the aircraft.

Bleed Air Functions

Pressurization

Air conditioning

Engine start

Aircraft ice protection systems

Water system pressurization

Hydraulic system reservoir pressurization

Boundary layer control systems

Pressurization Systems

The cabin needs to be pressurized so that the crew and passengers can breathe without supplemental oxygen

Bleed air is ducted into the cabin and air is released by an outflow valve

A regulator controls the outflow to maintain a set differential pressure or set cabin altitude

Cabin altitude kept at or below 8000’

Cabin Altitude

Cabin pressure expressed as an equivalent altitude above sea level

Differential Pressure

The difference between cabin pressure and atmospheric pressure

A safety valve acts as a:

Relief valve, releasing air from cabin to prevent cabin pressure exceeding maximum differential pressure

Vacuum relief valve, allowing air into the cabin when the ambient pressure exceeds cabin pressure

dump valve, allowing crew to dump cabin air manually

Air conditioning

Bleed air routed to air conditioning packs where it is filtered and cooled using an expansion process.

Cabin temp is regulated by mixing uncooled bleed air (200-250 C) with conditioned air

APU

A small gas turbine normally located in the tail cone and sometimes in the engine nacelle or wheel well.

Provides autonomous source of electrical power and bleed air without reliance on main engines or ground support equipment.

If certified for use in flight, APU can

provide redundancy in the event of an engine driven generator loss

Source of bleed air for starter assist for in-flight relight

power the air conditioning packs and pressurization system

Difference between De-Icing systems and Anti-Icing systems

De-Icing removes ice after forming

Anti-Icing prevents ice from forming

De-Icing systems remove ice through use of

Mechanical energy

Electrical energy

Thermal energy

Pneumatic De-Ice boots

Pneumatic rubber boots on leading edges of airfoil surfaces use relatively low pressure air to rapidly inflate and deflate boot to break up ice

Engine Start

Bleed air from either APU or another operating engine may be used to power an air turbine starter motor.

A lot of torque can be generated compared to an electrically or hydraulically powered starter motor.

Water System Pressurization

Used to pressurize the potable water holding tank eliminating the requirement for a pump to feed water to galleys and lavatories

Hydraulic Reservoir Pressurization

Used to pressurize hydraulic system reservoirs reducing likelihood of pump cavitation and resulting loss of system pressure

Boundary Layer Control

A small amount of bleed air is piped to channels running along the rear of the wing where it is forced through slots in the flats when they reach certain angles. It delays boundary layer separation from the airfoil to increase the stalling angle of attack and maximise lift.