Picture the scene. You are boarding a passenger jet bound for sunnier climes. The flight is running a little late, and it’s full. You squeeze into your middle seat between two overly chatty other passengers. It’s warm… Very warm. Despite it being a pleasant 15 degrees celsius outside, here it feels more like 50. You close your eyes and think of the deck chair that awaits you by the pool in just a few short hours. Then comes the most welcome sound known to the expectant passenger. The whoosh of air around the cabin as the air conditioning kicks in. The temperature starts to drop. You feel more alive, more human. You are comfortable – maybe even zen. You are ready for your holiday.
What you might not know is what to thank for that moment of relief. What magical system has rescued you from the doldrums of humidity? The answer is Bleed Air.
Bleed Air is one of the most critical systems in modern passenger aircraft, facilitating the most noticeable parts of your passenger experience – namely air conditioning (not sweating too profusely on the ground) and pressurization (being able to breathe at 35,000 feet). Let’s take a dive into what bleed air is and how we use it.
What is Bleed Air?
Bleed Air is high pressure air that is extracted (or ‘bled’) from the aircraft’s engine compressors and Auxiliary Power Unit (APU) and is used in a myriad of crucial systems on board. From ensuring cabin pressurization and providing breathable air to passengers at high altitudes, to powering the anti-icing systems that prevent ice formation on wings and engine inlets, bleed air is indispensable. It also facilitates the starting of engines, pressurization of hydraulic reservoirs, and the operation of various pneumatic systems.
How is Bleed Air used on the ground?
The most noticeable use of bleed air on the ground is in air conditioning. Before starting the main engines, the aircraft often starts its APU, which provides electrical power to the aircraft and air circulation to the cabin. As the APU is essentially a jet engine in the tailcone of the aircraft, it burns fuel, and it is increasingly common for aircraft to rely on ground based electrical power from a GPU unit to reduce costs and CO2 emissions. Bleed air can also be used to drive an air starter motor and start the main engines.
How is Bleed Air used in the air?
Maintaining cabin pressurization
Maintaining an aircraft’s cabin at a certain pressure requires a constant flow of air both into and out of the cabin. This is achieved by bleeding the air from the engines and using it to force air into the cabin. An automatically regulated outflow valve then dumps excess pressure to ensure the cabin remains at the target pressure.
Commercial aircraft cabins are typically pressurized to between approximately 11 and 12 pounds per square inch (PSI), which replicates an altitude of around 6000-8000 feet. Maintaining the cabin at these levels is vital to allow passengers the best levels of oxygen to breathe normally.
Powering anti-icing and de-icing systems
Airframe icing is a formidable hazard that affects aviation all year round. The phenomenon occurs primarily in atmospheric conditions where supercooled water droplets exist, in clouds, or when flying through freezing rain and drizzle. It is vital that aircraft can prevent, detect and remove ice buildup from critical areas such as the wings, tail and engines. Once again, bleed air comes to the rescue.
Different aircraft employ different methods to remove ice. Whilst light aircraft often use a fluid-based system, larger aircraft typically employ a system based on electricity or bleed air.
- On jet aircraft – an anti-icing bleed air system (AI-BAS) uses hot air from an aircraft’s engine to prevent ice from forming on the wings and engine intake. Anti-ice valves direct the hot air through ducts to vulnerable surfaces, such as the wing’s leading edge, where it can melt or dislodge ice.
- On turboprop aircraft – bleed air is used to inflate a series of rubber ‘boots’ located on areas of likely ice accumulation, such as the leading edge of the wing. This physically breaks off ice once a certain amount has accumulated.
How does bleed air get from the engines to the cabin?
Air is bled from the compressor stage of a gas turbine (jet) engine. This is the stage at which air is compressed and increased in speed before being mixed with fuel and ignited in the combustion chamber. Let’s take the Boeing 737-800 as an example.
On the 737-800, air is tapped from the 5th and 9th stages of the low pressure and high pressure compressor. This air is both high in speed and high in temperature, so passes through a cooling fan before entering the air conditioning pack. Once prepared, the air enters the cabin.
The air conditioning packs are units that ready the air for use in the cabin by passing it through a series of heat exchangers, cooling and slowing the airflow. (The heat exchangers cool the air by using cold ram air from outside the aircraft). A water extractor removes excess moisture and the air passes through another cooling turbine before entering the passenger cabin. The Boeing 737 and Airbus A320 divide the passenger cabin into three temperature zones which can be manually or automatically controlled by the three knobs on the control panel. Air-conditioning packs are designed to deliver at the coldest temperature demanded by any of the zones. Since that air will be a bit too cold for the other zones, trim air valves add small amounts of hot air into those zones to keep them at the selected temperature.
Bleed air from the APU follows a similar process, and bleed air destined for anti-icing systems is not run through cooling packs as its high temperature is essential in removing ice build-up.
Other uses of bleed air systems
In addition to pressurization, air conditioning and anti-icing systems, bleed air can also be used to pressurize an aircraft’s hydraulic and water systems.
Some military aircraft have employed an innovative system that uses bleed air to improve aerodynamic performance. Referred to as ‘Boundary Layer Enhancement’, this involves using bleed air to power a series of ‘blow flaps’. Air is forced through slots in wing flaps at certain angles of attack, forcing high energy air into the ‘boundary layer’ air on top of the wing. This has the effect of increase in the stalling angle of attack and the maximum lift coefficient the wing can create.
The future of Bleed Air systems
With the 787 Dreamliner program, Boeing attempted a true shakeup of the norms of passenger jet design. The 787 incorporates a ‘no-bleed systems architecture’ that eliminates the pneumatic system and bleed manifold. As such, the air-conditioning packs and wing anti-ice systems are now also electrically powered. The aircraft has even replaced the traditional hydraulic fluid system with an electrically-based alternative.
Boeing quotes the following benefits of this approach.
- Improved fuel consumption (because less air is being removed from the engine/combustion process)
- Reduced maintenance costs (as bleed air systems are notoriously maintenance-intensive)
- Improved reliability and fewer components in the engine installation.
- Expanded range and reduced fuel consumption (as the aircraft is lighter due to the removal of the system).
Despite these developments, the Dreamliner program suffered numerous early setbacks due to other complexities that derived from the aircraft’s all electric approach, specifically associated with the aircraft’s batteries.
Despite Boeing’s bleed-free approach in the 787, other new generation aircraft continue to use a bleed air system design that has remained largely unchanged from the previous generations of airliners. The Airbus A350 program elected to use a typical bleed air system in its design, and the 737 MAX family all use the system. The versatility and experience of using bleed air systems mean they will likely be with us for decades to come.
Cofer photo: Marius Hoepner, JetPhotos.
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