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Airplanes have all kinds of different systems that work with one another to make the plane operate the way it’s designed. Let’s review the bleed air system here.
Maybe you’ve heard the term before or you’re an up and coming aviation enthusiast that just wants to learn as much as you can about airplanes. In either case, we’ve got you covered. It can be amazing to think about how airplanes work at all and how we’re able to take to the skies thanks to the various onboard systems. So what is bleed air and how does it work?
Bleed air is the excess air created by the compressors in an aircraft engine. Most of the air goes towards starting the engine, but the excess air (the bleed air) is redirected to other systems on the airplane. It’s used for pressurization, de-icing, HVAC, starting other engines, and more.
For the vast majority of people in the world, bleed air won’t ever really mean anything. But to those of us that are obsessed with aviation, it’s an important topic to understand. In this article, you’ll learn everything you want to know about bleed air in airplanes. From what it is and how it works, to what it’s used for and how it affects the airplane.
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What is a Bleed Air System in an Airplane?
A bleed air system is a system that takes excess compressed air from the engine and uses it to power various systems on the airplane. It’s essentially an air-powered hydraulic system. The bleed air comes from the compressor stage of the engine, which is the section that compresses the incoming air into a usable fuel for combustion.
With all of this excess compressed air, engineers and aviation experts over the years have devised different ways to use it. Since the engines produce this air naturally during the compression process, there’s no reason not to use the potential energy to power some of the other systems on the plane. But before we get into what it’s used for, let’s take a look at how the bleed air system works.
How Does Bleed Air Work?
As mentioned above, bleed air is taken from the compressor stages of the engine, upstream of the fuel burners. The bleed air system starts with air being drawn into the engine at the front by a large fan, which is easy to find in turbofan engines. The air then flows through a series of compressors where it is compressed to a higher pressure.
The compressed air then flows through cooling turbines where it is cooled before entering bleed valves which take small amounts of air off for use in pressurizing and powering various aircraft systems. The remainder of the compressed air then enters combustion chambers where it is mixed with fuel and burned to drive the engine.
In other words, the bleed air system is one that takes excess air created in the compressor section of the turbine and redistributes it to other parts of the plane. The vast majority of the compressed air is used for turning and starting that same engine, but the extra air can be collected and redirected. This is, in essence, how a bleed air system works.
What is Bleed Air Used For?
It is used for many other things that are beneficial to the plane and its passengers, such as pressurization of the cabin, aircraft environmental control systems (air conditioning and heating) and operation of some engines' auxiliary power units.
Arguably the most important job of bleed air is to power the pressurization system. This system is what keeps the cabin at a comfortable level of pressure and oxygen for the passengers and crew.
The pressurization system has two main parts, the outflow valve and the compressor. The compressor is powered by one of the plane’s engines and it pulls air from outside of the airplane. This air is then compressed and sent to the outflow valve.
The outflow valve regulates how much air flows into the cabin. It also mixes in a small amount of oxygen so that everyone onboard can breathe properly. Without a pressurization system that’s working properly, planes would be uncomfortable at best, and impossible for sustained human flight at worst.
Air Conditioning (HVAC)
Another important job for bleed air is to power the airplane’s air conditioning and overall HVAC (heating, ventilation, and air conditioning) systems. The bleed air for the HVAC often works alongside the APU (auxiliary power unit), which is a small engine that provides power to the airplane when it’s on the ground.
Bleed air and the APU are also both used to power the environmental control systems, which provide additional heating, cooling, circulation, and humidity control for the passenger cabin. The air conditioning system uses a compressor to cool the bleed air, and then it’s distributed through the cabin via ducts.
The heating aspect of the HVAC system does pretty much the same thing, but with heat instead of cold. In most cases, the HVAC system will use the engine’s exhaust heat to warm up the cabin. But in some cases, it will use bleed air from the engines’ compressor section to heat up the cabin as well.
One of the most common uses for bleed air is in the de-icing system. The airplane has a number of pneumatic valves that open when the pilot selects the “de-ice” mode on the cockpit controls. Bleed air is then directed onto the wings, tail and propellers to help remove any ice or frost that may have built up. This helps improve aerodynamics and keep the plane flying safely.
Pneumatic controls are ones that are controlled using compressed air. For example, if you have an air compressor at home for some power tools, you’re using pneumatics to power them. For an airplane, this is done by redirecting the bleed air and using the pressure to de-ice the plane when necessary. This is probably the best example of how bleed air is basically an air-powered hydraulic system, as mentioned above.
Lastly, bleed air is also used to start other engines on the airplane. Even though we mentioned it last, this is probably the most common use of bleed air on larger planes such as commercial airliners.
By pumping bleed air into the second (or third and forth) engines, it can begin to spin the engine on its own without having to use all the energy necessary to fire up the turbine from a stand-still. Once it gets spinning, it acts as a sort of kickstart for the turbine, which can then take over and get the engine fired up.
Using the bleed air to get the engine started is much more energy efficient while also protecting the parts of the engine from the harsh forces experienced when going from a dead stop to all-out turbine rotations.
Why is Bleed Air So Important?
Bleed air is essential for powering all of these systems on the plane. If it wasn’t for bleed air, we wouldn’t be able to have pressurized cabins, heated and cooled cabins, de-iced airplanes, or engine starters. In fact, without bleed air most modern airplanes couldn’t even fly!
But bleed air isn’t just important for these system, it also plays a very important role in the safety of the airplane. Bleed air is used to pressurize the fuel tanks, which helps prevent any potential explosions or fires. It’s also used to power the fire suppression system on the airplane, which can put out a fire in just a few seconds.
How is Bleed Air Monitored?
Bleed air is monitored in a few different ways. The first way is through the use of sensors. There are sensors located throughout the airplane that monitor the bleed air pressure and temperature. These sensors send data to the aircraft’s computer, which then displays it on a screen for the pilots to see.
The second way that bleed air is monitored is through visual inspections. When an aircraft is on the ground, there are people called mechanics that inspect all of the systems, including the bleed air system. They look for any leaks or other problems that might be present.
Why Is It Important To Monitor Bleed Air?
It’s important to continuously monitor bleed air for two main reasons: safety and performance.
The safety reason is pretty self-explanatory. If there is a problem with the bleed air system, it could lead to a loss of cabin pressure. And if the cabin pressure is not maintained, it can be dangerous for everyone on board the aircraft.
The performance reason has to do with how the engine works. The engine needs bleed air to function properly. If there is a problem with the bleed air system, it can cause the engine to lose power and performance.