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Air Systems Bleed Air Script Writer: Roshan P. Bhojwani

Lesson Introduction Welcome to the PMDG 777-200LR Bleed Air Groundwork lesson, from Angle of Attack. In todays lesson, you will learn all the important characteristics of bleed air including how it is utilised, and why it is important. It is an interesting topic to study, but youll find the simplicity of bleed air operation does not do justice to how essential the system is for the safe completion of a flight in the 777. For this reason, we are going to look at the following topics:

- Bleed Air description, application and sources. - Bleed Air ducting system, - Engine Bleed Air supply, - APU Bleed Air supply, - Ground air connection.

Controls & cockpit indicators associated to the 777 will be discussed throughout the lesson.

Bleed Air System Overview Bleed air is essentially highly compressed, highly pressurized, very hot air that is used to power certain aircraft systems that make use of pressurized air as opposed to fuel, electricity or hydraulic power. The pressure and temperature properties of bleed air make it a useful and reliable source of energy for many different aircraft, ranging from small turboprops all the way to the 777. Normally, bleed air will also be referred to as pneumatic power. In the 777 specifically, bleed air is used to power a considerable amount of aircraft systems including:

- Air conditioning, - Pressurization and trim air, - Wing anti-ice, - Engine anti-ice, - Aft and bulk cargo compartment heat, - Air driven hydraulic pumps, - Hydraulic reservoir pressurization, - Potable water tank pressurization, - TAT probe aspiration, - Engine and APU start systems.

The air conditioning and pressurization systems depend on bleed air because of its high pressure and density. The wing and engine anti-ice systems take advantage of bleed airs high temperature to eliminate and prevent icing formation. Many other systems depend on bleed air, and one of them is especially important: engine and APU start

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Air Systems Bleed Air Script Writer: Roshan P. Bhojwani

systems. Bleed air provides the necessary pressure to turn the N2 rotor and start compressing air so that it may then be burned and the energy transferred to the turbines to turn the N1 rotor and thus the engine fan. Now where can we extract bleed air from? Either of four sources may supply pneumatic power.

- Engine (1) bleed air system, - Engine (2) bleed air system, - Auxiliary Power Unit (APU) bleed air, - Ground Power Unit (GPU) bleed air.

Bleed air for engine starts is provided by the APU or ground air.

Bleed Air Ducting System All four sources feed into a heavily insulated ducting system known as pneumatic manifold that collects all the bleed air and supplies it to the respective user systems whenever there is demand for it. For example whenever the wing anti-ice system switch is placed in the AUTO position, and the ice detection system senses possible icing conditions, the anti-ice bleed air valve opens and bleed air is allowed to flow from the pneumatic manifold to the wing leading edges in order to heat them up and minimize the risk of ice development. Lets look at a diagram of the pneumatic system. We can right away notice that there are three main sections in the duct: the left, right and center pneumatic ducts. The center one has a center-left and a center-right portion. The different sources of bleed air feed into different sides of the manifold, and different user systems extract bleed air from different sides. Its hard to remember the diagram entirely, therefore you can rely on the air systems synoptic display to aid you. This display is reached by pushing the AIR synoptic display switch on the display select panel. You can monitor parameters associated to the pneumatic duct pressures, as well as which source is actively providing bleed air to the aircraft. What we do have to emphasize is that the left engine supplies bleed air to the left side of the manifold. Similarly the right engine supplies to the right side. APU bleed air is ducted into the center-left side and lastly, ground air is supplied to the left portion of the manifold. Three isolation valves, named Left, Right and Center, divide the manifold into the respective segments. During normal operation, the system keeps the left and right isolation valves open, but the center valve closed. Only in abnormal conditions, such as during single bleed source operation or engine starts, does the center isolation valve open. Why are there left and right isolation valves then, if they are always kept open? Thats because in case any form of bleed air leak were to develop in the left or right sides of the manifold, it may be isolated from the rest of the system to prevent the remaining air being lost from the duct. Obviously the user systems attached to the failed side of the bleed air duct would stop receiving a bleed air supply if the respective isolation valve is closed.

Script Document

Air Systems Bleed Air Script Writer: Roshan P. Bhojwani

Even though the pneumatic manifold ducts are very heavily insulated, leaks can develop. It is fairly easy to detect a bleed air leak, because the structure and components surrounding most parts of the pneumatic manifold are at normal temperatures, sometimes a bit colder. Whenever these start to show excessive temperature readings, it is most likely because bleed air is leaking from the ducts into these other aircraft parts. The pneumatic ducts pressure varies as a function of aircraft altitude. Normal duct pressures are the following:

- 40psi when altitude is lesser than 27,000ft. - Decreasing value between 40psi and 32.5psi when altitude is more than 27,000ft but less than or equal to

43,000ft. - 32.5psi when altitude is greater than 43,000ft.

This normal duct pressure suffers momentary variations as demand for bleed air changes. For example, the values are biased whenever the cabin temperature controller requests more bleed air, up to a maximum of 6psi. In the case of an engine cross-bleed start, duct pressure may vary up to 9psi. Engine cross-bleed starts are an interesting topic covered in FlightWork, but essentially have to do with one running engine supplying the necessary pneumatic pressure to start the other engine. In this case obviously, all three isolation valves must remain open, so that the air is allowed to travel through the ducts into the other side, as well as a few other considerations youll see during FlightWork. Duct pressure transmitters relay the duct pressure in psi to the indications in the cockpit. Slight differences between right and left duct pressures are permissible, so long as there is sufficient air for cabin pressurization. Certain limitations govern the minimum duct pressure required to start an engine on ground. Remember that the required pressure may be provided by either the APU, ground air or an engine cross-bleed start. The minimums are:

- 15psi for an APU start, - 25psi for a Ground start, or a cross-bleed start.

Now that we know about bleed air and the pneumatic duct that transport it, lets dig deeper into the different sources of bleed air. Starting with the primary source, obviously, the engines.

Engine Bleed Air Supply Engines on the 777 supply regulated bleed air into the pneumatic manifold for use by the different aircraft systems. Even though the 777 engines are extensively discussed in the 777 Groundwork Engines lesson, we are going to quickly summarize their operation so you can understand where bleed air is actually bled from. Keep in mind this explanation is only a generalization. Ram air is sucked into the engines and is split into two types: primary air, which enters the engine core and passes through a series of low and high pressure compressors, turbines and a combustion process. Secondary air, which is about 80% of the total air, bypasses the engine core and is ducted around it. Given the shape of the engine cowling

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Air Systems Bleed Air Script Writer: Roshan P. Bhojwani

and fan ducts, bypassed air is enormously accelerated and rushed out of the engine tailcone, producing a great deal of thrust. Lets focus on primary air. Once it enters the engine core, it passes through a low pressure compressor, then the high pressure compressor after which it is mixed with fuel and ignited to generate energy in the form of heat. This energy is transformed to mechanical energy and is used to turn low and high pressure turbines and thus the engine fan. After the air has passed through several stages of compression, its pressure, density and temperature are greatly higher than their initial values, so that the air is adequately prepared for the combustion process. This air is also adequate enough to be used as bleed air, so bleed air is extracted from the high pressure compressor stages of the engine. Obviously this extraction of air would take away part of the total air that is used to generate thrust, however there is always a trade-off when a benefit is meant to be achieved. And the benefit, in this case, is of course the pneumatic power available to allow aircraft systems to function. Now, the high pressure compressor has several stages of compression. Air is extracted from either the 4th, or the 9th stages, constituting low and high stage air respectively. In cruise and under normal conditions, low-stage air is sufficiently capable of supplying the aircrafts pneumatic demand. During descent, and especially in low power settings, low-stage air becomes insufficient and thus a high-stage valve opens, and high-stage air, which is further compressed than low-stage air, starts to flow into the manifold. Look at the diagram of the engine bleed system. Were going to explain a few of the components here. Firstly we can see the low-stage and high-stage ducts. Notice there is a check valve in the low stage duct. A check valve controls the flow of air from the low-stage duct into the pneumatic duct and prevents air in the high-stage manifold from entering the 4th stage source. Moving on, if you follow the high-stage line, youll find a high pressure shutoff valve. Further downstream, is a bleed air pressure regulating and shutoff valve. The HPSOV and PRSOV are the same, they control the flow and direction of bleed air from the source into the pneumatic manifold. These valves modulate between their fully open and fully closed position to adjust the amount of bleed air flowing from the engine into the duct, according to demand of the user systems. The HPSOV and PRSOV valve positions are governed between open and closed by Air Supply Cabin Pressure Controllers. This control is based upon the selection of engines as a bleed source, and the demand for pneumatic power by the aircraft systems that need it, like the anti-ice system for example. To select engines as a bleed source, the respective ENG bleed switch in the overhead panel must be pushed. The switch has two positions:

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Air Systems Bleed Air Script Writer: Roshan P. Bhojwani

- ON: engine PRSOV and HPSOV are armed and operate whenever there is an adequate pressure of bleed air coming from the engine high-pressure compressors.

- OFF (ON not visible): both bleed valves in the related engine are commanded closed. Further downstream, a precooler decreases bleed air temperature. Also known as a cross-flow heat exchanger, it does so by taking bleed air, making it flow through a series of coils in the precooler, and exchanging its heat with cold air from the engine fan. Fan air takes away some of the thermal energy in bleed air and lowers its temperature, making it more suitable for use by the aircraft systems. A fan air modulating valve (FAMV) regulates the amount of fan air to be used by the precooler. Another series of coolers help in bringing down the bleed air temperature. These are known as controller air coolers, or CACs, and are located before the PRSOV. Bleed air temperature after it has passed through the precooler should be about 193C. In case the cabin temperature controllers were to request lower bleed air temperature for air conditioning purposes, then temperature downstream of the precooler is limited to 121C. If at any time the temperature exceeds 232C, the position of the PRSOV is changed to limit the amount of bleed air allowed through the precooler. In case of system logic failure where a bleed air overpressure condition should occur, a duct vent valve allows air to be disposed overboard. This component is located downstream of the HPSOV, and activates at about 185psig. The valve resets at 145psig. A bleed air overpressure condition could heavily damage the ducting system and increase the risk of air leaks, resulting in a system failure. Keep in mind that the engine bleed air valves will automatically close in any of the following conditions:

- During engine start, - Loss of bleed source, - Bleed air temperature exceeding 254C, or pressure exceeding 242psig. These two conditions are also

known as trip-offs. - Any form of leaking in the duct, - Whenever an engine fire switch is pulled, to prevent propagation of fumes into the air conditioning

systems and to protect engine integrity. - Whenever a ground cart is supplying external air to pressurize the pneumatic duct. Well discuss ground

air later on in this lesson. In an automatic closure of the engine bleed air valves, both the OFF and ON lights in the ENG bleed switch illuminate. All our discussion about the engines and their bleed system assumes that the engines are up and running, however, how do we receive bleed air to power the pneumatic system whenever either or both engines are turned off? The answer lies in the Auxiliary Power Unit, or APU, and its pneumatic capabilities. Lets talk about these a bit now.

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Air Systems Bleed Air Script Writer: Roshan P. Bhojwani

Auxiliary Power Unit Bleed Air Supply The APU is deeply discussed in its respective GroundWork lesson, so today we are going to focus only on its capacity as a source of pneumatic power. Even though the APU is capable of starting and electrically powering the 777 all throughout its flight envelope, bleed air from the APU can only be provided up to about 22,000ft. On ground, there is no limitation as to its pneumatic capabilities. APU bleed air feeds into the aircrafts pneumatic manifold, and powers the following user systems:

- Air conditioning and pressurization, - Air-driven hydraulic pumps, - Air pressure for engine starts, - A few other little items.

Engine bleed air supply is a direct function of thrust lever position, however, the APU normally runs at a constant speed. Therefore, the supply of APU bleed air is controlled with an auxiliary power unit controller, or APUC. The APUC controls the angular position of a series of vanes to allow for an increase or reduction of airflow. These vanes are called inlet guide vanes, and there are twenty-eight of them. Similarly to the case of the engines, the APU has a check valve that prevents reverse flow of air from the pneumatic duct back into the APU system. Remember that the APU feeds bleed air into the center-left side of the pneumatic manifold. Now, the APUC controls the position of the inlet guide vanes based on one of the following logics:

- Idle, - Duct pressurization, - Main engine start, - Air-Driven pump. - ECS - ADP/ECS.

The APUC selects the idle mode when there is no pneumatic system demand. The IGVs are fully closed. Idle mode is also selected whenever the aircraft is above 22,000ft. Duct pressurization mode is selected when the APU bleed air valve is open, but there is still no pneumatic system demand. The IGVs are commanded only open enough to pressurize the pneumatic ducts. During a main engine start, the APUC operates the main engine start mode. The IGVs open enough to meet the high pneumatic demand of an engine start, especially because this mode has priority over all other modes.

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Air Systems Bleed Air Script Writer: Roshan P. Bhojwani

Similarly, the ADP mode is selected whenever either ADP is operative. Likewise, the IGVs open to supply enough airflow to operate the ADPs. This is also the same case for the other modes, where if either or both air conditioning packs operate, the IGVs open to supply air to the airplane environmental control system.

Now that we understand how the APU provides bleed air, its important to know how do the pilots select it. Sometimes in your simulator flights, APU supplied bleed air will be your only pneumatic source to get the engines running and be able to fly away! An APU bleed switch in the overhead panel is located between the two engine bleed switches we talked about earlier. The switch has the following positions:

- AUTO: the APU bleed air valve is opened when system logic senses positive bleed air availability. - OFF (AUTO not visible): the APU bleed air valve is commanded closed.

If during normal operation the OFF light illuminates in amber, the APU bleed air valve closes automatically due to:

- A bleed air leak in the APU duct, - If the bleed valve is failing closed, - Or the APU fire switch has been pulled out.

We mentioned earlier that engine bleed air is extracted from the total air that enters the engine fan and is meant to come out of the engine exhaust and that this has a nominal impact on thrust and performance. Runways in the world are mostly long enough to accommodate for this loss of thrust and yet allow the aircraft to speed up and rotate with an important safety margin. However, there are certain cases such as during bad runway surface condition, smaller length runways, high outside air temperature and high density altitude, that a bleeds-off takeoff may have to be performed. In this scenario, engines are no longer used as a bleed source during the takeoff and initial climbout phases, so that the maximum thrust may be obtained for a particular takeoff condition. This also means that air conditioning packs, which are deeply discussed in the 777 Air Conditioning GroundWork lesson, can either be used ON or OFF. If the takeoff is made with packs OFF, no form of cabin temperature control would be necessarily achieved, however, these would have to be turned on during the initial climb and this is normally done at thrust reduction altitude. If OAT is high enough that a packs ON takeoff is imperative, the APU may supply the required bleed air to do so. This is known as an APU-to-Pack takeoff. The exact procedures to perform either a bleeds-off or an APU-to-pack takeoff is covered in Flightwork, however there are a few considerations you should keep in mind now: In an APU-to-pack takeoff only the left pack operates. The left engine bleed valves are automatically commanded closed, meaning the L ENG OFF light also illuminates. Because the system also closes the center isolation valve, the APU only powers the left pack and the C1 ADP. Right engine bleed may be used to power the C2 ADP, but the right pack valves are automatically commanded closed and the R PACK OFF light illuminates.

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Air Systems Bleed Air Script Writer: Roshan P. Bhojwani

Were now going to move on to the final segment of this video, a short explanation of the third available source of bleed air.

Ground Air Connection If on ground, at a gate, your engines are off and you cant keep your APU running for a long period of time otherwise your flight simulator airlines manager will complain about you wasting precious fuel, you may want to choose an external ground air connection to pressurize the pneumatic system. The high pressure ground connectors allow an external air source to feed pressurized air into the aircrafts pneumatic manifold. These ground connectors are located in the under wing fuselage area, forward of the environmental control system access door. There are a few considerations about ground air though. These are not of direct significance to the pilots, but are important to keep in mind anyways. Ground air must not exceed 50psig of pressure and/or 232C of temperature. Not complying with this may result in significant damage to the pneumatic ducting system, leading to a loss of insulation and the potential risk of bleed air leaks. Another important point to note is that you must make sure that the aircraft is electrically energized before actually allowing a ground air pneumatic supply. This is to prevent damage to the air conditioning system, and to ensure that the packs can startup and shutdown according to their normal sequence.

Lesson Summary To sum up this lesson, weve had a discussion about the 777s bleed air system. Firstly we explained the concepts behind bleed air, pneumatics, the pneumatic manifold, and others. After which, the three sources of bleed air were discussed in detail. When it comes to the actual operation of the bleed air system, it is fairly simple, as it does not require constant pilot input throughout the flight phases. However, the importance of pneumatics and the entire ducting system is such that you should try and fully understand how everything works in it. From our lesson, we can conclude that engines are evidently the more important producers of bleed air, but that this has a nominal impact on engine and aircraft performance. Some aircraft, like the 777, counter this by significantly optimizing the amount of air that is taken in and is processed in the engine with the of thrust management computers. Other aircraft, like the modern B787 for example, eliminate the need for bleed air and only rely on electrical power generation for their air conditioning, pressurization, and other systems. To finish off, heres the list of aircraft items that depend on bleed air for their functions:

- Air conditioning, - Pressurization and trim air,

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Air Systems Bleed Air Script Writer: Roshan P. Bhojwani

- Wing anti-ice, - Engine anti-ice, - Aft and bulk cargo compartment heat, - Air driven hydraulic pumps, - Hydraulic reservoir pressurization, - Potable water tank pressurization, - TAT probe aspiration, - Engine and APU start systems.

Stay tuned to the following lesson in the Air series. We will be talking about the Air Conditioning system as well as its importance and operations that rely heavily on bleed air. Thanks again for watching and Throttle On!