aoa_777_groundwork_ac-pressurization_transcript.pdf

Script Document

Air Systems Air Conditioning & Pressurization Script Writer: Roshan P. Bhojwani

Lesson Introduction Welcome to the PMDG 777-200LR Air Conditioning & Pressurization Groundwork lesson, from Angle of Attack. Today in this lesson we are going to run through the important aspects of the Air Conditioning and Pressurization systems aboard the 777. This lesson will be divided in two main areas. On one hand, well look at all the intricacies of the fundamental air conditioning system. On the other hand, well talk about how the 777 maintains its cabin pressurized to a comfortable level so that your FS passengers can breathe normally and not freeze when flying at twenty or thirty thousand feet high. Heres the list of topics were going to discuss today:

- Air Conditioning System Introduction, - Air distribution and temperature control, - Air conditioning packs, - Cabin pressure control, - Typical pressurization profile, - Outflow valves, - Cockpit indications.

There are several small portions of the air conditioning system that we will only explain at a basic level today, such as the ventilation system, gasper fans and electronic equipment fans. We encourage you to grab a PMDG 777 manual and read through those paragraphs, youll surely enjoy it!

Air Conditioning System Overview The 777 air conditioning system controls the interior environment of the aircraft for flight crew, passengers and equipment. These are the systems three primary functions:

- Control the flight deck and passenger cabin temperature, - Control fresh air flow for cabin pressurization and ventilation, - Recirculate cabin air for ventilation.

Two main components create the supply of fresh conditioned air. These are known as the left and right Air Conditioning Packs. We will simply be referring to them as packs during this lesson. Did you know that the word pack actually stands for pressurization and air conditioning kit? The entire pack process involves cooling, heating, cleaning, filtering and metering the amount of air that goes into the cabin. Packs work with bleed air supplied from the aircrafts pneumatic system and, as we know from the 777 Bleed Air Groundwork lesson, bleed air is mainly obtained from the high pressure compressor stages of the engines, therefore it is probably not clean or cool enough to be delivered raw to the passengers.

Script Document


Obviously, packs and their air cooling and cleaning process impose a large air demand over the pneumatic system, therefore recirculation fans in the passenger cabin help to minimize pack workload by taking cabin air, running it through cleaning filters and then pumping it back to the cabin. Conditioned air is not only provided by the air conditioning packs. Ground supplied pre-conditioned air may also be fed into the aircraft through a series of ports in the lower fuselage.

Air Conditioning Distribution & Mix Manifold Air from all the sources flows into a common pool known as mix manifold. From the mix manifold, air supply to the aircraft is metered and controlled according to the demand for cool air by the aircrafts cabin sections. For distribution and temperature control purposes, the aircraft is divided into the following zones:

- Flight Deck, - Six passenger cabin zones named A through F.

The flight deck receives 100% fresh conditioned air from the left pack. The cabin zones receive conditioned air from both packs, but also a share of recirculated air to minimize the pack workload. As we mentioned before, the recirculation process involves drawing air from the cabin, running it through a series of physical and chemical filters, mixing it with pack air and then re-inserting it back into the distribution ducts. As far as zone temperature control is concerned, heres an interesting point. Both the flight deck and passenger cabins have temperature selectors located in the overhead air conditioning panel. These have automatic temperature control logics that work in the following way: There is one selector for the flight deck, and one selector for the six passenger cabins. The air-conditioning packs are commanded to produce air according to the coldest of all temperature zones. Why does it do that? Because rather than packs producing air with different temperatures for each cabin zone, it is more efficient to have the packs produce air of a single temperature in the mix manifold according to the coldest cabin zone, and then add warm air to all the other cabin zones that require air with a higher temperature. This warm air that is added to pack air is known as trim air. Trim air is bleed air taken from the aircrafts pneumatic manifold. Remember that it does not flow into the mix manifold; instead, it is mixed with pack air downstream of the mix manifold. Cabin temperature controllers use trim air modulating valves to adjust the temperature in cabin zones that need more heat. Trim air from the left engine bleed system works with the flight deck, three passenger cabin zones and the aft cargo compartment. Trim air from the right engine bleed instead is added to the three other passenger cabin zones, forward cargo compartment and the electronic equipment compartments. TRIM AIR switches in the overhead panel command the operation of trim air modulating valves. The TRIM AIR switches have the following positions:

- ON: trim air modulating valves are commanded open and regulated by the cabin temperature controllers. - OFF (with ON not visible): Trim air valves are commanded closed and the FAULT light illuminates.

Script Document


Whenever the FAULT light illuminates, it is a signal of a problem with the trim air valves. This could be triggered by trim air valves failing closed, a zone supply duct overheat, or simply if the related trim switch is selected off.

Referring back to the temperature selectors, the first one being the FLT DECK TEMP selector have the following positions:

- AUTO: provides automatic temperature control for the flight deck, maintaining an average temperature of 24C.

- MAN: provides manual control of the trim air modulating valve. Turning the valve from the MAN position to the C detent drives the flight deck zone trim air modulating valve to closed, so air from the mix manifold is higher in proportion to trim air. Similarly, turning the valve from the MAN position to the W detent drives the flight deck zone trim air modulating valve to open, so there is a higher amount of trim air flowing into the flight deck. The temperature limits during automatic and manual operation are 18C and 29C.

The CABIN TEMP selector works similarly, but has a more automatic logic than the FLT DECK TEMP selector. The CABIN TEMP selector provides automatic temperature control, with the normal master reference temperature being 24C. Turning the knob toward C or W changes the master reference temperature between 18C and 29C. If either temperature selection is impossible to achieve by the pack temperature controller because of system fault, the pack outlet temperature is regulated to achieve the last commanded temperature, or an average temperature of 24C. For the cabin sections at least, there are auto comfort correction features that automatically compensate for temperature changes during different flight phases. For example, a cruise condition would cause the cabin temperature controllers, which well now refer to as CTCs, to adjust the cabin temperature up, to ensure that the cabin stays at a comfortable temperature when the aircraft skin is exposed to about -50C at high altitudes. The last bit of the distribution system has to do with ventilation. The ventilation system:

- Removes odours from lavatories and galleys, - Causes air flow across the different temperature sensors in the flight, passenger and cargo compartments. - Supplies air flow into the bulk cargo compartment.

Now, thats as far as were going to take it in terms of air distribution and temperature control. Weve mentioned the word pack a lot during this lesson, and its now time to have a specific look at what packs are and how they operate.

Air Conditioning Packs The word pack, as we mentioned earlier, stands for pressurization and air conditioning kit. There 777 has two identical packs that essentially provide the housing for all the cleaning, filtering and cooling of the ram air that is

Script Document


conditioned to enter the cabin. Packs discharge air at a value that is sufficient to maintain the flight deck, passenger cabin and cargo areas to their automatically or manually commanded temperatures. Two redundant channels in two separate controllers control each of both packs. If one channel in a controller fails, a system logic commands the other channel to assume command immediately. And so, we have bleed air, but what exactly happens to it when it goes through either pack? The air flow control system includes the following items in the packs, in the following sequence:

- Primary heat exchanger, - Air Cycle Machine (ACM) compressor, - Secondary heat exchanger, - Water collector, - Reheater, - ACM turbine 1, - Condenser, - ACM turbine 2, - Check valve, - Mix Manifold.

Air from the packs is mixed with recirculated air in the mix manifold. This air moves through the air ducting system to the passenger zones. The flight deck receives air directly from the left pack. So firstly, bleed air from the pneumatic system is run through a primary heat exchanger, where it is put into contact with ram air in order to lower the bleed airs temperature. A quick refresh: Remember that bleed air is very hot, highly pressurized compressed air that is extracted from the high pressure compressor stages of the engines in order to feed the aircraft with pneumatic power. After the first stage of heat exchange, air flows into the ACM compressor stage. The compressor stage compresses air from the primary heat exchanger to increase its pressure. In the process, heat is also produced therefore the airs temperature increases once again. Because of this, and after air has passed through the ACM compressor, it passes through a secondary heat exchanger where again it is put into contact with ram air. Air is then ducted into the condenser and reheater stage. In here is where air flows over both heat exchange cores so that they absorb heat from the air. This temperature decrease induces condensation of moisture in the air into water, so that then, a water collector can remove it from the air. During the reheater stage, air is heated once again to add thermal energy to it so that it can later be converted to mechanical energy to operate the turbines in the ACM. The energy that is released in the turbines turns the shaft of the air cycle machine. Finally, downstream of all the components we just mentioned, is a check valve. The check valve controls the direction of the flow from the packs into the mix manifold and prevents backward flow.

Script Document


Ok, lets talk a bit about the actual operation of packs, which is a more important concern at this stage of your 777 training. There are two pack switches, L & R. The switch is a pushbutton that has the following positions:

- AUTO: the pack is automatically controlled by the system. - OFF: the pack flow control valves are commanded closed denying the flow of bleed air through them.

Whenever the OFF light illuminates in amber, it is a signal of some form of fault in the related pack. This could be:

- The pack valves are failed closed, - An overtemperature condition may have occurred downstream of the compressor or the pack itself. Duct

temperature transmitters aid in determining the airs temperature and relaying them to the computers that need the information.

- Bleed air pressure is inadequate to commence flowing through the air flow control system. - The related pack switch is pushed OFF. - Or finally the R PACK valve is closed during an APU to PACK takeoff procedure. For more information on

this, check out the 777 GroundWork Bleed Air lesson. The detection of these faults, overheat protection, and pack control in general, are all automatic processes that do not require significant crew input. Furthermore, in any overtemperature condition or significant pack fault, the system commands an immediate shut down of the related pack. In this situation, the flight crew can attempt to restore pack operation by pressing the AIR COND RESET switch. Pushing it resets any closed pack flow control or trim air valves that have been held closed due to any of the fault conditions we mentioned before, and also resets fault protection. If there is any fault in the recirculation fans, the AIR COND RESET switch can also restart them. The air conditioning packs also protect themselves from faults whenever there are certain system malfunctions. Pack control in this case automatically enters a standby cooling mode in order to backup normal mode, instead of fully shutting off. If one pack is normal and the other is in standby cooling mode, the standby mode pack shuts off at lower altitude where outside air temperature is not as cold so as to be uncomfortable for passengers and crew. The important bit to emphasize is that if both packs are in standby cooling mode, or if one pack is inoperative for any reason, the remaining pack can adequately maintain the cabin pressurized. The cabin temperature will probably not be maintained as comfortably as during normal operation, but for sure the cabin will stay pressurized. In fact, during standby cooling mode, because of the lower cooling capacity, temperatures in the flight deck or cabin may be slightly higher at low altitudes, or when OAT is high. Speaking about cabin pressurization, lets now move on to the other main portion of this lesson: pressurization.

Script Document


Cabin Pressurization Overview As you may remember from your meteorology manual, or from old time high school lessons, the atmosphere is made up of air. In the tropospheric region of the atmosphere, the volume of air decreases at a nearly constant rate as altitude increases. They are inversely proportional, because air density becomes thinner and thinner as we climb. Above the troposphere, there is the stratosphere, which is where most airliners fly due to the relatively calm air and cold temperatures. Unfortunately in the lower stratospheric region between 30,000ft and 40,000ft the amount of ambient air is very less for humans to be able to survive without being exposed to the risks of hypoxia, and other hazards. Hypoxia is an oxygen deficiency condition that presents symptoms only after a period of time since the supply of oxygen has been detained. Just to clarify, this does not happen only between 30,000 and 40,000ft, but can happen during any phase of flight. This is particularly dangerous for the flight crew, as it may lead to a pilot incapacitation. There is no emergency worse than a pilot incapable of safely operating his aircraft. For this reason, the 777s cabin must be pressurized so that an adequate and constant supply of breathable air is provided to the flight crew, passengers and or any pets you may have chosen to carry in your FS flights.

Cabin Pressure Control From the earlier section of this lesson we saw that normally, the air conditioning packs would supply conditioned air into the flight deck and passenger cabin. Regulating the discharge of air from the aircraft overboard controls cabin pressure throughout the aircrafts operating altitude envelope, between -2000ft and 43,100ft. Air supply cabin pressure controllers, or ASCPCs, control this discharge of air from the aircraft by modulating between open and closed a pair of valves in the lower fuselage. These valves are called outflow valves, and the 777 has one forward and one aft outflow valve. Here are two concepts you should become familiar with: Cabin altitude and pressure differential. Cabin altitude is the altitude at which the aircraft pressurizes its cabin to. The aircraft may be flying as high as FL400, but cabin altitude may be only 8000ft. This means that cabin air is pressurized to a level similar to that you may encounter at 8000ft under ISA conditions. It may sound like a simple clarification, but keep in mind that as altitude increases, ambient pressure decreases therefore as cabin altitude increases, cabin pressure decreases. And so, the difference between cabin pressure and outside ambient pressure when the aircraft is in flight is known as the pressure differential. There is a limit pressure differential that an aircraft can withstand without sustaining damage to the fuselage or other components.

Script Document


Now, because of certain unwanted conditions that may develop, the pressurization system has relief valves, one positive and one negative. The positive pressure relief valves come into action when the aircrafts internal air pressure exceeds ambient pressure by a considerable amount. In this case, they discharge bleed air overboard to try and match ambient pressure. The negative pressure relief valves do the opposite. They vent bleed air into the airplane whenever ambient pressure is considerably higher than the aircrafts cabin pressure. This condition can occur during a rapid aircraft descent to low altitudes.

Outflow Valves As we talked about earlier, one forward and one aft outflow valve permit the discharge of air overboard in order to maintain the required cabin pressure. Both valves are identical, but one is obviously located in the rear and the other in the forward fuselage. Both valves are controlled with two separate channels with backup power sources, to provide system redundancy. The discharge of air is sequenced so that about 20% of air is bled from the forward outflow valve, and the remaining 80% from the aft valve. This distribution improves cabin ventilation and smoke removal. In case of a single valve failure, the remaining valve can adequately maintain cabin altitude and full ventilation rates. Now, both outflow valves can be controlled automatically or manually. Automatic operation hardly requires any input from the pilots, except during pre-flight Lets talk about each one, starting with automatic control.

Automatic Cabin Pressure Control The ASCPCs supply automatic operation of the cabin pressure control system. To engage the automatic control mode, the OUTFLOW VALVE switches in the overhead pressurization panel must be pressed, obviously checking that the AUTO light is illuminated. In this scenario, the cabin altitude controllers design a pressurization schedule for the cabin. Maximum cabin altitude for the 777 is 8,000ft, and if cabin altitude ever exceeds 11,000ft, the cabin altitude controller closes both outflow valves completely to bring the cabin pressure back up to a normal value. The cabin pressurization schedule is based on a standard flight profile comprising the following flight phases:

- Ground mode, or takeoff mode, - Climb, - Cruise, - Descent, - Landing mode.

Script Document


Lets talk about each case in particular starting with the ground mode.

Flight Profile On ground, the ASCPCs command the outflow valves to a full open position, to ensure that the cabin does not pressurize whenever the aircraft is stationary or during taxi to the runway. At speeds above 25kts, and once the pilots have commanded the engines to higher than 60% N1 for a few seconds, the ASCPCs understand a takeoff condition and do the following: They move the outflow valves to a slightly less open, more closed position. This pre-pressurizes the cabin to 0.1psi above takeoff field pressure to avoid any uncomfortable pressure bumps for the passengers during rotation. Whenever the air ground sensing system switches from ground to air, or when the aircraft climbs beyond 1,000ft, the ASCPCs enter climb mode. When engaged, the climb mode calculates and sets a target cabin altitude during cruise, keeping the 8,000ft limitation in mind. Cabin rate of climb is also calculated and limited to lesser than 500 ft/min. Outflow valves are modulated to give a smooth change in cabin pressure. Another limitation you should keep in mind, is that cabin to ambient pressure differential may not exceed 8.6 psi. The climb pressurization mode has two submodes: FMC climb and internal climb mode. Whenever FMC data is correct, the autopilot is engaged and VNAV is on, FMC climb mode takes priority. In this scenario, airplane cruise altitude, landing field elevation and cabin limits are taken into account to design the pressurization schedule. When FMC climb is not engaged, internal climb takes place. In this mode, cruise altitude is set to 43,100ft and target cabin altitude is set to 7,950ft. If the takeoff was made from an airport higher than 8,000ft, the cabin descends to the cabin cruise altitude while the aircraft if climbing. Now, once the climb has stopped and aircraft altitude does not change more than 100ft for more than three minutes, the ASCPCs enter the cruise mode. At cruising altitude, the ASCPCs adjust the outflow valves to keep the cabin pressure constant at the value calculated for the cabin altitude target. As simple as that. When the aircraft starts descending and has descended at least 1000ft below cruise altitude, the ASCPCs command the descent mode. During the descent mode, the outflow valves gradually open to give a smooth change in cabin pressure. Target cabin altitude is set so that the cabin pressure value is slightly higher than landing field pressure.

Script Document


Similarly to during climb, when the aircraft descends, the pressurization system also follows one of two logics: FMC descent, or internal descent. During FMC descent, landing altitude data is used to design a descent pressurization schedule so that the cabin reaches its target cabin altitude when the aircraft reaches landing altitude. The normal FMC descent schedule provides a 300fpm cabin altitude descent rate until a cabin altitude that is slightly lower than the landing altitude has been reached. This ensures a slightly positive cabin pressurization during touchdown. When FMC information is lacking, the pressurization logic during descent follows the internal descent mode, where the cabin pressure is changed at a maximum rate until either the target cabin altitude has been reached, or the 8.6psi pressure differential limit imposes a restriction for cabin pressurization. If the landing is planned at an airport at above 8,000ft, the descent schedule allows the cabin altitude to exceed the 8,000ft limitation in order to match the field altitude. If for any reason the landing altitude data from the FMC becomes unavailable, the flight crew may select a landing altitude with a knob located in the pressurization overhead panel, however this becomes part of the manual way of controlling cabin pressure. If no landing altitude data has been input by the flight crew, the system assumes 1000ft as a standard value. Lastly, when the aircraft lands, the ASCPCs command the outflow valves to open fully and depressurize the cabin at a rate of about 500fpm. If 45s have passed after touchdown and the outflow valves are not sensed fully open, the cabin commands a 2000fpm depressurization rate.

Manual Cabin Pressure Control In case of any significant system fault, or if it becomes imperative for safety reasons, the outflow valves may also be controlled manually. This would imply that the whole pressurization system would enter manual control. Cabin rate of change and cabin altitude may be controlled by modulating between open and closed the position of both outflow valves. The OUTFLOW VALVE MANUAL switches allow this. If the switch is moved up to the OPEN position, the related outflow valve is driven to move towards open, similarly for when the switch is in the CLOSE position. Intermediate valve positions can be selected, it does not have to be either fully open or fully closed. Whenever the manual mode is selected in either outflow valve, pressurization system indications automatically appear in the EICAS display, which leads us to the cockpit indications part of this lesson.

Pressurization Cockpit Indications Pressurization system indications include:

- Duct pressure, - Cabin altitude,

Script Document


- Cabin altitude rate of change, - Outflow valve source of control, - Outflow valve position, - Cabin pressure differential, - Landing altitude selection, - Landing altitude.

Automatic display of the pressurization system indications occurs when either outflow valve switch is operating in manual mode, or when:

- Landing altitude is MAN, - Cabin altitude is above a normal range, - Cabin pressure differential is above the normal range, or is excessive, - Pneumatic duct pressure is below normal range, when the engine is running,

Lesson Summary This lesson covered all the main aspects of the air conditioning and pressurization systems aboard the 777. It is interesting to see that a considerable level of redundancy is built into both systems, with multiple controllers, channels, and a manual backup in case things go wrong. Here are a few key points to remember about air conditioning and pressurization:

- The CTCs set the packs to supply the coldest air necessary for the zone that needs the lowest mix manifold temperature.

- Cabin zones that need a higher temperature receive mix manifold air mixed with trim air. - Recirculation fans help bring down the air demand over the pneumatic system and the packs by taking

cabin air, running it through a series of physical and chemical filters and then re-inserting it back to the cabin.

- Packs are automatically shutdown whenever there are significant system faults such as an overtemperature condition.

- Any one normally operating pack can adequately maintain cabin pressure. - As aircraft altitude increases, ambient pressure decreases therefore as cabin altitude increases, cabin

pressure decreases. - 20% of air is bled overboard from the forward outflow valve, and the remaining 80% from the aft outflow

valve. Either outflow valve can adequately maintain the cabin pressurized. - Outflow valves are normally operated automatically, following the typical flight profile. Manual control is

achieved with cockpit switches. Remember: Drive the outflow valve to closed if you need more air in the cabin. This will increase cabin air pressure, therefore reduce cabin altitude. Vice versa when you drive the outflow valve to open.

This lesson wraps up the air systems section of the 777 Groundwork. We hope you enjoyed it, and stay tuned to the following sections! Until then, thanks for watching and Throttle On!

Script Document


aoa_777_groundwork_ac-pressurization_transcript.pdf

Documents