The air conditioning system provides a mix of outside air and recirculated cabin air, maintaining a regulated temperature throughout the aircraft. This system also delivers conditioned air to the flight deck's shoulder heaters for additional comfort. Furthermore, it ensures ventilation to the passenger cabin, including the lavatories, galleys, and, if opted, the overhead crew rest compartments. The system's features, such as pack control, zone temperature regulation, cabin air recirculation, fault identification, and overheat prevention, operate automatically. In case of any system malfunctions, backup control modes are also automatically engaged. The aircraft is segmented into seven distinct temperature zones, comprising the flight deck and six zones within the passenger cabin.
Air from the outside is channeled to four electric cabin air compressors (CAC) via two exclusive inlets located in the wing-to-body fairings. To prevent debris from entering the CACs during typical ground operations and the landing stages of a flight, deflector doors are deployed at the CAC inlets. These doors may be retracted on the ground if the outside temperature falls below 35°F (2°C) or rises above 95°F (35°C).The air is then compressed and its flow is regulated by the CACs before being delivered to two identical air conditioning packs, with each pack being served by two dedicated CACs. A single CAC is capable of supplying enough air to support its corresponding pack across all modes of operation. The packs are managed by two identical control systems, ensuring that if one fails, control is seamlessly transferred to the alternate system.
The airflow to the packs is adjusted by managing the output of the cabin air compressors. The CAC output is automatically increased during times of high demand (for instance, to compensate for a malfunctioning pack) or reduced during instances where electrical power must be conserved for other essential systems. Under normal conditions, the flow of outside air is regulated to guarantee a consistent minimum rate of ventilation.
Recirculation fans support the air conditioning packs in keeping the cabin's ventilation rate steady. These fans pull air from the cabin through filters and then reintroduce it back into the air distribution system, ensuring the air is conditioned.
The flight deck is supplied with 100% outside air that has been conditioned, whereas the passenger cabin receives a blend of outside air and filtered, recirculated air. To prevent any smoke from infiltrating the flight deck, its pressure is kept marginally higher than that of the passenger cabin. In scenarios where one air conditioning pack is deactivated and its corresponding lower recirculation fan is operational, some recirculated air is also directed to the flight deck.In cases of system failure or when it's necessary to conserve electrical power, the flow of outside air is diminished. This measure helps safeguard the electrical power distribution system and ensures there's enough power for other vital aircraft systems.
Cabin crew have the capability to decrease the amount of outside air flowing into the cabin, depending on the number of occupants. However, this reduction is controlled to make certain that a minimum level of airflow is always maintained in the cabin.
Air that is expelled from the passenger cabin is either directed into the upper recirculation system or down to the lower deck. From there, it can be either released outside through outflow valves or pulled into the lower recirculation system, where it's mixed with air from the packs before being sent through the distribution ducts.
The lower recirculation system features liquid-to-air heat exchangers for additional cabin cooling, which lessens the cooling load on the CACs. This cooling function is part of the Integrated Cooling System (ICS), also used for cooling the galleys. Additionally, a dehumidification system in the passenger compartment's crown area automatically activates when electrical power is available. It employs two zonal dryers (forward and aft) to extract moist air from the area, recirculating dry air back while venting moist air below the cabin floor.
The Alternate Ventilation System (AVS) offers a backup method for ventilating both the cabin and the flight deck when both air conditioning packs fail to operate. This system is made up of a switch located on the flight deck and a specialized flush inlet valve and duct that channel outside air into the system, bypassing the left pack outlet. When the aircraft is not pressurized, activating the AVS switch to the ALTN (alternate) setting triggers a valve to open, permitting the inflow of fresh air from outside directly into the air distribution network.
Warm trim air from each air conditioning pack is blended with the conditioned air from the pack to manage the temperature within each zone. The trim air system is designed to serve two cabin zone ducts and one flight deck zone duct specifically. The temperature in the cabin zones is adjusted by modulating the temperature at the pack's outlet and introducing warm trim air into the supply ducts for each zone via the trim air valves. This process ensures that the desired temperature is achieved across all seven temperature zones.
The air conditioning system in the forward cargo compartment is designed for temperature regulation and delivering conditioned air, catering to the needs of perishable, live, or other types of temperature-sensitive cargo. This compartment receives ventilation and conditioned air through the forward electronic/electrical (E/E) cooling system's exhaust, a specialized refrigeration unit, an in-line electric heater, and an exhaust fan specifically for the forward cargo. With automatic ventilation and the option to select between LOW and HIGH flow settings, the system effectively prevents odors from the cargo area from reaching the flight deck or passenger cabin.
The forward and bulk cargo compartments are equipped with their own temperature management systems, allowing for independent temperature control. In contrast, the aft cargo compartment receives heating, but its temperature is not regulated. An insulated curtain is used to divide the aft and bulk cargo compartments, maintaining separation between the two areas.
The forward cargo area is equipped to transport live animals, with ventilation and heating supplied through the forward electronic/electrical (E/E) cooling system's exhaust, an electric heater installed in-line, and a dedicated exhaust fan for the forward cargo. The system's automatic ventilation feature is specifically designed to prevent odors from the cargo space from reaching the flight deck or passenger cabin.
Ventilation and heating for the bulk cargo compartment are supplied by cabin air, which is directed through a supply fan and an in-line electric heater. Additionally, the compartment benefits from residual heat originating from the aft electronic/electrical (E/E) cooling system's exhaust, which enters from beneath the compartment floor.
The aft cargo compartment receives ventilation and heating from the exhaust of the aft electronic/electrical (E/E) cooling system, which enters from beneath the compartment's floor. Further, residual heat is also present due to air leaking through the insulated curtain from the bulk cargo compartment. The temperature in the aft cargo area is not regulated, making it unsuitable for transporting live animals.
The cooling and ventilation systems for the forward equipment are designed to maintain optimal temperatures and airflow for the electrical and electronic devices located on the flight deck and within the forward electrical and electronic (E/E) compartment's equipment racks. These systems utilize internal fans and valves to route cabin air over the equipment, dissipating heat by either expelling the warmed air through a specialized vent valve to the outside or redirecting it to the forward cargo compartment when additional heating is needed there.
There are two fans provided for the cooling system supply: a primary fan and a secondary backup fan. In the event that the primary fan ceases to function, the backup fan is automatically activated to ensure continuous cooling and ventilation.
The cooling system for the aft equipment mirrors the design and operation of the forward equipment cooling system. It serves to cool and ventilate the aft electronic/electrical (E/E) equipment compartment, as well as to provide heating and ventilation for the aft cargo compartment. Utilizing internal fans and valves, the aft system channels cabin air towards the equipment in the aft bay, then discharges the heated exhaust air either through a specific exhaust valve to the outside or into the aft cargo compartment, depending on the need.
This system is equipped with two supply fans for cooling: a main fan and an alternative backup fan. Should the main fan cease to function, the backup fan is designed to start automatically, ensuring the system continues to operate efficiently.
The Power Electronics Cooling System (PECS) is a dedicated liquid-based cooling mechanism designed for the large motor power distribution setup situated within the aft electronic/electrical (E/E) equipment compartment. Additionally, it extends its liquid cooling capabilities to the auxiliary cooling units and their corresponding motor controllers that are part of the Integrated Cooling System. This system is comprised of two separate liquid cooling circuits, with each circuit being serviced by a pump package that includes two fully redundant pumps.
Control of this liquid cooling system is not facilitated from the flight deck; its functioning is entirely automated, eliminating the need for crew intervention under both standard and irregular operational scenarios.
The Integrated Cooling System (ICS) operates as a unified refrigeration mechanism, tasked with cooling galley carts and aiding in the cooling of recirculated cabin air. This system is seamlessly interconnected with both the air conditioning system and the liquid cooling system, playing a pivotal role in the overall management of the aircraft's thermal loads. Control of the ICS is not accessible from the flight deck, as there are no flight deck controls for this system.
The miscellaneous equipment cooling system is designed to regulate the temperature of devices scattered throughout different sections of the aircraft. It specifically targets the in-flight entertainment (IFE) systems and other non-critical devices positioned in the crown and lower lobe regions that are not covered by other cooling systems. This system operates autonomously, activating whenever the aircraft is powered on. The power supply to the miscellaneous cooling system can be disconnected through the use of the IFE switch located on the overhead panel of the flight deck.
Cabin pressurization is managed by adjusting the release of conditioned cabin air via outflow valves. The system includes two such valves, positioned at the front and rear of the cabin. Under normal circumstances, the release of cabin air is evenly distributed between these two valves, with either one capable of sustaining cabin altitude and full ventilation rates on its own.
To safeguard the aircraft's fuselage from excessive pressure differences, both positive and negative pressure relief valves are in place.
The pressurization system operates in both automatic and manual modes. For automatic operation, no particular action is required from the flight crew beyond the standard procedures for inputting data into the Flight Management Computer (FMC).
During flight, the Cabin Pressure Control System (CPCS) functions in one of three modes: climb, cruise, or descent.
The system leverages external pressure readings and the flight plan information from the Flight Management Computer (FMC) to establish a pressurization schedule. This schedule ensures a comfortable ascent of the cabin to the cruising altitude.
Before takeoff, the system initiates a slight positive pressurization to facilitate a seamless transition to the cabin's altitude climb schedule upon aircraft rotation.
In climb mode, the cabin altitude incrementally rises in accordance with the aircraft's climb rate and the planned cruise altitude in the flight plan. If the FMC's climb path includes a level-off segment, this period is factored into the overall time for reaching the climb's apex, with cabin altitude continuing to rise during this segment. Should there be an unplanned level-off or if VNAV is deactivated, the cabin altitude holds steady, provided there's no change in the aircraft's altitude. If the aircraft's ascent deviates above the FMC's climb trajectory and reaches the maximum cabin pressure differential, the cabin's rate of change then aligns with the aircraft's climb rate to avoid surpassing the maximum cabin pressure differential.
During cruise mode, the cabin altitude is maintained at no higher than 6,000 feet. As the aircraft begins its descent, the pressurization system transitions to descent mode, gradually reducing cabin altitude to slightly below the FMC's planned landing altitude, ensuring the aircraft lands while still pressurized. The landing altitude correction for barometric pressure is derived from the captain's altimeter settings.
For takeoffs from airports situated above 8,000 feet, the cabin pressure decreases to a predetermined altitude as the plane ascends. Conversely, for landings at airports exceeding 8,000 feet in elevation, cabin altitude is adjusted to 6,000 feet post-takeoff and maintained during cruise until it appropriately ascends to match the elevation of the destination airport.
Bleed air, sourced from the engines, is exclusively utilized for anti-icing operations on the engine cowls.
Bleed air is drawn from a specific bleed port on the engine. The anti-ice valves are activated upon the detection of icing conditions by the ice detection system or when the selector is manually set to ON. This allows hot bleed air to circulate through the inlet cowl, preventing ice formation. The anti-ice valves then close automatically once the icing conditions cease to be detected.