Air Systems - Air Conditioning 
Introduction
The air-conditioning system supplies conditioned bleed air and recirculated cabin air at a controlled temperature throughout the airplane, by processing bleed air from the airplane Pneumatic System (Engines or APU) or from a ground source.
The system supplies conditioned air to the flight deck shoulder heaters [and] ventilation for the cabin:
- Lavatories
- Galleys
- Individual passenger seat gaspers
Pack control, zone temperature control, cabin air recirculation, fault detection and overheat protection are all automatic
The airplane is divided into four temperature zones: the flight deck and three cabin zones.
Air Conditioning Packs
Two identical air conditioning packs cool bleed air from the airplane Pneumatic System (Engines or APU) or high pressure air from a ground source. Bleed air is pre-cooled before entering the pack.
Pack Controllers
The packs are controlled by two identical Pack Controllers. Pack output is automatically increased (High Flow) during high pack demand periods (to compensate for a failed pack or recirculation fan) or limited during high bleed air demands (such as with an engine inoperative).
Positioning the selector to AUTO or STBY sends an open signal to the Pack Valve.
Pack Valves
The Pack Valve is a flow control valve and requires air pressure to open. When open, the valve limits air flow to a scheduled rate based on airplane altitude.
Air Conditioning Automatic Mode
With the Pack Selector in the AUTO position, pack output temperature is determined by the Left and Right Cabin Temperature Controllers. Zone temperature is then satisfied by the Zone Temperature Controllers using Trim Air.
Conditioned air from the packs flows into a common manifold where it mixes with air from two recirculating fans. The mixed air is then ducted into the temperature controlled compartments of the airplane.
Pack High Flow
Certain operating conditions require the pack(s) to provide higher than normal air flow for adequate air conditioning and pressurisation. This is accomplished by a pack high air flow system. The system is automatic and armed for operation at all times.
The pack high flow system controls the pack valve to open to a second schedule and provide more airflow for:
- During ground operation whenever the APU or ground source is supplying air for air conditioning (both packs HIGH).
- If one pack is INOP.
- If a recirculation fan is INOP.
The pack high flow system is inhibited when:
- Wing anti-ice is ON and only one engine is supplying bleed air
- an Engine is INOP
- Flaps are not UP
- aft cargo compartment ARMED switch pushed
When a pack is operating in high flow, the displayed Maximum Thrust Limit for the associated engine is automatically reduced.
Air Conditioning Standby Mode
With the Pack Selector in the Standby mode, pack output temperature is determined by the position of the Pack Selector:
- N (Normal) - constant, moderate temperature (approximately 24°C}.
- C (Cool) - full cold (as dictated by ambient conditions).
- W (Warm) - full warm. Outlet temperature is controlled by ram air flowing across the pack heat exchangers and will be dependent upon ambient conditions.
Ground Conditioned Air Operation
When a ground source of conditioned air is available, it may be used to supply conditioned air directly to the cabin air distribution system, eliminating the need for pack operation. Air from the ground source enters the left air conditioning manifold and is routed through the mix manifold to the cabin distribution ducts.
Pack Non-Normal Operation
Pack Control, zone temperature control, cabin air recirculation, fault detection and overheat protection are automatic.
Pack INOP
When an automatic control fault or a pack overheat is detected, the pack automatically shuts down. The pack INOP illuminates and the EICAS advisory message L/R PACK TEMP displays.
• Pack Automatic Control System Fault
If the INOP Light extinguishes immediately upon selecting STBY, the fault is in the Automatic Control System. The pack temperature control valves freeze in the last controlled position. The pack cannot be reset in this case and must be operated in Standby.
• Overheat in Pack Output Air
If the INOP light remains after selecting STBY, the fault is a pack overheat. The Pack will automatically set to maximum cooling.
After the pack has cooled (5 minutes), an attempt to restore pack operation may be made by pushing the Pack Reset switch.
• Internal Pack Overheat
If the PACK OFF light illuminates together with the INOP light and the EICAS advisory message L/R PACK OFF, a pack trip due to an internal pack overheat is indicated.
The PACK OFF Light illuminates and the EICAS message L/R PACK OFF displays whenever the Pack Valve is Closed. The Pack Valve will Close if:
- The Pack Control Selector is OFF.
- A pack trip has occurred.
- (With associated Engine Bleed Air OFF light illuminated) - Loss of air to the pack.
Packs Off Take-Off
During a Packs Off Take-Off, both engine Bleed Air Valves may close due to low air demand, with associated L/R ENG BLEED OFF advisories.
Air Distribution
Conditioned air from the packs flows into a mix manifold where it mixes with air from two recirculation fans. Recirculation fans maintain overall cabin air circulation while allowing a reduction of cabin air ventilation, permitting the packs to be operated at a reduced flow. The mixed air is then ducted into the cabin.
Note: The 767 flight deck receives the same conditioned air (from both packs) as the passenger cabin.
Recirculation Fans
The Recirculation Fan Switches control Left and right recirculation fans. Each fan draws air which has been exhausted from the cockpit and passenger cabin through filters and returns that air to the mix manifold. There it mixes with conditioned air from the packs and is distributed again to the cabin. The purpose of recirculating cabin air is to maintain the desired level of passenger cabin ventilation while minimising the use of bleed air.
Both recirculation fans can be turned OFF for several minutes to provide a more rapid exchange of air.
The Recirculation Fan INOP Light illuminates and the EICAS advisory message L/R RECIR FAN displays whenever a Recirculation Fan fails or is not operating. A slight increase in fuel consumption occurs for each fan that is OFF.
Compartment Temperature Control
The airplane is divided into four temperature control compartments:
- Flight Deck
- Forward Cabin
- Mid Cabin
- Aft Cabin
The Cabin Temperature Controllers regulate the pack output air temperature to satisfy the temperature requirement of the compartment requiring the coolest air.
Trim Air
Hot Trim Air from the bleed air system (obtained downstream of the Pack Valves) is added through Trim Air Valves to control the temperature in each of the other compartments.
Each of the four temperature control compartments has an associated Temperature Controller which modulates Trim Air to satisfy the increased temperature requirements..
- AUTO - Automatic temperature control in the range 18° to 30° C.
- OFF - Closes the Compartment Trim Air Valve and the INOP light illuminates.
Compartment Temperature INOP
A compartment INOP light illuminates and the EICAS advisory message FLT DECK TEMP, FWD CABIN TEMP, MID CABIN TEMP, AFT CABIN TEMP displays to indicate:
- A fault in the Zone Temperature Controller
- the Trim Air switch is OFF
- the Compartment Temperature Control is in the OFF position (FWD, MID and AFT cab ONLY).
- excessive temperature of the conditioned air in the distribution ducting associated with the light.
Temperature Control with Loss of Trim Air System
During operation with the Trim Air System OFF, the packs attempt to maintain all compartments at an average temperature (24°C). The Trim Air OFF Light illuminates and the EICAS advisory message TRIM AIR displays when the Trim Air switch is OFF. The compartment temperature INOP lights also illuminate.
Shoulder and Foot Heaters
Flight crew shoulder and foot heat is provided by Electric heaters in the ducts that supply conditioned air to the side windows and rudder pedal areas respectively. The heaters operate inflight only.
