Boeing 787 Air Conditioning, Pressurisation and Oxygen

The Boeing 787 air-conditions and pressurises its cabin without engine bleed air. Outside air enters through dedicated inlets, four electrically driven cabin air compressors (CACs) raise it to cabin pressure, and two air-conditioning packs condition it before it is mixed with recirculated air and distributed. Forward and aft outflow valves control the cabin pressure, and fixed and portable oxygen systems protect the occupants if it is lost.

Why the 787 does this electrically is explained in Boeing 787 no-bleed systems architecture; this article covers how the air and oxygen systems are built and what the crew sees. It draws on the FAA Master Minimum Equipment List (MMEL), the FAA Flight Standardization Board (FSB) reports, the EASA type-certificate data sheets, Boeing's airport planning document and Boeing's 2007 AERO article, written before entry into service. The A220's equivalent is described in A220 air conditioning, pressurisation and oxygen, and the 787's other systems in articles such as APU and fire protection and landing gear and electric brakes.

Cabin air without bleed

Electrically driven compressors provide the cabin pressurisation function, with fresh air brought on board through dedicated cabin air inlets. The MMEL lists two left and two right CACs on every model: two feed each pack, and with both CACs on one side inoperative, that side's pack is treated as inoperative as well. Each CAC is associated with one of the engines' variable frequency starter generators (VFSGs): with one left CAC inoperative, for example, the VFSG associated with the other left CAC must work. The cabin air supply therefore depends on the electrical system.

Boeing's case for the compressors was efficiency. A bleed system takes air at more pressure than needed and wastes energy regulating it down in pre-coolers and modulating valves; adjustable-speed motor compressors produce air at the pressure required, and their inflow can be matched to the number of occupants. The engine installation is simplified by removing the pre-coolers, control valves and pneumatic ducting, and the Rolls-Royce Trent 1000 data sheet confirms that no Trent 1000 supplies air for cabin ventilation. The compressor inlets have their own ice protection, described in Boeing 787 ice and rain protection. For pilots coming from the 777-300ER, FSB Revision 7 rates the air conditioning differences, the electric CACs included, at level B, aided instruction.

Packs and temperature control

The CAC output flows through what Boeing calls low-pressure air-conditioning packs. For them the MMEL lists two air cycle machines, left and right ram air fans, each with its own motor controller, left and right pack control units of two channels each, and PACK switches with OFF and AUTO lights. Pack bay overheat detection has two channels and four loops, two for each pack; with the loops of one pack inoperative, that pack counts as inoperative.

Temperature is trimmed by zone. Two trim air pressure regulator valves, each with a TRIM AIR switch carrying FAULT and ON lights, work with zone trim valves, two for the flight deck and four for the cabin. Recirculation fans return cabin air to the distribution system: one upper and two lower fans on the 787-8 and 787-9, two upper and two lower on the 787-10; the other model differences are listed in Boeing 787 family and flight deck. Two ozone converters are fitted, and the original FSB report found the 787 qualified for operations in areas of high ozone concentration without special flight procedures.

On the ground, low-pressure conditioned air from a ground unit enters through an 8-in (20.3 cm) connection and goes directly to the passenger cabin, bypassing the air cycle machines. The 787-8 has one such port, the 787-9 and 787-10 two. The cargo compartments have their own heating, and the MMEL also lists forward cargo air conditioning as optional equipment. General principles are in air conditioning packs.

Equipment and power electronics cooling

The electronics bays are cooled by separate forward and aft equipment cooling systems, each with two supply fans, an exhaust fan and smoke detectors. Each system has an EQUIP COOLING switch with OVRD and AUTO lights, and the FSB lists a smoke EICAS message for equipment cooling. The power electronics are cooled differently, by a pumped liquid loop, the power electronics cooling system (PECS), for which the MMEL lists four pumps.

The 787-8 also has an alternate ventilation system, which the original FSB report lists as a new function compared with the 777; the 787-9 and 787-10 deleted its valve. Cabin air distribution in general is covered in cabin air distribution and equipment cooling.

Pressurisation and cabin altitude

Cabin pressure is controlled by two outflow valves, forward and aft. Each can be driven automatically or manually and has an OUTFLOW VALVE switch with MAN and AUTO lights. Automatic control has four channels, two in each of two valve control units, and each valve's manual control is associated with one of the valve control units. Two positive pressure relief valves and four negative pressure relief valves protect the fuselage against excessive positive and negative differential pressure.

For 777 pilots little changes: the original FSB report lists the pressurisation system, controls and indicators as the same as the 777's, with no change in procedures. The 787-9 and 787-10 add a 1:00 position to the outflow valve indication.

The flight deck indications listed in the MMEL are cabin altitude, cabin differential pressure and cabin rate of climb, with outflow valve position indications on the 787-8. Cabin altitude and differential pressure back each other up: either may be inoperative for dispatch if the other works and a chart converts one into the other.

The EASA certification basis of the 787-8 and 787-9 includes a special condition titled "High Altitude Operation / High Cabin Heat Load". Principles are in pressurisation principles and control and pressurisation safety valves and warnings.

Humidity and air quality

The MMEL lists, as optional equipment, a flight deck humidification system with a HUMID switch and ON light, and humidification systems for the overhead flight crew rest (OFCR) and overhead flight attendant rest (OFAR). Each may be inoperative for dispatch with its water supply valve closed. Optional zonal dryer systems are listed too.

On air quality, the original FSB report states that the environmental control system is designed to keep carbon monoxide concentrations in the passenger and crew compartments within the limits of 14 CFR 25.831, and that the flight deck door has no louvres or other means of ventilation. Hazards such as fumes are covered in carbon monoxide and cabin air hazards.

Abnormal pressurisation and dispatch

The MMEL's dispatch conditions show how much redundancy the system has, and what the crew loses with each failure:

• One pack inoperative. The CACs of the other pack must be verified before each departure, their VFSGs and the opposite trim air system must work, and, among other conditions, the flight must remain within 60 minutes of a suitable airport; with a crew rest occupied, the aircraft must also stay at or below FL 350.
• One outflow valve inoperative. It is locked in a fixed cruise position, manual control must be verified on the other valve, both packs must work and at least three CACs must be verified before each departure.
• Automatic control. One channel in each valve control unit may be inoperative if manual control is verified on both outflow valves before each departure.
• Both outflow valves, or all four automatic channels. Only unpressurised flight is allowed, with the cargo compartments empty or holding only ballast, empty cargo handling equipment or fly-away kits.

In flight, a loss of pressurisation brings in the oxygen systems below; the physiology is covered in decompression and hypoxia and hyperventilation.

Crew and passenger oxygen

For the flight crew, the MMEL lists a crew oxygen pressure indication system and an overboard discharge indicator disc. If the pressure indication is inoperative, the supply must be verified above the required minimum before each departure and the crew oxygen shutoff valve or valves verified open. In its compliance checklist, the original FSB report, written in 2011 for the 787-8 before entry into service, states that two hours' worth of oxygen is provided to meet the crew requirement of 14 CFR 121.333(b), and that flight crew oxygen masks are provided. Mask rules in general are covered in crew oxygen systems.

For passengers, the same report states that twelve minutes of passenger oxygen is available as basic equipment. The passenger service units carry more masks than there are seats, so cabin crew between their seats can use them, and portable oxygen equipment is installed for each attendant. The MMEL lists one passenger oxygen automatic presentation system, which may be inoperative only if the flight stays at or below FL 300, a passenger oxygen ON light, and power channels for each passenger service unit zone. With the whole passenger oxygen system inoperative, dispatch needs portable oxygen supplies, both packs and the pressurisation system working, appropriate altitude adjustments and a passenger briefing.

In the overhead crew rests, oxygen controllers are fitted at each seat and bunk, and an inoperative controller takes its seat or bunk out of use. Before occupying a rest, crew members must be familiarised with its emergency oxygen and decompression procedures. Portable oxygen bottles or units and portable protective breathing equipment (PBE) complete the system (passenger and portable oxygen).

The EASA data sheet lists, among the equivalent safety findings of each model, the crew's determination of the quantity of oxygen in the passenger oxygen system, the minimum mass flow of supplemental oxygen and a first aid portable pulse oxygen system. An equivalent safety finding means that the design meets the safety intent of a certification rule by other means.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.