Where does the air for the climate control system in an aircraft come from?

The airlines seem to compete with one another to make flying at 30,000 or 40,000 feet above the earth as comfortable, homey, and as similar to life on the ground as possible.

They boast good food and wine, provide magazines and pillows, and, something we take for granted wherever we are, supply us with air at a comfortable temperature and pressure.

Remarkably, this air comes from right outside the airplane, but before it reaches you it passes through a complex pressurization and air conditioning system, which in turn has several backup systems to assure maximum safety.

Fans in the engine of a Boeing 747 suck outside air into the engine’s compressor. A small percentage of this air is issued directly into the pneumatic system for air conditioning and pressurization, avoiding the combustion chamber so as not to pick up noxious exhaust elements.

Since the air reaches a temperature of about 400 degrees Fahrenheit in the compressor, it is necessary to cool it with new, cold air from the outside. Scoops under the fuselage draw in outside air (“ram air”), which at an altitude of 45,000 feet has a temperature of approximately –70 degrees Fahrenheit.

The 747 can control and maintain different temperatures in four different zones of the jet: the flight deck, main deck nose, main deck center, and main deck aft. Three air cycle packs, each consisting of a fan, a compressor, and a turbine on a common shaft, work in unison to cool the air in the pneumatic system. They are adjusted to cool the air to the maximum required by the zones; this air then feeds into a common “cold plenum,” or storage tank.

From there air goes directly to the zone calling for the coldest temperature. For zones requiring warmer temperatures, hot air may be added along the way to raise the cool air to the appropriate temperature.

The air cycle packs provide 8,000 cubic feet per minute of fresh air, and a distribution system adds another 2,000 cubic feet per minute of recirculated air.

Electrically operated valves control the amounts of hot or cold air entering the ducts and can route the air through any combination of engines and any combination of packs; that flexibility ensures safety in the event one or even two packs should become inoperative.

In the main cabin, air enters along the side walls at hat rack level. About 20 percent of the air goes up into the ceiling area, where fans direct it back into the supply ducts. Exhaust air leaves through side wall openings just above floor level and is circulated down to warm the cargo compartments, which are usually at 40 to 65 degrees Fahrenheit.

Just as heating, cooling, and recirculating are performed constantly while the plane is in flight, pressurization is maintained and, again, carefully monitored. The pilot has a selector on the flight panel by which he may control cabin pressure manually, or it may be controlled automatically.

Air pressure drops as the plane ascends; in order to counteract this, the aircraft must be “pumped up” to provide necessary comfort. Since the thin air pumped in from the stratosphere contains less oxygen, nitrogen, and other gases than we are accustomed to, more air is continuously pumped in, compressed, cooled, and released through the air conditioning system. A fully automatic system measures the amount of pressure required to avoid structural damage or passenger discomfort. At 45,000 feet, the atmosphere inside the airplane is comparable to that at 8,000 feet, which is perfectly comfortable since extreme and prolonged physical exertion isn’t necessary.

Upon descent, a system of valves releases the inside pressure gradually, so that passengers and crew are acclimated to the airport landing altitude, and the doors of the airtight airplane can be opened safely without air rushing in or out.