Stale Recycled Airliner Cabin Air The Myth and the Facts

             Stale Recycled Airliner Cabin Air

                      The Myth and the Facts

                                                                                                      Third Edition, Vol. 2

OK.! Now this is a topic that always spurs a debate over whether you can get sick or catch some sort of virus flying in an Airliner cabin. So, I’m going to clear up any confusion or misconception anyone might have regarding Airline cabin air quality right here, right now.

First, we have to have a little education in aircraft pressurization systems and environmental systems. These two systems work together to create a comfortable cabin atmosphere for the multitudes of paying customers on every scheduled airline flight. I’m not talking about a charter flight in a non-pressurized Piper Chieftain or any aircraft like that. Only pressurized cabins. Whether it is “Big Iron” stuff, or a GA Cessna P-210 or a Beechcraft 58 “P-Baron”.  This topic is really just for the “Big Iron” though, as this is where the misinformed usually speak out about this subject the most. Although, the P-210 and the “P-Baron” utilize essentially the same concept of cabin pressurization, the environmental systems are not integrated on the GA aircraft like they are on the Jet Airliner.

This gets pretty technical, so try to follow along if you can. On jetliners, bleed air is supplied to the Environmental Control System (ECS) from a compressor stage of each turbine engine. This source of air is taken from or “bled” off of the engine, hence the term “bleed air”. The temperature and pressure of this bleed air varies according to which compressor stage is used, and the power setting of the engine, but several hundred degrees Fahrenheit in temperature is not uncommon. A Manifold Pressure Regulating Shut-Off Valve (MPRSOV) restricts the flow as necessary to maintain the desired pressure for downstream systems, and to give the engine priority at high power settings such as at take-off. A certain minimum supply pressure is needed to drive the air through the system. A low supply pressure as possible is used, because the energy the engine uses to compress the bleed air would not be available for propulsion. For this reason, air is commonly drawn from one of two ports at different compressor stage locations. When the engine is at low pressure (low thrust or high altitude), the air is drawn from the highest pressure bleed port. As pressure is increased (more thrust or lower altitude) and reaches a predetermined crossover point, the High Pressure Shut-Off Valve (HPSOV) closes and air is selected from a lower pressure port. The reverse happens as engine pressure decreases. Follow me so far?

To achieve and maintain the desired cabin temperature, the bleed-air is passed through a heat exchanger called a “pre-cooler”. Air bled from the engine fan is blown across the pre-cooler, which is generally located in the engine pylon, and absorbs excess heat from the service bleed air. A Fan Air Modulating Valve (FAMV) varies the cooling airflow to control the final air temperature of the service bleed air. The heart of the “Cold Air Unit” (CAU) is the “Air Cycle Machine” (ACM). Some aircraft, including early 707 jetliners, used vapor-compression refrigeration not unlike that used in our home or automobile air conditioners. These are called “Vapor Cycle Machines”, (VCM). The ACM type uses no Freon. The air itself is the refrigerant. The ACM is preferred over VCM’s because of reduced weight and maintenance requirements. Most jetliner’s CAU’s are called cooling “PACKs” which stands for Pressurization Air Conditioning Kits. These cooling PACKs are located in the wing to body fairing between the wing and the fuselage. On some jetliners, the cooling PACKs are located in the tail. Others are located in the front of the aircraft beneath the flight deck. Nearly all jetliners have two cooling PACKs, and larger aircraft have three.

These air conditioning PACKs receive clean filtered, compressed air from the compressor stages of the aircraft’s jet engines while in flight, or when on the ground, from the Auxiliary Power Unit (APU) or a ramp side Air Conditioning Pack cart. The quantity of bleed air flowing to the cooling PACK is regulated by the Flow Control Valve (FCV). One FCV is installed for each PACK. A normally closed “isolation valve” prevents air from the left bleed system from reaching the right PACK (and vice versa), although this valve may be opened in the event of loss of one bleed system. Downstream of the FCV is the CAU. In the ACM type of air conditioner, the bleed air enters the primary “ram air heat exchanger”, where it is cooled by either ram air, expansion or a combination of both. The cold air then enters the compressor, where it is re-pressurized, which reheats the air. A pass through the secondary “ram air heat exchanger” cools the air while maintaining the high pressure. The air then passes through an “expansion turbine”, which expands the air to further reduce heat. Similar in operation to a turbocharger unit, the compressor and turbine are on a single shaft. The energy extracted from the air passing through the turbine is used to power the compressor. The air flow then is directed to the re-heater before it passes to the condenser to be ready for water extraction by the water and vapor extractor. The air is then sent through a water separator, where the air is forced to spiral along its length and centrifugal forces cause the moisture to be flung through a sieve and toward the outer walls where it is channeled toward a drain and sent overboard. Then, the air usually will pass through a water separator coalescer and then the “sock”. The sock traps the dirt and oil from the engine bleed air to keep the cabin air cleaner. This water removal process prevents ice from forming and clogging the system, and keeps the cockpit and cabin from fogging during ground operation and at low altitudes. The temperature of the cooling pack outlet air is controlled by adjusting the flow through the “ram air system” and modulating a Temperature Control Valve (TCV) or commonly called a “mixing valve”, which bypasses a portion of the hot bleed air around the ACM and mixes it with the cold air downstream of the ACM turbine. The cooling PACK exhaust air is ducted into the pressurized fuselage, where it is mixed with filtered air from the recirculation fans, and fed into the “mixing chamber”. On nearly all modern jetliners, the airflow is approximately 50% “outside air” and 50% “recirculated filtered air”. Modern jetliners use High Efficiency Particulate Arresting (HEPA) filters, which trap more than 99% of all bacteria and clustered viruses. This recirculated filtered air is usually employed based on the bleed air available at that particular phase of the flight. Conditioned air distribution utilizes two entirely independent systems. The “Conditioned Air Distribution System” routes the mixture of hot and cold air to the passenger cabin and cockpit. This pressurized and “conditioned” air from the “mixing chamber” is directed into the cabin via the ventilation ducts covered by inconspicuous “grills” displaced throughout the cabin within the various “zones” of the aircraft. Temperature in each zone may be adjusted by adding small amounts of “trim air”, which is low-pressure, high-temperature air tapped off the cooling PACK upstream of the TCV. The “Individual Air Distribution System” routes only cooled air from the ACM air conditioning PACKs to the individually controlled overhead nozzles, called “Gaspers”, (the small, circular vents above each passenger seat). A master control for gaspers is located in the cockpit, and gaspers may be temporarily turned off during certain phases of flight, when the load on the engines from bleed air demands must be minimized (e.g. take-off and climb). This is when the recirculation air fans would normally be turned on.

Airflow into the fuselage is fairly constant, and cabin altitude/pressure is maintained and regulated by varying the opening of the Out Flow Valve (OFV). The OFV is really a cabin pressure regulator that “ventilates” the pressurized cabin air overboard to maintain the selected “cabin altitude”. Most modern jetliners have a single OFV located at the aft pressure bulkhead or near the bottom aft end of the fuselage. Some larger aircraft have two OFV’s. In the event the OFV should fail closed, at least two Positive Pressure Relief Valves (PPRV) or “Safety Valves” and at least one Negative Pressure Relief Valve (NPRV) are provided to protect the fuselage from over and under pressurization. Aircraft cabin pressure is commonly pressurized to a “cabin altitude or pressure altitude” of 8,000 feet or less. That means that the differential pressure is 10.9 pounds per square inch (75 kPa), which is the ambient pressure at 8,000 feet (2,400 m). (Note that a lower cabin altitude is a higher differential pressure). The cabin pressure is controlled by a “Cabin Pressure Schedule”, or “Cabin Altitude Selector” which associates the aircraft altitude with the cabin altitude and adjusts the Outflow Valve accordingly. The cabin Outflow Valve regulates the amount of air that exits the cabin to maintain the cabin altitude, (pressure altitude), at a comfortable level at varying aircraft altitudes. Without the regulating aspect of the Outflow Valve, the aircraft fuselage would blow up like a balloon until it burst. The Outflow Valve is like a variable, calibrated “leak” and varies the area of the opening to account for ascents and descents, (pressure differentials). Even rapid changes in aircraft altitude can be compensated for by the OFV. New airliners have lower maximum cabin altitudes which help in reducing passenger fatigue during longer flights. The primary purpose of the pressurization is to maintain the oxygen percentage to acceptable levels within the cabin for human and cargo, (animals), health and comfort. The cargo hold beneath the passenger cabin is also pressurized and heated. Still with me?

The atmosphere at typical Jetliner cruising altitudes is generally very dry and cold. The outside air pumped into the cabin on a long flight has the potential to cause condensation or a high humidity atmosphere which may cause corrosion and is thus eliminated as mentioned earlier. Consequently, when humid air at lower altitudes is encountered and drawn in, the ECS dries it through the warming and cooling cycle and the water separator mentioned earlier also, so that even with high external relative humidity, the relative humidity inside the cabin will usually not be much higher than 10%. Although low cabin humidity has health benefits of preventing the growth of fungus and bacteria, the low humidity causes drying of the skin, eyes and mucosal membranes and contributes to dehydration, leading to fatigue, discomfort and health issues. Because of the high output the pressurization system is capable of, the volume of air in a modern airliner cabin is constantly exchanged at a rate of approximately every two to three minutes. This means that every two to three minutes, the air in the cabin has circulated in and out and fresh air has taken its place. Only in some cases is 50% of the cabin air recirculated, but it’s mixed with fresh, conditioned outside air, as discussed above. Think about it. If the air in the cabin never circulated in and out, eventually all of the oxygen within the cabin would be depleted and exchanged for carbon dioxide by all of the breathing humans on board, and we’d all pass out and eventually die. Understand?  So, there really is no “stale recycled cabin air”. Not possible! Can’t happen! I haven’t lost you yet have I?

So, unless some chucklehead behind you, or in the close proximity of you, sneezes and the resultant spray goes airborne, and you inhale some of it, your chances of getting sick from the air inside that pressurized aluminum tube, hurling through the air at 400+ miles per hour, are slim to none at best. You’re more likely to pick up an unwanted virus from the surface of the seats, tray table, the Emergency Procedure Card, the In Flight magazine, or the lavatory.

Let’s recap in simpler terms. The “Pressure Vessel” or cabin/fuselage of the aircraft, is pressurized by a continuous flow of fresh bleed air from the compressor section of the turbine engine. This freshly compressed, heated air then passes through an assortment of heat exchangers to cool the heated air before it passes through some valves and into an air conditioning unit where it is cooled even further. This cooled air is sometimes mixed with bypassed heated engine compressor bleed air to achieve a comfortable temperature that is then filtered and pumped into the cabin. The air that is pumped into the fuselage is constantly being exchanged about every two to three minutes via fresh bleed air. This cycle is continuous even while sitting on the ramp while you’re boarding the aircraft and while seated waiting for push-back. The APU at the tail of the airplane is providing the necessary air to operate the cooling PACKs. These PACKs are basically always operating until the aircraft is completely shut down. The only time these CAU’s are shut down during service, is that moment when the main engines are started and the APU is shut down. The CAU is then restarted on main engine bleed air. The pumped up cabin pressure is regulated to the selected “cabin altitude” by the Outflow Valve(s). Thus providing a comfortable, oxygen enriched, fresh air atmosphere within the “Pressure Vessel” or passenger cabin and cockpit of the aircraft. So, simply put. The cabin altitude and temperature is kept at a comfortable level by the pressurized “conditioned” air provided by the engine bleed air that has passed through the CAU and all of its heat exchangers and valves, that is then forwarded on to the pressure vessel, which, is maintained at a safe pressurization differential by the Outflow Valve. Got it?

So, there you go. I hope I didn’t lose you along the way. You can breathe easier now because you know the facts about stale cabin air in the modern and not-so-modern Jet Airliner. And on your next flight, when your seat mate complains about the possibility of getting sick from the recirculated stale air he’s breathing in the aluminum can you’re riding in, you can just smile at him and chuckle at his ignorance, because now you know better. Remember, knowledge is power.