2
Environmental Control

Commercial jet aircraft are designed to carry passengers safely and comfortably from one point to another. The external environments of the aircraft include taxiing, takeoff, cruise, and descent; outside temperature from below −55°C (−65°F) to over 50°C (122°F); ambient pressure from about 10.1 kPa (1.5 psi) to 101 kPa (15 psi); and water content from virtually dry to greater than saturation. For aircraft to transport people in those extremes of external environment, they are equipped with environmental control systems (ECSs) that provide a suitable indoor environment.

A number of aircraft systems are involved in meeting the environmental needs, including the propulsion system (engines), which is a source of pressurized air; the pneumatic system, which processes and distributes the pressurized air; and the ECS, which conditions the pressurized air and supplies it to the cabin. For the purposes of this report, each component or subsystem that is integral in providing the necessary environmental conditions in the aircraft cabin is considered to be part of the ECS, even if it is technically part of another aircraft system.

This chapter first describes the important functions of the ECS, including background information on principles of ventilation, temperature control, and humidity control. It then describes the equipment and subsystems that make up the ECS; the descriptions of aircraft systems in this chapter apply principally to large aircraft (more than 100 passengers) and might not be applicable to all aircraft. Finally, standards that are potentially related to aircraft cabin environments and aircraft ECSs are examined.



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The Airliner Cabin Environment and the Health of Passengers and Crew 2 Environmental Control Commercial jet aircraft are designed to carry passengers safely and comfortably from one point to another. The external environments of the aircraft include taxiing, takeoff, cruise, and descent; outside temperature from below −55°C (−65°F) to over 50°C (122°F); ambient pressure from about 10.1 kPa (1.5 psi) to 101 kPa (15 psi); and water content from virtually dry to greater than saturation. For aircraft to transport people in those extremes of external environment, they are equipped with environmental control systems (ECSs) that provide a suitable indoor environment. A number of aircraft systems are involved in meeting the environmental needs, including the propulsion system (engines), which is a source of pressurized air; the pneumatic system, which processes and distributes the pressurized air; and the ECS, which conditions the pressurized air and supplies it to the cabin. For the purposes of this report, each component or subsystem that is integral in providing the necessary environmental conditions in the aircraft cabin is considered to be part of the ECS, even if it is technically part of another aircraft system. This chapter first describes the important functions of the ECS, including background information on principles of ventilation, temperature control, and humidity control. It then describes the equipment and subsystems that make up the ECS; the descriptions of aircraft systems in this chapter apply principally to large aircraft (more than 100 passengers) and might not be applicable to all aircraft. Finally, standards that are potentially related to aircraft cabin environments and aircraft ECSs are examined.

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The Airliner Cabin Environment and the Health of Passengers and Crew ENVIRONMENTAL CONDITIONS During flight, the aircraft cabin is a ventilated, enclosed environment whose occupants are totally dependent on the air provided by the ECS. The ECS is designed to provide a healthy and comfortable environment for the aircraft occupants from the time crew members and passengers first board for a flight until all passengers and crew members deplane after a flight. The ECS must pressurize the aircraft cabin and maintain its temperature within tolerable limits. Most other functions are subordinate to those requirements at cruise altitudes. The aircraft ECS is different from ECSs used in most other applications, such as buildings and surface vehicles, in that it must be able to operate in extremes of temperature, ambient air quality, and air pressure. The primary role of an aircraft ECS is to protect the occupants of the aircraft from those extreme conditions. Commercial aircraft operate over a broad range of temperatures from −55°C to 50°C (−65°F to 122°F) at ground level and as low as −80°C (−112°F) at an altitude of 12,000 m (39,400 ft). As shown in Figure 2–1, at a typical cruise altitude of 11,000 m (36,000 ft), the air temperature is usually about −55°C (−65°F) but can range from about −70°C to −30°C (−92°F to −20°F) (ASHRAE 1999a). More critically, at a typical cruise altitude of 11,000 m (36,000 ft), the atmospheric pressure is only about one-fifth that at sea level (Figure 2–2). Although the relative concentration of oxygen at that altitude is nearly the same as at sea level, the partial pressure of the oxygen (PO2) is only about 4.7 kPa(0.69 psi) compared with 21 kPa (3.1 psi) at sea level and is far below what is necessary to sustain human life. Furthermore, the ambient air quality on the ground and at low altitudes can range from pristine to extremely polluted in urban environments. The ECS meets those needs through integrated subsystems that pressurize the cabin when in flight, control thermal conditions in the cabin, and ventilate the cabin with outside air to prevent a buildup of contaminants that might cause discomfort or present a health hazard. Pressure In flight, the ECS maintains the cabin pressure and therefore the oxygen partial pressure at acceptable levels by compressing the low-pressure outside air and supplying it to the cabin. The air pressure in aircraft cabins is com-

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The Airliner Cabin Environment and the Health of Passengers and Crew FIGURE 2–1 Typical temperature design conditions for aircraft. Source: Adapted from ASHRAE 1999a.

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The Airliner Cabin Environment and the Health of Passengers and Crew monly expressed as a “pressure altitude” equivalent. Cabin pressure altitude is the distance above sea level at which the atmosphere exerts the same pressure as the actual pressure in the aircraft cabin. The relationship between atmospheric pressure and pressure altitude is based on the corresponding curve in Figure 2–2. The minimal cabin pressure is set by Federal Aviation Regulation (FAR) 25, which requires the pressurization system to “provide a cabin pressure altitude of not more than 8000 ft [2,440 m]” under normal operating conditions. This limit of 2,440 m (8,000 ft) corresponds to a cabin pressure of 75 kPa (10.9 psi) as shown in Figure 2–2. Thus, the cabin pressure can range from a maximum of 101 kPa (14.7 psi) on the ground at sea level to a minimum of 75 kPa (10.9 psi) in flight regardless of the altitude at which the aircraft flies. The primary purpose of the pressurization is to maintain the PO2 at acceptable levels. Figure 2–2 shows that PO2 values at sea level and at a pressure altitude of 2,440 m (8,000 ft) are 21 kPa (3.1 psi) and 16 kPa (2.3 psi), respectively. Thus, the minimal PO2 allowed in the aircraft cabin at the maximal allowed cabin pressure altitude of 2,440 m (8,000 ft) is 74% of the sea level value (Federal Aviation Regulations (FAR) Section 25.841). In addition to generating enough pressure to maintain the necessary PO2 in the cabin, the ECS must prevent rapid changes in cabin pressure. Rapid changes in pressure can cause changes in the volume occupied by gases in the body cavities and result in discomfort. Controlling the rate of change in pressure is particularly important during ascent and descent. During normal operation, the rate of change in cabin pressure altitude is limited to not more than 5 m/s (about 1,000 ft/min), sea-level equivalent, during climb and 2.3 m/s (450 ft/min) during descent (ASHRAE 1999a). The aircraft skin and pressure bulkhead at the rear of the cabin form a pressure hull that allows the aircraft to withstand the pressurization necessary during flight. In flight, pressurized air from the engine compressors is supplied continuously to the cabin, and the cabin pressure is controlled by outflow valves; the valves are automatically controlled to maintain cabin pressure but can be manually overridden by controls in the cockpit. For structural reasons, the difference between internal and external pressures is not allowed to exceed about 55–62 kPa (8–9 psi), depending on the aircraft. On some aircraft, the cabin pressure altitude is controlled to the lowest possible value (highest cabin pressure) and would not reach 2,440 m (8,000 ft) until the aircraft reaches its maximum operational altitude (e.g., 14,300 m [47,000 ft]). On other aircraft, the cabin pressure altitude is con-

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The Airliner Cabin Environment and the Health of Passengers and Crew FIGURE 2–2 Effect of altitude on atmospheric pressure.

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The Airliner Cabin Environment and the Health of Passengers and Crew trolled to the highest allowed value (lowest pressure) to minimize structural loads from pressurization. Because of the interrelationships between flight altitude, cabin pressure, and structural load on the aircraft, any change in pressure altitude requirements will affect the altitudes at which many aircraft can operate. Contamination Contaminants generated in the aircraft cabin air are eliminated by ventilating the cabin with outside air. The compressed outside air that is used for pressurization in the cabin is the same air that is used for ventilation. Pressurization and ventilation, however, serve very different purposes. For ventilation, outside air is used to dilute contaminants in the air and flush them out of the cabin. As described below, the rate of flow of outside air has a substantial and direct impact on the concentration of contaminants in the cabin air. The flow rate has a negligible effect on the PO2, in that only a tiny portion of the oxygen in this air is consumed by the aircraft occupants. A typical sedentary adult consumes oxygen at about 0.44 g/min (0.001 lb/min) (Nishi 1981). With the FAR 25 minimal design outside-air flow rate of 0.25 kg/min (0.55 lb/min) per cabin occupant, oxygen is brought into the cabin at 0.058 kg/min (0.127 lb/min) per person. Oxygen consumption by the occupants reduces the PO2 levels by about 0.8% in this case, compared with a PO2 reduction of up to 25% due to the reduced cabin pressure, as explained earlier. Thus, adequate oxygen concentrations in the cabin are maintained, even at ventilation rates far below those specified in FAR 25, as long as the cabin is adequately pressurized. Contaminants can originate in the cabin itself or in sources outside the cabin. Furthermore, the concentrations of contaminants in the cabin are subject to change as a result of fluctuations in the source emission and ventilation rates. Some contaminants degrade or react with other chemicals in the cabin. The following sections discuss the generation, distribution, and elimination of contaminants in cabin air. Contaminants Originating in the Cabin The basic steady-state ventilation equation for a particular contaminant “i” may be expressed as follows (derived from ASHRAE 1997a): Dc,i=Do,i+Si/Vo, (2–1)

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The Airliner Cabin Environment and the Health of Passengers and Crew where Dc is contaminant density in cabin air, kg/m3 (lb/ft3), Do is density of contaminant in outside air used for ventilation, kg/m3 (lb/ft3), S is strength of contaminant source, kg/s (lb/s), and Vo is ventilation rate of outside air, m3/s (ft3/s). To be accurate, both Vo and Do,i should be evaluated at cabin temperature and pressure (see Box 2–1). For gaseous contaminants, it is easier to work in terms of concentrations rather than densities, and the above equation can be expressed as follows: Cc,i=Co,i+(Si MWa)/(mo MWi), (2–2) where Cc is volume fraction of contaminant in cabin air, Co is volume fraction of contaminant in outside air used for ventilation, S is strength of contaminant source, kg/s (lb/s), mo is ventilation rate of outside air, kg/s (lb/s), MWa is molecular weight of air (28.96), and MW is molecular weight of contaminant. An example of the application of Equation 2–2 for carbon dioxide (CO2) is as follows. The CO2 concentration in the cabin air may be related to the rate at which outside air is supplied to the cabin by the ventilation system. A typical sedentary person will generate CO2 at about 7.7×10−6 kg/s (ASHRAE 1999b). The concentration of CO2 in clean outdoor air is about 0.037%. The molecular weight of CO2 is 44.01 g/mol. If the occupants are the only source of CO2 in the cabin, Equation 2–2 becomes Cc,CO2=0.00037+N(7.7×10−6)(0.658/mo), (2–3) where N is the number of occupants and 0.658 is the ratio of the molecular weights of air and CO2. Equation 2–3 can be used to relate ventilation rates to measured values of CO2 concentrations as long as respiration is the dominant source of CO2 in the cabin and the outside CO2 concentration is not above typical values. The CO2 concentration with the FAR 25 minimal design ventilation rate for aircraft of 0.0042 kg/s per person (0.25 kg/min) can be estimated as

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The Airliner Cabin Environment and the Health of Passengers and Crew Cc,CO2=0.00037+(7.7×10−6)(0.658/0.0042)=0.00158=1,580 ppm. Other ventilation rates will result in higher or lower CO2 concentrations according to Equation 2–3. It should be pointed out that CO2, at the concentrations present in this example is not noticeable by the occupants, nor is it considered hazardous. However, occupant-generated CO2 is produced roughly in proportion to other occupant-generated bioeffluents that can affect perceived air quality. The concentration of CO2 is sometimes used as an indicator of the concentration of other contaminants (ASHRAE 1999b). The ventilation requirements in the FAR are given in terms of mass flow. However, it is common to state ventilation flows in volumetric terms such as liters per second or cubic feet per minute; this practice can lead to confusion in that the relationship between mass flows and volumetric flows depends on the ambient pressure and temperature (see Box 2–1). Contaminants Originating Outside the Cabin The preceding discussion dealt with contaminants that are generated in the cabin and that can be effectively controlled by ventilation. However, other contaminants can be in the outside air, such as ozone (O3) or can be picked up in the air supply system, such as leaking oil. Obviously, it is not possible to control or eliminate those contaminants through an increased ventilation flow rate. If the source of the contaminant exists for only a short time (e.g., during deicing), effective control can be achieved by turning off the flow of outside air while the source is present. That control measure is not an option in flight, because of the requirements for pressurization; nor is it an option when the source is present for more than a short time (e.g., 15 min). Some reduction in concentrations of such cabin air contaminants can be achieved by using the minimal practical flow of outside air and increasing the flow of recirculated air if the recirculation filters are effective at removing the contaminants in question (see recirculation section later in this chapter). At high altitudes, especially at high latitudes, O3 concentrations in the outside air can be high enough for their introduction into the cabin to result in O3 concentrations that exceed the FAR 25 limit of 0.25 ppm by volume at any time above 32,000 ft (9,800 m) or above a time-weighted average of 0.1 ppm during any 3-h flight above 27,000 ft (8,200 m). Therefore, catalytic destruction of the O3 in the incoming air is used on some aircraft ECS to meet the FAR requirement.

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The Airliner Cabin Environment and the Health of Passengers and Crew BOX 2–1 Units for Expressing Ventilation It is common to express ventilation flow rates in volumetric terms—liters per second (L/s) or cubic feet per minute (cfm). However, this practice leads to ambiguity unless the pressure and temperature are also stated. Air density is proportional to atmospheric pressure and inversely proportional to absolute temperature. Thus, air that will occupy 1 m3 (35 ft3) at sea level will expand to over 3 m3 (106 ft3) at pressures and temperatures typical of outside air at cruise altitudes. The importance of this effect can be demonstrated by examining the ventilation requirement of 0.25 kg/min (0.55 lb/min) in FAR 25. At sea level and the standard atmospheric temperature of 15°C (59°F), the corresponding volumetric flow rate is 3.4 L/s (7.2 cfm); at the maximal allowed cabin pressure altitude of 2,440 m (8,000 ft) and a typical cabin temperature of 22°C (72°F), it is 4.7 L/s (9.9 cfm). At ambient atmospheric pressure at an altitude of 12,000 m (39,300 ft) and standard atmospheric temperature of −63°C (−81°F), the flow rate is 13.0 L/s (27.6 cfm). For ventilation purposes, the mass flow rate, not the volume flow rate, is most important. Thus, FAR 25 is correctly stated in this regard. Unfortunately, the data on most ventilation systems, including aircraft ventilation systems, are expressed in terms of volumetric flow. Temperature and pressure information generally is not included with those data, so there can be uncertainty as to the amount of air flowing. For aircraft, it can be assumed that the flow data do not correspond to outside ambient conditions at cruise altitudes. However, it is not always clear whether they correspond to cabin conditions, standard conditions, or some other conditions. With the exception of O3, the outside air at cruise altitudes is generally quite pure and requires no additional cleaning. The outside air at or near ground level, however, can contain a wide variety of contaminants from industrial and urban sources. In addition to outside air contaminants, leaking hydraulic fluid, spilled fuel, or deicing fluid can be entrained in the air supply systems; few, if any, aircraft have cleaning systems to remove any of these contaminants. Transient Response of Cabin Environmental Conditions Equations 2–1 through 2–3 describe contaminant concentrations under steady-state conditions. Contaminant concentrations in the cabin do not

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The Airliner Cabin Environment and the Health of Passengers and Crew change immediately when the controlling characteristics of ventilation flow rate and contaminant source strength are changed. It takes time for contaminant concentrations to build up to steady-state conditions after introduction of a source and to decline after the source is removed or ventilation begins. The time it takes for contaminant concentrations to approach steady-state conditions in aircraft is short, typically around 5–15 min, and is proportional to the quantity derived by dividing the volume of the space being ventilated by the ventilation rate. Such a rapid response means that there is only a short lag in the buildup of contaminants once they are introduced. It also means that the contaminants are flushed from the cabin quickly once the source is eliminated. In that respect, aircraft differ from buildings, in which it can take several hours to reach a steady state when ventilation rates are those recommended in American Society of Heating, Refrigeration, and Air-conditioning Engineers (ASHRAE) Standard 62 (1999b). Because contaminants can concentrate so quickly in an aircraft cabin, it is important that the ECS not be shut down for an extended period when the aircraft is occupied (except in an emergency). When the ECS is not operating, contaminant concentrations can become excessive and temperature uncomfortably high rapidly—in less than 15 min for a fully loaded aircraft in a hot environment. Reactive Contaminants Some contaminants react with other substances or decompose after they enter the cabin environment. Whether the contaminants decompose or combine with other chemicals once they are in the cabin can have an important effect on the contaminant concentrations in the cabin (Weschler and Shields 2000). The residence time of a contaminant in the cabin (the average length of time from introduction of the contaminant until it is flushed from the cabin by ventilation) has an important influence on the concentration of reactive contaminants in that it determines how long a contaminant has to decompose or react. As with transient responses discussed previously, residence time is proportional to the quantity derived by dividing the volume of the space by the ventilation flow rate, so residence time in aircraft is typically much shorter than in buildings. Residence time is particularly important for O3 and its byproducts (see Chapter 3 for additional discussion).

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The Airliner Cabin Environment and the Health of Passengers and Crew Temperature Control Temperature control in the aircraft cabin is critical for safety at high altitudes and is important for occupant comfort at all altitudes. Comfortable conditions are maintained in the cabin by supplying cool or warm air to the cabin as needed. Because of the high occupant density, cooling of the cabin is required in most circumstances, particularly on the ground and at low altitudes in warm climates. Supplying air to the cabin at an appropriate temperature is a key function of the ECS. The temperature of the air that must be provided to the cabin can be determined from the steady-state heat balance for an aircraft cabin, which in equation form is analogous to Equation 2–2: Tc=Ts+(Q/ms)(1/cp), (2–4) where Tc is the temperature of the air in the cabin, °C (°F), Ts is the temperature of the air supplied to the cabin, °C (°F), Q is the amount of heat that must be removed from the cabin, W (Btu/s), ms is the flow rate of conditioned air supplied to the cabin, kg/s (lb/s), and cp is the specific heat of the air, 1000 J/kg·°C, (0.25 Btu/lb·°F). The ECS designer must determine the combination of ms and Ts that will result in the desired Tc for a given Q. The air conditioning systems can provide air at a wide range of temperatures. However, there is a lower limit at which air can be supplied to the cabin without creating uncomfortable cold drafts near the inlets, typically about 10°C (50°F), and even at that temperature only with good air circulation in the cabin and good diffuser design (ASHRAE 1997a). Consequently, the system must be designed with an air flow rate that is adequate to meet the largest heat load with this temperature of supply air. An example will demonstrate this principle. The average heat generation by a comfortable, sedentary person, excluding heat loss due to evaporation of moisture, is about 70 W (ASHRAE 1997b). The total heat load in an aircraft cabin will include heat loads from electronics and heat gain through the aircraft skin. For the purpose of this example, the total heat load will be taken as twice the occupant-generated amount, or

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The Airliner Cabin Environment and the Health of Passengers and Crew Ground-Based Systems It is sometimes desirable to provide pneumatic or conditioned air to the aircraft from ground-based sources rather than use the aircraft engines or APU. Ground-based equipment can be less expensive to operate, can prevent overheating of aircraft due to prolonged operation of packs on the ground, and can substitute for inoperable ECSs. Depending on the airport, one or more ground-based systems may be available. Ground-based systems include the high-pressure ground cart, the low-pressure ground cart, and the low-pressure fixed source. The high-pressure ground cart is essentially an APU on wheels with a filter on the inlet to protect the cart from debris that might be entrained in the airflow. A pneumatic air line from the ground cart is attached to a fitting on the belly of the aircraft that connects to the aircraft pneumatic manifold (see Figure 2–8). This air is conditioned by the aircraft air-conditioning packs before being supplied to the cabin. Airlines also have the option of supplying air to the aircraft downstream of the packs by using a low-pressure ground cart, which is essentially a conventional air conditioner on a mobile cart. With this method, air is supplied only to the cabin air supply system, not to the pneumatic system, and no filter is used (see Figure 2–9). The temperature of the air is adjustable and controlled by the cart. The flow rate is fixed but generally is nearly equal to the normal total output of the packs. A variation of the low-pressure ground cart operates by replacing the air conditioner with a heater. This approach is more economical and is typically used when only warm air is needed. Again, the air is normally not filtered. Airlines have also begun to install fixed, low-pressure systems at each gate to replace the low-pressure ground carts. Rather than use an air conditioner mounted on a cart, the fixed system uses an air conditioner that is permanently mounted on the ground or in the terminal building and is connected to the aircraft by a flexible duct. The fixed system connects to the aircraft in the same manner as the low-pressure ground cart. ALTERNATIVE ENVIRONMENTAL CONTROL SYSTEMS Aircraft manufacturers and aircraft systems manufacturers might be investigating the use of alternatives to bleed-air-based ECSs, but any such work is highly confidential because of the competitive nature of this industry,

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The Airliner Cabin Environment and the Health of Passengers and Crew and no information about alternative systems was provided to the committee. Nor did the committee encounter any information about new alternative systems in its literature review. Consequently, the committee had little information to review in preparing this report. However, a brief examination of the potential for alternative systems is given below. As explained earlier in this chapter, the ECS functions to pressurize the cabin, provide an adequate supply of clean air throughout the cabin, control the air temperature in the cabin, and to some degree regulate the humidity in the cabin. Any alternative to a bleed-air-based ECS must meet the same requirements. The key features that distinguish the bleed-air-based ECS from other possible systems are the use of engine bleed air as the source of outside air and the use of rotating air-cycle equipment to cool this air. Those two features are fully integrated; not only does the high-pressure bleed air provide enough pressure to pressurize the cabin, but the pressure is high enough to operate the rotating air-cycle cooling equipment without external power sources. An ECS that uses an alternative source of outside air (e.g., ram air) must, at a minimum, include compression equipment that can compress the outside air enough to meet the pressurization needs of the cabin. The compression equipment must be lubricated, and it must have power sources to drive it, such as electric motors, gas-turbine engines, or compressed-air turbines that use bleed air as the power source. Just as the engine compressors have the potential for contaminating the supply air, any alternative means of compressing the air also has the potential for contamination, although the nature of the potential contamination can be different. Thus, using an alternative to the bleed air as a source of outside air does not automatically ensure uncontaminated air. Because of the need for a separate compressor and compressor drive mechanism, they can impose a substantial weight penalty as well. If the air is compressed just enough to pressurize the cabin rather than being compressed to a point where it can operate rotating air-cycle equipment, the temperatures attained in the compression could possibly be limited enough to avoid pyrolysis of contaminants, such as hydraulic fluid or lubricating oils. However, an alternative means of cooling the air would be required. The most obvious method is vapor-compression air-conditioning equipment similar to that used in most buildings and in many land vehicles. That equipment works well in ground-based applications, but its size and weight might be unacceptable for aircraft. The first passenger jet aircraft, introduced in the early 1960s, did not use bleed air as the source of outside air. Ram air was compressed and used to

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The Airliner Cabin Environment and the Health of Passengers and Crew pressurize and ventilate the cabin. Bleed air was used to power a turbocompressor, which compressed the ram air to the proper pressure. That air was cooled by a Freon vapor compression-cycle air conditioner for temperature control before being distributed to the cabin. The arrangement was heavy, expensive, and inefficient because of the inefficiencies of the turbocompressor (E.Marzolf, retired, Douglas Aircraft Co., personal communication, April 29, 2001). Even more important was the high amount of maintenance that the systems required (R.Kinsel, retired, AlliedSignal, personal communication, May 7, 2001). For those reasons and because increasing the altitude of flights would have required an even larger turbocompressor and there was no discernable difference in quality between ram air and bleed air, it was decided in the late 1960s to use bleed air directly (E.Marzolf, op. cit.). New large passenger aircraft have since used bleed-air-based ECSs. Essentially every large commercial passenger aircraft in use today is equipped with an alternative system, the APU. The APU provides a source of air that is independent of the propulsion engines. Although some APUs provide compressed air in much the same manner that the main propulsion engines provide bleed air, by extracting compressed air from the turbine-engine compressor, some APUs use a compressor, that is independent of the engine compressor, as described previously. Definitive data are not available, but the committee saw no evidence that contamination from the APU or brought into the APU is any less likely to affect cabin air adversely than is bleed air. The committee did not investigate alternative ECSs in depth. To do so would take person-years of effort because reliable information is not readily available in the public domain, if at all. Although it is possible that alternative systems are being developed or could be developed, the committee saw no evidence that bleed-air-based systems cannot be designed, maintained, and operated to provide adequate clean air to the aircraft cabin. That statement does not imply that they always provide uncontaminated air to the cabin. As described in Chapter 3, there is evidence that air is sometimes contaminated; but the measurements that have been taken during routine operation show no contamination of concern arising from bleed-air systems (Nagda et. al. 2001). Data are available on only a few aircraft and conditions, but they are adequate to make the point that bleed air can be clean. The committee does not want to discourage the pursuit of alternative systems, but it finds that the best method to ensure good cabin air is to have the Federal Aviation Administration (FAA) focus on ensuring that bleed-air-based systems are designed, maintained, and operated properly and that prob-

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The Airliner Cabin Environment and the Health of Passengers and Crew lems with them are identified and resolved. The reasons for the committee’s conclusion as follows: Many uncertainties and potential disadvantages are associated with alternative ECSs, the alternative systems might have problems as yet unknown. Alternative systems have been used and abandoned. There is a slow turnover of the aircraft fleet, and aircraft with existing systems almost certainly will be in service for a long time. There is good reason to believe that bleed-air systems can be designed and operated as to provide uncontaminated air to the cabin. ENVIRONMENTAL CONTROL SYSTEM DESIGN AND OPERATIONAL STANDARDS In this section of the report, the committee reviews guidelines, standards, and specifications that can have a bearing on aircraft ECS design and operation. Some of the guidelines and standards were developed primarily for building environments but might have relevance to aircraft as well. The FARs that are applicable to aircraft ECSs are discussed in Chapter 1. The FARs determine the design and operation requirements for aircraft; none of the guidelines and standards described below are legally enforceable for commercial aircraft. For buildings, the primary indoor air-quality guideline in the United States is ASHRAE Standard 62–1999 (ASHRAE 1999b). As with all ASHRAE standards, it is a voluntary consensus guideline and is not legally binding unless adopted by the applicable regulatory body for a particular application. FAA has not adopted it for aircraft. Standard 62–1999 was developed for and applies primarily to buildings. However, its scope claims applicability to all occupied indoor or enclosed spaces. There is no specific mention of aircraft or aircraft systems anywhere in the standard. Table 2 of the standard, which sets outdoor air requirements for various applications, has a single entry for vehicles but with no explanation as to type of vehicles (automobile, trains, buses, aircraft, and so on). In a requested interpretation, ASHRAE confirmed that aircraft are not specifically excluded from the scope of the standard but also stated: “Whether the requirements of Standard 62 should be applied to specific vehicles would be a decision for the authority having enforcement jurisdiction” (ASHRAE 2000a). In the case of aircraft, FAA is that authority.

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The Airliner Cabin Environment and the Health of Passengers and Crew If ASHRAE Standard 62–1999 were applied to aircraft, it would require outside air at 7 L/s (15 cfm) for each occupant. The standard does not specify the temperature and pressure that apply for this requirement. For most buildings, that omission is not critical, inasmuch as the overwhelming majority of buildings are at altitudes between sea level and 1,000 m (3,300 ft) and the density of outside air does not vary greatly over this range. However, the standard does not have any special requirement for buildings at high altitudes where the density of the air can be substantially less than that at sea level. For aircraft, that omission is more serious because they routinely operate at very high altitudes. Using standard thermodynamic conditions of 101 kPa and a temperature of 25°C (14.7 psi and 77°F) (Howell and Buckius 1992), the Standard 62–1999 requirement translates to outside air at 0.50 kg/min (1.1 lb/min) per occupant. At the minimal allowed cabin pressure of 75 kPa (10.9 psi) and a temperature of 25°C (77°F), that would be 0.37 kg/min (0.82 lb/min). For outside conditions at a cruise altitude of 12,000 m (39,000 ft) and the standard atmospheric temperature of −63°C (−81 °F) for that altitude, it would mean only 0.067 kg/min (0.15 lb/min). Basing the requirement on outside conditions at that altitude is unrealistic, but Standard 62–1999 does not specifically prohibit it. The numbers highlight the potential folly of applying to aircraft a standard developed for terrestrial applications. Assuming that the cabin temperature and pressure apply, and not the outside conditions, it is seen that ASHRAE Standard 62–1999 would require 50–100% more outside air than the current requirement in FAR 25. However, a provision in Section 6.1.3.4 of Standard 62–1999 might allow the outdoor air flow rate to be as little as half that listed above for some flights less than 3 h long. The provision is meant to reflect the fact that human-generated contaminants in the indoor air take some time to rise in response to occupancy. That phenomenon probably is not effective in aircraft, because of their high ventilation rates, but there is nothing in current interpretations of the provision that would prevent it from being applied to aircraft (see, for example, ASHRAE 2000b or ASHRAE 2000c) ASHRAE Standard 62–1999 is continuously maintained. A revision being considered would divide the ventilation requirements into occupant-generated and building-generated requirements. If the revision is adopted and if aircraft are treated similarly to how buildings are treated, the ventilation requirements for aircraft might drop considerably, with the minimal outside-air requirement for a fully loaded aircraft possibly falling to 2.4–3.3 L/s (5–7 cfm) per person.

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The Airliner Cabin Environment and the Health of Passengers and Crew Standard 62–1999 sets maximal concentrations of contaminants in the outside air used for ventilation. If the concentrations are exceeded, the outside air must be cleaned before it is used for ventilation. Most of the requirements are based on averages over periods of 24 h to 1 yr and so have little meaning for an aircraft that spends much of its time cruising in pristine air or on the ground not operating. However, ASHRAE’s limits on carbon monoxide (CO) and O3 could apply. The limits for CO are averages of 35 ppm for 1 h and 9 ppm for 8 h. The O3 limit is an average of 0.12 ppm for 1 h. It is not possible to compare the above requirements from ASHRAE Standard 62–1999 with those in FAR 25 directly; because the FAR 25 requirements are for cabin air and the ASHRAE requirements are for inlet air. O3, in particular, will continue to react after the air is in the cabin. An optional provision of Standard 62–1999 does have a guideline for the maximal allowed concentration of O3 in the indoor air: 0.05 ppm on a continuous basis. CO, in contrast, is mostly inert in the cabin; it is present continuously in the inlet air, the cabin concentration ultimately will match that of the inlet air. Thus, ASHRAE Standard 62–1999 is generally more restrictive than FAR 25 with respect to both O3 and CO. The optional provisions in Standard 62–1999 also include a guideline for the maximal concentration of CO2: 700 ppm above the outside air concentration, or about 1,000–1,100 ppm in most situations. That limit is strictly for the purpose of limiting discomfort related to odors from human bioeffluents and is not meant to be a limit on exposure to CO2 (ASHRAE 2000d). It uses respiration-generated CO2 as a proxy for the human-generated odors and is approximately comparable with the 7 L/s (15 cfm) per person requirement at sea level. Obviously, it is not applicable if other sources of CO2, such as dry ice, are present. The guideline of 1,000–1,200 ppm that comes from Standard 62–1999 should not be compared with the 5,000-ppm limit in FAR 25; the latter is a limit on exposure to CO2 itself and is not intended to be a measure of ventilation. ASHRAE Standard 62–1999 does not mandate temperature or humidity requirements. Such requirements are, however, covered in ASHRAE Standard 55–1992, Thermal Environmental Conditions for Human Occupancy (ASHRAE 1992), which sets ranges of temperature and humidity that are generally found comfortable as a function of activity level and clothing. The standard’s scope claims applicability to environments “at atmospheric pressure equivalent to altitudes up to 3,000 m (10,000 ft) in indoor spaces designed for human occupancy for periods not less than 15 minutes” and thus appears to apply to aircraft. The lower relative-humidity limits range from about 20% to

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The Airliner Cabin Environment and the Health of Passengers and Crew 30%, depending on the temperature, and might be difficult to attain in most aircraft on all but the shortest flights. A public review draft of a proposed revision of the standard removes the lower humidity limits (ASHRAE 2001). ASHRAE is developing a standard specifically for aircraft cabin air quality and has established the special-project committee on Air Quality within Commercial Aircraft, SPC 161P, to complete the task. The scope under which the committee is working states that it “applies to commercial passenger air-carrier aircraft carrying 20 or more passengers and certified under Title 14 CFR Part 25.” Although the committee has been working on the standard for several years, the first public review draft is not expected before to 2002. The Society of Automotive Engineers (SAE) recommended practice Procedure for Sampling and Measurement of Engine Generated Contaminants in Bleed Air Supplies from Aircraft Engines Under Normal Operating Conditions, ARP4418 (SAE 1995), includes a table from AIR4766, Air Quality for Aircraft Cabins that specifies the maximal concentrations of contaminants in engine bleed air. Those limits are presented in Table 2–3. AIR4766 is under development by SAE and has not been released to the public (SAE, personal communication, May 7, 2001). Military Specification MIL-E-5007D, General Specifications for Aircraft Turbojet and Turbofan Engines (1973), also includes limits on contaminants in bleed air, which are presented in Table 2–4. Neither the limits in ARP4418 TABLE 2–3 Limits on Engine-Generated, Bleed-Air Contaminants Under Normal Operating Conditions Contaminant Maximal Allowable Concentration Above Ambient CO2 400 ppm CO 5 ppm Hydrogen fluoride 1 ppm Oxides of nitrogen (nitrogen dioxide equivalent) 1 ppm Formaldehyde 0.3 ppm Acrolein 0.05 ppm Organic material (synthetic oil equivalent, MW 600) 0.4 ppm Respirable particles 0.5 mg/m3   Source: Adapted from SAE (1995).

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The Airliner Cabin Environment and the Health of Passengers and Crew TABLE 2–4 Limits on Engine-Generated, Bleed-Air Contaminants Under Normal Operating Conditions Substance Maximal Allowable Concentration Above Ambient, ppm CO2 5,000 CO 50 Ethanol 1,000 Fluorine (as hydrogen fluoride) 0.1 Hydrogen peroxide 1.0 Aviation fuels 250 Methyl alcohol 200 Methyl bromide 20 Nitrogen oxides 5 Acrolein 0.1 Oil breakdown products (e.g., aldehydes) 1.0 O3 0.1   Source: Military Specification (MIL-E-5007D) (1973). nor the limits in MIL-E-5007D appear to be a factor in the commercial aircraft industry, because modern engines are associated with much lower contaminant concentrations under normal operating conditions when there are no equipment failures or malfunctions. Note that both specifications are for engine-generated contaminants; any contaminants in the engine inlet air would not be covered. CONCLUSIONS The adequacy of oxygen in the cabin is determined by the PO2. The cabin air pressure is the dominant factor in determining the PO2 in the cabin. The ventilation rate has little effect on PO2, and ventilation rates well below those normally present in aircraft would not seriously affect PO2. The flow of outside air must be adequate for contaminant control in the cabin, whether or not recirculation is used. As long as the outside-air flow rate is appropriate for contaminant control in the cabin and the recirculation system is properly designed, operated, and maintained, recirculation does not normally affect cabin air quality adversely.

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The Airliner Cabin Environment and the Health of Passengers and Crew As aircraft have moved from using 100% bleed air for cabin ventilation to incorporating recirculated air into ECS design, the amount of outside air supplied to the cabin has decreased. The mixing of the air that occurs in the cabin, with or without recirculation, and the proximity of cabin occupants makes it impossible to eliminate exposure to infectious agents and other contaminants in the cabin air. The low humidity in aircraft cabins cannot be increased through ventilation controls without raising questions about the effect on air quality. A number of problems are associated with humidification of cabin air. Consequently, the low humidity common in aircraft cabins is not readily corrected. The environmental conditions in an aircraft cabin respond quickly to changes in ECS operation. Consequently, it is important that the ECS not be shut down for a long period when the aircraft is occupied except in the case of an emergency, because excessive contaminant concentrations and uncomfortably high temperatures can occur quickly. RECOMMENDATIONS FAA should rigorously demonstrate in public reports the adequacy of current and proposed FARs related to cabin air quality and should provide quantitative evidence and rationales to support sections of the FARs that establish air-quality-related design and operational standards for aircraft (standards for CO, CO2, O3, ventilation, and cabin pressure). If a specific standard is found to be inadequate to protect the health and ensure the comfort of passengers and crew, FAA should revise it. For ventilation, the committee recommends that an operational standard consistent with the design standard be established. The committee reiterates the recommendation of the 1986 NRC report that a regulation be established that requires removal of passengers from an aircraft within 30 min after a ventilation failure or shutdown on the ground and that full ventilation be maintained whenever on-board or ground-based air-conditioning is available. REFERENCES ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers). 1992. Thermal Environmental Conditions for Human Occupancy. ANSI/

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The Airliner Cabin Environment and the Health of Passengers and Crew ASHRAE 55–1992. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Atlanta, GA. ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers). 1997a. Ventilation and Infiltration. Chapter 25 in 1997 ASHRAE Handbook: Fundamentals. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Atlanta, GA. ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers). 1997b. Non-Residential Heating and Cooling Load Calculations. Chapter 28 in 1997 ASHRAE Handbook: Fundamentals. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Atlanta, GA. ASHRAE (American Society of Heating Refrigerating and Air-Conditioning Engineers). 1999a. Aircraft. Chapter 9 in 1999 ASHRAE Handbook: Heating, Ventilating, and Air-Conditioning Applications. American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Atlanta, GA. ASHRAE (American Society of Heating Refrigerating and Air-Conditioning Engineers). 1999b. ASHRAE Standard—Ventilation for Acceptable Indoor Air Quality. ANSI/ASHRAE 62–1999. American Society of Heating Refrigerating and Air-Conditioning Engineers, Atlanta, GA. ASHRAE (American Society of Heating Refrigerating and Air-Conditioning Engineers). 2000a. Ventilation for Acceptable Indoor Air Quality. Interpretation IC 62– 1999–37 of ANSI/ASHRAE 62–1999 . American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Atlanta, GA. ASHRAE (American Society of Heating Refrigerating and Air-Conditioning Engineers). 2000b. Ventilation for Acceptable Indoor Air Quality. Interpretation IC 62–1999–18 of ANSI/ASHRAE62–1999. American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Atlanta, GA. ASHRAE (American Society of Heating Refrigerating and Air-Conditioning Engineers). 2000c. Ventilation for Acceptable Indoor Air Quality. Interpretation IC 62–1999–20 of ANSI/ASHRAE 62–1999. American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Atlanta, GA. ASHRAE (American Society of Heating Refrigerating and Air-Conditioning Engineers). 2000d. Ventilation for Acceptable Indoor Air Quality. Interpretation IC 62–1999–05 of ANSI/ASHRAE 62–1999. American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Atlanta, GA. ASHRAE (American Society of Heating Refrigerating and Air-Conditioning Engineers). 2001. Thermal Environmental Conditions for Human Occupancy. BSR/ ASHRAE 55–1992R. ASHRAE Standard, Proposed Revision to American National Standard. First Public Review Draft. American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Atlanta, GA. February. Boeing Company. 1988. Air Conditioning—Description and operation. P. 1 in Boeing 747–400 Maintenance Manual. 21–00–00. Boeing Company. October 10, 1988. Boeing Company. 1995. Ventilation—Description and operation. P.1 in Boeing 747– 400 Maintenance Manual. 21–26–00. Boeing Company. February 10, 1995.

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The Airliner Cabin Environment and the Health of Passengers and Crew Fanger, P.O. 1982. Thermal Comfort: Analysis and Applications in Environmental Engineering. Malabar, FL: R.E.Krieger Publishing. Howell, J.R., and R.O.Buckius. 1992. Fundamentals of Engineering Thermodynamics. New York: McGraw-Hill. Jones, B.W., M.H.Hosni, and H.Meng. 2001. The Interaction of Air Motion and the Human Body in Confined Spaces. ASHRAE Research Project 978. Final Report. Institute for Environmental Research, Kansas State University, Manhattan, KS. May 1, 2001. Lorengo, D., and A.Porter. 1986. Aircraft Ventilation Systems Study. Final Report. DTFA-03–84-C-0084. DOT/FAA/CT-TN86/41-I. Federal Aviation Administration, U.S. Department of Transportation. September 1986. Military Specification. 1973. General Specifications for Engines, Aircraft, Turbojet and Turbofan. MIL-E-5007D. October 15, 1973. Nagda, N.L., H.E.Rector, Z.Li, and E.H.Hunt. 2001. Determine Aircraft Supply Air Contaminants in the Engine Bleed Air Supply System on Commercial Aircraft. ENERGEN Report AS20151. Prepared for American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Atlanta, GA, by ENERGEN Consulting, Inc., Germantown, MD. March 2001. NASA (National Aeronautics and Space Administration). 1976. U.S. Standard Atmosphere, 1976. National Oceanic and Atmospheric Administration, National Aeronautics and Space Administration, and the U.S. Air Force, Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. Nishi, Y. 1981. Measurement of thermal balance of man. Pp. 29–40 in Bioengineering, Thermal Physiology, and Comfort, K.Cena, and J.A.Clark, eds. New York: Elsevier. SAE (Society of Automotive Engineers). 1995. Procedure for Sampling and Measurement of Engine Generated Contaminants in Bleed Air Supplies from Aircraft Engines Under Normal Operating Conditions. SAE ARP4418. Society of Automotive Engineers, Warrendale, PA. Weschler, C.J., and H.C.Shields. 2000. The influence of ventilation and reactions among indoor pollutants: Modeling and experimental observations. Indoor Air 10(2):92–100.