ENVIRONMENT CONTROL SYSTEM FOR AIRCRAFT HAVING INTERIOR CONDENSATION PROBLEM REDUCTION, CABIN AIR QUALITY IMPROVEMENT, FIRE SUPPRESSION AND FIRE VENTING FUNCTIONS

Abstract: An environment control system for a body of an aircraft that provides controlled ventilation of the interior space of an aircraft body (1), facilitating reduction of volatile organic compounds (VOCs) within cabin air, dehumidifying and reducing moisture condensation and thus corrosion and other moisture related problems within the envelope (5), allowing increased humidification of cabin air, and allowing suppression of fires within the envelope. The environment control system includes at least a cabin (3) and an envelope (5). It includes supply means (23, 23a) for supplying a flow of dry ventilation air to the aircraft body (1). An airflow control device (13) is capable of dividing the flow of ventilation air into an envelope ventilation air stream (25) and a cabin ventilation air stream (26). An envelope ventilation duct system (14, 16, 27, 29) directs the envelope ventilation air stream (25) into the envelope (5), and a cabin duct system (20, 21) directs the cabin ventilation air stream (26) into the cabin (3). An anti-corrosion/sorption treatment (41) is applied to surfaces subject to condensation in the envelope (5). A return air control unit (17) is provided for selectively drawing return air from one of the envelope (5) and the cabin (3). The environment control system can be incorporated into new aircraft construction, or can be installed as a retro-fit into existing aircraft.

Description: ENVIRONMENT CONTROL SYSTEM FOR AIRCRAFT HAVING INTERIOR CONDENSATION PROBLEM REDUCTION. CABIN AIR QUALITY IMPROVEMENT. FIRE SUPPRESSION AND FIRE VENTING FUNCTIONS TECHNICAL FIELD The present invention relates to a method and apparatus for controlling the environment within an enclosed space. More particularly, the present invention relates to an environmental control system for providing controlled ventilation of the interior space of an aircraft body, such that interior condensation and corrosion is reduced, cabin air quality is improved, the cabin can be humidified to healthy levels without increasing condensation and associated deleterious effects, and envelope fires can be directly suppressed and vented. BACKGROUND OF THE INVENTION In the embodiments of the invention described below and illustrated in the appended drawings, the "body" of an aircraft is comprised entirely within the fuselage, and excludes the wings and tail surfaces, as well as those portions of the nose and tail cones which extend beyond the respective nose and tail pressure bulkheads. However, it will be understood that the present invention is equally applicable to other aircraft geometries (such as, for example flying wing and lifting body designs). Thus in general, and for the purposes of the present invention, the "body" of an aircraft will be considered to be that portion of the aircraft which is pressurized during normal cruising flight, and within which it is desirable to control the environment in order to enhance safety and comfort of passengers and crew. For the purposes of the present invention, the body of an aircraft is considered to be divided into a cabin, one or more cargo bays, and an envelope which surrounds both the cabin and the cargo bay(s). The terms "cabin" and "aircraft cabin" shall be understood to include all portions of the interior space of the aircraft which may be occupied during normal flight operations (i.e. the passenger cabin plus the cockpit). The term "envelope" shall be understood to refer to that portion of the aircraft body between the cabin (and any cargo bays), and the exterior surface of the pressure shell (including any pressure bulkheads) of the aircraft. In a conventional jet transport aircraft, the envelope typically comprises inter alia the exterior fuselage skin; nose, tail and wing root pressure bulkheads; insulation blankets; wire bundles; structural members; ductwork and the cabin (and/or cargo bay) liner. The term "ventilation air" is defined as outside air typically introduced as bleed air from an engine compressor. For the purposes of this invention, "ventilation air" shall be understood to be outdoor air brought into the cabin by any means, for example, engine bleed air, either with or without filtering. "Ventilation air" does not include recirculation air or cabin air, filtered or otherwise reconditioned, which is supplied back into the interior space of the aircraft. For the purposes of this invention, "recirculation air" shall be understood to comprise air drawn from the interior space of the aircraft, possibly conditioned, and then returned to the cabin. To facilitate understanding of the present invention, the following paragraphs present an outline of condensation/corrosion, air quality, and fire problems encountered in typical jet transport aircraft, and conventional measures taken to address such problems. Moisture Condensation Problems Aircraft are subjected to sub-zero temperatures (e.g., - 50°C) when flying at cruising altitudes. While the aircraft skin is slightly warmer than outside air due to air friction, temperatures behind and within the insulation blankets (particularly adjacent the skin) cool to 0°C to -40°C, depending upon flight duration and altitude. When cabin air passes behind the insulation, it can reach the temperature at which its moisture starts to condense (i.e., its dew point). Further cooling beyond this temperature will result in additional condensation (as liquid water or ice) on the skin and other cold sinks. Cabin air circulates behind the insulation, drawn through cracks and openings by pressure differences created when the cabin is depressurized during ascent, for example, and during flight by stack pressures (buoyancy effect). Stack pressures are created by density differences between the cooler air behind the insulation and the warmer air in front of the insulation. The density difference creates a slight negative pressure in the envelope (relative to the cabin) near the ceiling of the cabin and a slight positive pressure in the envelope near the floor of the cabin. The effects of this condensation range from a simple nuisance through increased operation costs to decreased aircraft life. The more an airplane is used and the greater its occupant density, the higher its potential for condensation problems. Cases have been reported of water dripping from the cabin paneling. Wetting of insulation increases thermal conduction and, over time, adds weight, increasing operating costs. This condensation increases the potential for electrical failure. It can lead to the growth of bacteria and fungi. It causes corrosion, leading to earlier fatigue failure and reduced aircraft life. Some estimates place capital and maintenance costs attributable to such condensation at up to $100,000 annually for larger, heavily utilized passenger aircraft. Conventionally, passive measures have been used to cope with the envelope moisture problem. These include anti-corrosion coatings, drainage systems, and deliberately maintaining cabin humidity well below American Society of Air-Conditioning Engineers (ASHRAE) Standard recommended levels. United States Patent No. 5,386,952 (Nordstrom) teaches a method for preventing moisture problems by injecting dehumidified cabin air into the envelope. However, the installation of dehumidifiers, as taught by Nordstrom, increases electrical consumption, occupies additional volume, and adds dead weight. Thus in a recently published study ("Controlling Nuisance Moisture in Commercial Airplanes") Boeing Aircraft Company concluded that active dehumidification systems, such as those taught by Nordstrom, are not cost-effective, even though they can reduce moisture condensation within the envelope. Additionally, the dehumidification system taught by Nordstrom is incapable of addressing related cabin air quality issues, as described below. Cabin Air Quality Relative humidities above 65 percent, which commonly occur in aircraft envelopes for even relatively low cabin humidities, can support microbial growth under appropriate temperature conditions. Such growth can include Gram-negative bacteria, yeasts and fungi. Where sludge builds up, anaerobic bacteria may grow, producing foul smelling metabolites. Saprophytic microorganisms provide nutriment for Protozoa. Exposure to aerosols and volatile organic compounds (VOCs) from such microbial growth can result in allergenic reactions and illness. The relative humidity of outside air at typical cruising altitudes is frequently less than 1-2% when heated and pressurized to cabin conditions. Consequently, since cabin air normally is not humidified, on longer flights some passengers may experience dryness and irritation of the skin, eyes and respiratory system, while asthmatics may suffer incidences of bronchoconstriction. High air circulation velocities compound this problem. While humidification of the cabin air during flight would alleviate the "dryness" problem, it would also exacerbate the potential for microbial growth and damp material off-gassing in the envelope. Thus, although it would be of benefit for health purposes to maintain higher cabin air relative humidities which are within the ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) Standard, this is made impracticable by the envelope condensation problem. Other air contaminants in aircraft causing sensory irritation and other health effects can originate from ventilation air, passengers, materials, food, envelope anti-corrosion treatments, envelope microbial growth, etc. Ventilation air contaminants originate outdoors and within the engine (when bleed air is used). Potential contaminant gases and particulate aerosols include: • combusted, partially combusted and uncombusted hydrocarbons (alkanes, aromatics, polycyclic aromatics, aldehydes, ketones); • deicing fluids; • ozone, possibly ingested during the cruise portion of the flight cycle; and • hydraulic fluids and lubricating oils, possibly originating from seal leakage within the engine. Gas chromatography/mass spectrometry (GC/MS) head space analyses of engine lubricating oil (Figure 9a), jet fuel (Figure 9b), and hydraulic fluid (Figure 9c) indicate some of the potential VOCs that might be found in aircraft ventilation air. Figure 8a shows a GC MS plot of a ventilation air sample taken in a jet passenger aircraft during the cruise portion of the flight cycle (28000 ft and -34°C). The total concentration was 0.27 mg/m 3 at a cabin pressure altitude of approximately 8000 ft. For comparison, ventilation air VOC concentrations for downtown buildings typically are less than a third of this concentration. VOCs identified include 3 -methyl pentane, hexane, 3 -methyl hexane, toluene, hexanal, xylene, and many C9-C12 alkanes. Additional compounds reported by other researchers include formaldehyde, benzene and ethyl benzene. Many of the compounds in the jet fuel (Figure 9b) can be seen in this ventilation air sample. The total VOC (TVOC) concentration was 0.27 mg/m 3 at a cabin pressure altitude of approximately 8000 ft. Of this some 0.23 mg/m 3 could have a petroleum (combustion source). The TVOC concentration is equivalent to a TOC exposure of 0.36 mg/m 3 at sea level. In comparison, urban residential ventilation air TVOC concentrations are typically less than one-third this aircraft ventilation air concentration (i.e., <0.03 mg/m 3 ), and building room air TVOC concentrations typically are less than 0.5 mg/m 3 . One postulate for the high VOC concentrations found in aircraft is that periodic incidents of lubricating oil leakage produce aerosols which enter the ventilation system and progressively coat the interior surfaces of the supply ducts. This coating, in turn, could sorb VOCs ingested during taxi from the exhaust of other aircraft. These VOCs may subsequently be released into the cabin during flight. Contaminated ventilation air increases ventilation rate requirements to achieve any particular space concentration target. For example, a ventilation rate with TVOCs=0.36 mg/m 3 must be three times higher than one with TVOCs = 0.036 mg/m 3 to maintain a room TVOC concentration of 0.5 mg/m 3 . Cabin air contaminants can originate from materials and, possibly, microbial growth in the envelope as well as from cabin furnishings, food and passengers. Contaminants in the envelope enter the cabin when cabin air is circulated behind the insulation, drawn there by envelope stack pressures and by decreasing cabin pressures (for example, during ascent). Figure 8b shows a GC/MS plot of envelope air in an aircraft parked when the temperature in the air space between the skin and insulation was approximately 35 °C. The total (TVOC) concentration was 22 mg/m 3 . Of this, some 21 mg/m 3 had a petroleum source and 0.6 mg/m 3 could have had a microbial source. VOCs from one source of these envelope contaminants, an anti-corrosion treatment, is illustrated in Figure 9e. This head space sample was taken at -5°C, a temperature representative of the temperature behind the insulation during the early portions of cruising flight. This anti-corrosion treatment emitted many of the compounds seen in the envelope and the ventilation air, plus a number of cycloalkanes and aliphatics not seen in the other samples. Figure 9d shows the head space GC/MS plot of a general purpose cleaner (2-butanone or methyl ethyl ketone) used on this aircraft. This compound was also identified in the envelope, engine oil, ventilation air and anti-corrosion treatment samples. When the envelope is cooled in flight or warmed on the ground, envelope material off-gassing and sorption of contaminant gases change. For example, under ideal conditions, the deposition of VOCs of interest behind the insulation could increase a hundred-fold for temperature decreases over the typical flight cycle temperature range. Condensation of higher molecular weight compounds at higher concentrations may occur when the envelope is cooled. For example, the maximum concentration of dodecane (a compound found in the ventilation air and anti-corrosion treatment samples), at -40°C is 0.26 mg/m 3 . One implication of the above is that during the ascent and the early portions of the cruise flight cycle while the envelope is still relatively warm, envelope VOCs could pose an air quality problem for passengers. Another implication is that cabin air VOCs will be deposited (sorbed) in the envelope when it is cold, particularly during later stages of the cruise portion of the flight cycle. For example, both ventilation air VOCs (Figure 8a) and the cabin cleaner VOC (Figure 9d) can be found in the envelope air sample (Figure 8b). Some aircraft have high efficiency particulate filters (HEP A) filters which will remove human microbial aerosols that enter the circulation system. Some have catalytic converters to remove ozone. Very few have sorbent air cleaners to remove ventilation-air and cabin VOCs. Fire and/or Pyrolysis in the Envelope In the case of a fire, thermal and electrical insulation systems in the envelope as well as other materials in the cabin can undergo pyrolysis and burning, generating toxic smoke and combustion products. Conventionally, this problem is addressed by employing fewer combustible materials, and using hand-held containers with non-toxic fire suppressants. Currently, insulation is under review in this regard with a prevention program potentially involving more than 12,000 commercial aircraft. Under any cabin fire emergency, the objective is to exhaust the smoke from the cabin while suppressing the fire. There is currently no method in place to directly suppress or extinguish fire and/or pyrolysis within the envelope. Nor is there any effective means of preventing smoke within the envelope from penetrating into the cabin. Furthermore, exhaustion of air from the cabin is usually via grilles at the floor, which undesirably enhances smoke circulation throughout the cabin. United States Patent No. 4,726,426 (Miller) teaches a method of fire extinguishment in aircraft cabins using ventilation ducts in communication with the cargo fire extinguishment system. However, this system does not address envelope fires and/or pyrolysis, or the health and safety problems associated with exposing passengers to potentially lethal combinations of fire suppressants and their combustion products in combination with fire and smoke. SUMMARY OF THE INVENTION It is an object of the present invention to provide an environment control system that overcomes the above-noted deficiencies in the prior art. It is a further object of the present invention to provide an environment control system capable of inhibiting moist cabin air from contacting cold surfaces of the envelope, thereby reducing moisture condensation within the envelope, and associated "rain-in-the-plane", electrical failures, corrosion, microbial growth, and dead weight. It is a further object of the present invention to provide an environment control system capable of reducing infiltration of smoke from the envelope into the interior cabin space, thereby increasing passenger and crew safety during an in-flight fire situation. It is a further object of the present invention to provide an environment control system capable of improving cabin indoor air quality (IAQ) by at least partially removing contaminants from ventilation air prior to entering the cabin. Accordingly, an aspect of the present invention provides an environment control system for a body of an aircraft including at least a cabin and an envelope, such environment control system comprising: an air supply capable of providing a flow of dry ventilation air to the interior of the body; an airflow control device capable of dividing the flow of ventilation air into an envelope ventilation air stream and a cabin ventilation air stream; an envelope air distribution system capable of directing the envelope ventilation air stream into the envelope; a cabin air distribution system capable of directing the cabin ventilation air stream into the cabin; and a return air control unit capable of drawing a return air stream from a selected one of the envelope and the cabin. In an embodiment of the invention, the envelope is divided into an upper lobe and a lower lobe, the envelope air distribution system being adapted to provide a respective upper lobe and lower lobe envelope ventilation air stream. Each of the upper lobe and lower lobe can be further divided into port and starboard sides, to define four quadrants of the envelope, in which case the envelope air distribution system is preferably adapted to provide a respective portion of the envelope ventilation air stream to each quadrant. Preferably, the airflow control device is capable of controlling the upper and lower lobe envelope ventilation air streams to deliver respective different air flows to the upper lobe envelope and the lower lobe envelope. In an embodiment of the invention, the air supply includes an air supply duct adapted to conduct bleed air from a compressor stage of an engine of the aircraft into the body of the aircraft. In an embodiment of the invention, the envelope air distribution system comprises at least one envelope supply duct disposed longitudinally of the aircraft body, and a plurality of respective ventilation air branch lines distributed along the length of the envelope supply duct, whereby the envelope ventilation air can be distributed throughout the envelope in such a manner as to offset stack pressures. Preferably, each ventilation air branch line comprises at least one nozzle for injecting ventilation air into the envelope. In an embodiment of the invention, at least one nozzle is a shell-side nozzle capable of injecting envelope ventilation air between an insulation jacket and the exterior skin of the aircraft. Preferably, two or more shell-side nozzles are provided in communication with each ventilation branch line, the shell-side nozzles being disposed at spaced intervals around a circumference of the envelope in a manner which optimizes offsetting stack pressures. In an embodiment of the invention, at least one nozzle is a cabin-side nozzle capable of injecting envelope ventilation air between an insulation jacket and the cabin liner. Preferably, two or more cabin-side nozzles are provided in communication with each ventilation branch line, the cabin-side nozzles being disposed at spaced intervals around the circumference of the envelope. An embodiment of the invention further comprises an anti-corrosion/VOC sorption treatment applied to an interior surface of the aircraft structure possibly exposed to condensation. Preferably, the anti-corrosion/VOC sorption treatment is formulated to provide acceptable characteristics, namely: adhesion to metal surfaces; hydrophobic; low flammability; and low off-gassing at typical envelope temperatures during cruising flight. More preferably, the anti-corrosion/VOC sorption treatment is formulated to: resist solidification within the aircraft envelope; sorb ventilation air VOCs at typical envelope temperatures during cruising flight and desorb said ventilation air VOCs at warmer temperatures substantially without hysteresis. In an embodiment of the invention, the return air control unit is adapted to draw the return air stream from the cabin only. In a preferred embodiment, the return air control unit is adapted to selectively draw the return air stream from either the cabin or the envelope in accordance with an operating mode of the environment control system. In an embodiment of the invention, the return air control unit comprises a housing, an envelope opening defined in the housing and in communication with the envelope, a cabin opening defined in the housing and in communication with the cabin, and a damper capable of selectively closing the envelope opening and the cabin opening. An embodiment of the invention further comprises at least one flow blocker disposed in the envelope and capable of at least partially blocking a circumferential flow of air within the envelope. Preferably, there is at least one pair of flow blockers disposed within the envelope, members of each pair being symmetrically disposed on opposite sides of the body of the aircraft. The envelope air distribution system preferably comprises at least one nozzle capable of injecting at least a portion of the envelope ventilation air stream into a portion of the envelope below each flow blocker, and at least one nozzle capable of injecting at least a portion of the envelope ventilation air stream into a portion of the envelope above each flow blocker. In a particularly preferred embodiment, a flow blocker is disposed within the envelope at approximately mid-height of an upper lobe of the body of the aircraft. In an embodiment of the invention, the cabin air distribution system comprises: an air conditioner communicating with the airflow control device for receiving at least a portion of the cabin ventilation air, and operative to condition the cabin ventilation air to create cabin supply air; and a cabin supply air duct capable of directing the cabin supply air into the cabin. Conveniently, the air conditioner may be used to control a relative humidity of the cabin supply air. In such cases, the air conditioner is preferably operated to maintain a relative humidity of the cabin supply air of at least 20%, preferably between 20% and 80% (more preferably between 40% and 70%). An embodiment of the invention further comprises a return air duct in communication with the return air control unit, for conducting a flow of return air therefrom. An embodiment of the invention further comprises an outflow valve in communication with the return air duct, the outflow valve being capable of dividing the return air stream into an exhaust air stream and a recirculation air stream, the exhaust air stream being vented out of the aircraft, and the recirculation air stream being supplied back to the cabin. Preferably, the recirculation air stream is supplied to the cabin via an air conditioner. In a preferred embodiment of the invention, the airflow control device and the return air control unit are capable of operating cooperatively to maintain a predetermined pressure differential between the cabin and the envelope. An embodiment of the invention further comprises a fire suppression system in communication with the envelope air distribution system, the fire suppression system being capable of releasing a flow of chemical fire suppressant into at least the envelope air distribution system when smoke or fire is detected in the envelope. Preferably, the fire suppression system and the envelope air distribution system are adapted to cooperate to flood at least a portion of the envelope with the chemical fire suppressant. Conveniently, the fire suppression system comprises a container of chemical fire suppressant, a supply line in communication with the container and the envelope air distribution system for conducting the chemical fire suppressant thereto, and a valve capable of controlling a flow of chemical fire suppressant from the container. Conveniently, the chemical fire suppressant is any one or more of Halon, carbon dioxide, nitrogen, and other fire suppressant agents, or mixtures of these. A further aspect of the present invention provides a method of controlling the environment within an aircraft body including at least a cabin and an envelope, the method comprising: providing a flow of dry ventilation air; dividing the flow of ventilation air into an envelope ventilation air stream and a cabin ventilation air stream; supplying the envelope ventilation air to the envelope; supplying the cabin ventilation air to the cabin; drawing a return air stream from a selected one of the envelope and the cabin; and controlling the envelope ventilation air stream and the cabin ventilation air stream to maintain a predetermined pressure differential between the cabin and the envelope. In an embodiment of the present invention, during a cruising portion of a flight cycle, the envelope ventilation air stream and the cabin ventilation air stream are controlled to maintain the envelope at a higher pressure than the cabin, and the return air stream is drawn from the cabin. An embodiment of the invention further comprises the step of injecting at least a portion of the envelope ventilation air into a space between an exterior skin of the aircraft body and an insulation jacket. An embodiment of the invention further comprises the step of injecting at least a portion of the envelope ventilation air into a space between an insulation jacket and a cabin liner. An embodiment of the invention further comprises the step of humidifying the cabin ventilation air prior to supplying same to the cabin. An embodiment of the invention further comprises the steps of venting a portion of the return air stream out of the aircraft, and recirculating a remaining portion of the return air stream back into the cabin. In an embodiment of the present invention, during a taxi and ascent portion of a flight cycle, the envelope ventilation air stream and the cabin ventilation air stream are controlled to maintain the envelope at a slightly negative pressure relative to the cabin, and the return air stream is drawn from the envelope. Preferably, in such cases, the method further comprises the step of venting substantially all of the return air stream out of the aircraft. In an embodiment of the present invention, during an in-flight fire and/or pyrolysis within the envelope or in the cabin: the envelope ventilation air stream and the cabin ventilation air stream are controlled to maintain the envelope at a lower pressure than the cabin, and the return air stream is drawn from the envelope. Preferably, in such cases, the method further comprises the step of flooding at least a portion of the envelope with a chemical fire suppressant. Preferably, the cabin ventilation air comprises substantially all of the total flow of ventilation air. Still more preferably, substantially all of the return air stream is vented out of the aircraft. In an embodiment of the present invention, during ground operations of the aircraft, the ventilation air stream is provided by a conventional ground conditioned air supply unit, and the return air stream is drawn from the envelope. Preferably, in such cases, the method further comprises the step of venting substantially all of the return air stream from the upper lobe out of the aircraft. Still more preferably, the ventilation air stream is heated to accelerate volatilization of VOCs and any moisture within the envelope. In summary, the system of the present invention, at various stages of the aircraft flight cycle: • Removes at least in part contaminants from ventilation air entering the cabin; • Pressurizes the envelope with dry ventilation air; • Reduces entry into the cabin of contaminated envelope air; • Purges the envelope of some contaminant VOCs and moisture; • Reduces moisture condensation on cold surfaces of the envelope and thus reduces corrosion; • Dries the envelope; • In the event of a fire, prevents entry to the cabin of any smoke and other combustion products in the envelope, exhausts cabin air with any smoke and other combustion products directly to the envelope (rather than the floor grille), and ducts smoke and other combustion products from the envelope to the aircraft exhaust valve and thereafter out of the aircraft; • In the event of fire/pyrolysis in the envelope, ducts fire suppressants (existing or new supply) to the side of the upper and/or lower lobe segment of the envelope with the fire; The environment control system of the invention can be incorporated into new aircraft construction, or installed as an upgrade or retrofit in an existing aircraft. BRIEF DESCRIPTION OF THE DRAWINGS Further features and advantages of the present invention will become apparent from the following detailed description, taken in combination with the appended drawings, in which: Figure 1 shows a schematic cross sectional view through the body of an aircraft, showing components of an air handling system in accordance with an embodiment of the present invention; Figure 2 is an enlarged partial cross section illustrating a portion of the embodiment of Figure 1 in greater detail; Figure 3 is a schematic diagram illustrating the operation of the present invention during normal cruising flight; Figure 4 is a schematic diagram illustrating the operation of the present invention during taxi and ascent; Figure 5 is a schematic diagram illustrating the operation of the present invention during descent from cruising altitude and taxi after landing; Figure 6 is a schematic diagram illustrating the operation of the present invention during ground purging of the system; Figure 7 is a schematic diagram illustrating the operation of the present invention during an in-flight fire event; Figure 8a shows a gas chromatography/mass spectrometry (GC/MS) analysis plot of a ventilation air sample taken in a jet transport aircraft during flight (Temperature approximately 20°C); Figure 8b shows a gas chromatography/mass spectrometry (GC/MS) analysis plot of an envelope air sample taken in a jet transport aircraft on the ground at approximately 35°C; Figure 9a shows a gas chromatography/mass spectrometry (GC/MS) analysis plot of a head space sample of a jet engine lubricating oil at 100°C; Figure 9b shows a gas chromatography/mass spectrometry (GC/MS) analysis plot of a head space sample of a jet fuel at 90°C; Figure 9c shows a gas chromatography/mass spectrometry (GC/MS) analysis plot of a head space sample of an aircraft hydraulic fluid at 90°C; Figure 9d shows a gas chromatography/mass spectrometry (GC/MS) analysis plot of a head space sample of a general purpose cleaner used in aircraft at 90°C. Figure 9e shows a gas chromatography/mass spectrometry (GC/MS) analysis plot of a head space sample of an anti-corrosion treatment sprayed on metal surfaces in the envelope (- 5°C). It will be noted that throughout the appended drawings, like features are identified by like reference numerals. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to Figures 1-3, the body 1 of a typical jet transport aircraft is generally divided into upper and lower lobes. Figures 1 and 2 show a typical cross section between adjacent ribs. The upper lobe comprises that portion of the body (fuselage) 1 that generally extends above the floor 2 to enclose the cabin 3 (which may in fact have more than one level), and is normally occupied by crew and passengers during flight. Conversely, the lower lobe comprises that portion of the body 1 that generally extends below the floor 2, and normally houses cargo bays 4. Both lobes can conveniently be subdivided into port and starboard sides, which will be symmetrical with exceptions such as doors. As may be seen in Figure 1, the present invention can be used to provide controlled ventilation within all four quadrants of the body 1 (upper lobe-port side; upper lobe-starboard side; lower lobe-port side; and lower lobe-starboard side). For simplicity of description, the following discussion will focus on only one quadrant (upper lobe-port side) of the body, it being understood that the same provisions can be made (with appropriate substitutions of components) within each of the other quadrants as desired. An upper lobe envelope 5 encompasses the components of the body 1 between the outer skin 6 and the cabin liner 7. Similarly, a lower lobe envelope 8 encompasses the components of the body 1 between the outer skin 6 and the cargo bay liner 9. Conventionally, an anti-corrosion treatment 41 is applied on the interior surface of the skin and on structural members within the envelope. An insulation blanket 10 is normally provided within the upper and lower lobe envelopes 5, 8, and is typically secured to the stringers 11, so that a small gap 12 normally exists between the skin 6 and the outermost surface of the insulation 10. The present invention provides an environment control system which operates by controlling flow of air within both the cabin 3 and the upper and lower lobe envelopes 5 and 8. The system comprises an airflow control devicel3; upper and lower lobe envelope supply ducts 14P, 14S, 15P and 15S which communicate with the airflow control devicel3 and which run generally parallel to the aircraft longitudinal axis; one or more ventilation air branch lines 16 which communicate with each of the upper and lower lobe envelope supply ducts 14, 15 and extend into the respective upper and lower lobe envelopes 5, 8; a plurality of return air controllers 17 which communicate with a respective main return air duct 18P, 18S; an outflow valve 19 communicating with the main return air ducts 18 ; a cabin air conditioner 20; a cabin supply air duct 21; and a control unit 22. The lower lobe envelope supply ducts 15P and 15S and associated ventilation air branch lines 16 are independent of the main part of the system and can be omitted if desired. Referring now to Figure 3, air bled from the compressor section of an engine 23 in a conventional manner is conditioned (that is, cooled and possibly dehumidified by conventional air conditioning packs 23 a) to provide dry ventilation air 24 , which is supplied to the airflow control device 13. The airflow control device 13 operates in response to control signals A from the control unit 22 (or optionally is pre-set) to divide the flow of ventilation air 24 to create an envelope ventilation air stream 25, at least a portion of which is distributed to the upper lobe port side envelope 5 through the port-side upper envelope supply duct 14P and ventilation air branch lines 16, and a cabin ventilation air stream 26 which is supplied to the cabin air conditioner 20. In the illustrated embodiment, the airflow control device 13 is provided as a unitary control valve. However, it will be appreciated that the airflow control device 13 may be provided as any suitable combination of one or more valves; dampers, orifices or duct assemblies, which may be used in combination with conventional ventilation ducts previously existing within an aircraft. Similarly, the ventilation supply duct 14P may be a separate air supply duct, or may be a supply air duct, such as cabin or gasper ventilation air supply lines, previously installed in an aircraft. The ventilation air branch lines 16 are distributed at suitable intervals along the length of the upper envelope supply duct 14P so as to provide a distribution of ventilation air along the length of the upper lobe envelope 5. The number of ventilation air branch lines 16 will, in general, depend on the tightness of the envelope (i.e. leakage between cabin and envelope) and the presence of air-flow obstructions within the envelope. In aircraft with a particularly tight cabin liner and few obstructions to longitudinal flow within the envelope, as few as one ventilation air branch line 16 may be used. In other situations, a greater number of ventilation air branch lines 16 may be preferred. Conveniently, a single ventilation air branch line 16 can be provided in each rib space of the body 1. Each ventilation air branch line 16 includes a plurality (four are shown in the illustrated embodiment, see Fig. 1) of shell-side nozzles 27, which are designed to inject envelope ventilation air 25 behind the insulation 10, that is, into the space 12 between the skin 6 and the insulation 10. The shell-side nozzles 27 are distributed at suitable intervals around the circumference of the upper lobe envelope 5, so that envelope ventilation air 25 can be supplied to the envelope 5, behind the insulation 10. The number and spacing of shell-side nozzles 27 will depend on the tightness of the cabin liner, and the presence of obstructions to circumferential movement of air. Preferably, the envelope ventilation air flows are controlled to be sufficient to neutralize stack effect pressures of up to 1.5 Pa (with a least one flow blocker per side) and create slightly higher pressures in the envelope relative to the cabin (e.g., at least 0.5 Pa). The " stack effect" is a phenomenon which occurs within the envelope and which tends to cause a circumferential flow of air within the envelope. In general, envelope air between the insulation 10 and the cabin liner 7 tends to rise (because it is lower density); passes through the insulation 10 where it contacts the fuselage skin 6 and cools; the cold envelope air between the insulation 10 and the skin 6 tends to sink (because it is higher density), and passes back through the insulation 10 near the floor 2 of the cabin 3. The amount of this natural convective flow depends on cabin height, the temperature differential across the insulation 10, and the presence of flow restrictions. In a conventional aircraft fuselage, stack effect pressures of up to approximately 3 Pa or more can be encountered at cruising altitudes. In order to reduce stack effect, it is useful to provide at least one flow blocker 28 within the envelope 5, which serves to block circumferential movement of air within the envelope 5. Preferably, a flow blocker 28 is positioned between the panel 7 and the insulation 10, and squeezes the insulation against the skin 6 or stringer 11. In most conventional jet transport aircraft, a single flow blocker 28 will normally be sufficient. In such cases, the flow blocker 28 can advantageously be installed at approximately mid-height within the envelope 5 (i.e. just above the windows (not shown) on both sides of a conventional jet transport aircraft). This reduces stack effect pressures to approx. 3 Pa or less at cruising altitudes. In very large aircraft, particularly those with multi-level cabins, it may be necessary to install two or more flow blockers 28 on each side. Optionally, one or more cabin-side nozzles 29 (two are shown in the embodiment of Figure 1) can also be provided in order to inject envelope ventilation air 25 into the upper lobe envelope 5 in front of the insulation 10, that is, between the insulation 10 and the cabin liner 7. When the envelope ventilation air 25 is injected behind the insulation 10, the envelope ventilation air 25 will be cooled well below the cabin temperature (for example, by as much as 60°C, going from +20°C to -40°C). This cooling promotes ventilation air contaminant sorption and condensation in the envelope. In particular, most VOCs identified in cabin ventilation air (see Figure 8a) may condense at temperatures well above -40°C on cold envelope surfaces (for example the interior surface of the fuselage skin 6 and adjoining structural members), during cruising flight. Particles (e.g. oil aerosol) entrained within the envelope ventilation air stream may impact and adhere to the interior surface of the skin (or adjoining surfaces), and/or will be removed (by physical filtration or electrical forces) as the air passes through the insulation blanket 10 toward the cabin. It will be noted that any water vapor present in the envelope ventilation air 25 will also tend to condense on the cold surfaces within the envelope 5. However, because of the extremely low relative humidity of the envelope ventilation air 25, at least during the cruise phase of flight, the amount of moisture likely to accumulate within the envelope is negligible. Sorption of VOCs within the envelope can be enhanced by replacing the conventional anti-corrosion treatment 41 with an improved composition having both anti-corrosive and enhanced VOC sorbent properties. The combined anti-corrosion/VOC sorption treatment 41 on the skin and structural members in the envelope is formulated to: not freeze at temperatures above -50°C; maximize sorption of typical ventilation air VOCs in the temperature range 0 to -40°C; and maximize desorption of these compounds in the temperature range 10°C and higher. A particularly suitable formulation will be capable of performing multiple sorption/desorption cycles without hysteresis (i.e. it does not gradually become loaded with effectively permanently sorbed VOCs) or chemical degradation. It contains an anti-oxidant that ensures that it will not harden for several years and so will remain sorbent between regular maintenance cycles when it can be renewed. The envelope ventilation air 25, after being cooled, passes through the insulation 10 to the cabin liner 7. During this passage, the air is heated by the dynamic insulation effect before it enters the cabin 3. If the envelope ventilation air 25 is injected in front of the insulation, contaminant removal through sorption and condensation is reduced. However, the envelope 5 is still pressurized with dry air throughout, preventing humid cabin air entry and thus allowing the cabin 3 to be humidified to desirable levels. Nozzles placed behind the insulation 10 improve the efficiency of VOC contaminant removal during flight at cruising altitudes through sorption and condensation, removal of ozone through surface contact with reactive materials, and deposition of particles through centrifugal and electrical forces. Nozzles placed in front of the insulation 10 simplify the installation and reduce heat loss. Either option, taken alone or in combination, can be utilized as required. In order to ensure that ventilation air passes from the envelope and into the cabin, the cabin must be maintained at a slight negative pressure relative to the envelope. This can be accomplished by drawing return air from the cabin, by connecting the return air ducts 18 in communication with the cabin space, possibly via one or more simple return air grills. In order to provide enhanced system capability, one or more return air control units 17 are provided at suitable intervals along the length of body 1, as shown in Figures 1 and 2. The use of such return air control units 17 permits return air to be selectively drawn from either the cabin or the envelope, as desired, thereby facilitating smoke removal, envelope purging, and fire suppressant injection while maintaining a negative pressure in the envelope relative to the cabin. Conveniently, a return air control unit 17 can be provided in association with conventional return air ducting arrangements previously provided within an existing aircraft. In the illustrated embodiment, a return air control unit 17 is provided in each rib space, at the floor level of the upper lobe envelope 5. Each return air control unit 17 comprises a housing 30 having an envelope opening 31 communicating with the upper lobe envelope 5, and a cabin opening 32 communicating with the cabin 3. A damper 33 within the housing 30 enables a selected one of the envelope opening 31 and the cabin opening 32 to be opened and the other to be closed. Thus return air can be selectively drawn from within the envelope 5 or the cabin 3, as desired and in accordance with the operating regime of the aircraft. The position of the damper 33 can be controlled by any suitable drive means (not shown), such as, for example, a solenoid, servo motor or pneumatic actuator in response to control signals B received from the control unit 22. Each return air control unit 17 communicates with the main return air duct 18 through which return air 34 (whether drawn from the envelope or the cabin) can be removed from the upper lobe of the body 1. Return air 34 from the cabin 3 (or the envelope 5) flows through the main return air duct 18P and is supplied to the (conventional) outflow valve 19. The outflow valve 19 operates in response to control signals C received from the control unit 22 to maintain cabin pressurization, vent at least a portion of the return air 34 out of the aircraft as exhaust air 35, and (possibly) supply the remainder of the return air 34 to the cabin air conditioner 20 as recirculated air 36. The cabin air conditioner 20 generally comprises one or more conventional mixing and filtering units 20a and a humidity control unit 20b which operates in response to control signals D from the control unit 22. In operation, the cabin ventilation air stream 26 from the airflow control device 13, and recirculated air 36 from the outflow valve 19 are combined in a mixing unit 20a, then filtered, cooled (or heated) as required, and humidified by the humidity control unit 20b to create cabin supply air 37. The cabin supply air 37 is then supplied to the cabin through the supply air duct 21. In the illustrated embodiment, fire suppression is provided by means of a container of chemical fire suppressant 38 , such as, for example Halon (trade name) or an equivalent, connected to the envelope supply ducts 14 and 15 via a valve (or valves) 39 which is responsive to a control signal E from the control unit 22 . Upon opening the valve 39, chemical fire suppressant is supplied to the envelope 5 to extinguish the fire. This fire suppressant supply could be from an existing cargo fire suppressant system or it could be added. If desired, each of the envelope supply ducts 14P, 14S, 15P and 15S can be provided with its own valve 39, which can be independently controlled by the control unit 22. In this case, chemical fire suppressant 38 can be drawn from a single, common container, or from separate independent containers as desired. This arrangement has the benefit that chemical fire suppressant can be selectively delivered to any desired quadrant of the envelope 5P, 5S, 8P and 8S. Thus smoke/fire detectors can be strategically distributed within the envelope (for example near electrical devices or other potential sources of ignition) so that the approximate location of a fire can be detected. Upon detection of a fire, the flight crew can choose to flood only that quadrant of the envelope in which the fire has been detected, thereby conserving fire suppressant and/or facilitating the delivery of higher concentrations of fire suppressant to those areas of the envelope where it is most needed. The control unit 22 can suitably be provided as an environment control panel within the cockpit of the aircraft. The control unit 22 can be designed as a simple switch panel, allowing the flight crew to manually control the operation of the airflow control device 13, return air control units 17, outflow valve 19 , cabin air conditioner 20 and fire suppressant valve 39. Alternatively, the control unit 22 can be at least partially automated, such that the operation of the system can be controlled in accordance with one or more predetermined programs and signals. The environment control system of the invention can be incorporated into new aircraft construction, or installed as an upgrade or retrofit in an existing aircraft. Appropriate evaluation of the aircraft mission (e.g. requirements of moisture control, and whether or not air quality control and additionally fire/smoke suppression are required) and testing of the recipient aircraft type (e.g. configuration and geometry) will reveal the numbers, sizing and preferred locations for each of the elements of the system, as well as which ones (if any) of the optional elements (e.g. flow blockers, cabin-side nozzles, selectable flow return air control units, humidifiers etc.) are required in order to obtain desired operational characteristics. Upgrading an existing aircraft ventilation system in accordance with the illustrated embodiment, which incorporates all optional elements, can be accomplished by the following steps: • The cabin liner 7 and the insulation 10 are removed to obtain access to the envelope 5; • One or more lines of flow blockers 28 are installed on each side; • An anti-corrosion/VOC sorbent material 41 is applied on the metal in the envelope; • The insulation 10 is refitted as necessary to make a continuous blanket. Either new insulation can be used, or the existing insulation can be reinstated; • The fire suppressant container 38 (existing or new, if desired) and its control valve(s) 39 are installed; • Upper lobe envelope ventilation supply ducts 14 (and lower lobe envelope ventilation supply ducts 15 if desired) and the associated branch lines 16, including shell-side nozzles 27 and (if desired) cabin-side nozzles 29 are installed; • A cabin air conditioner (filter, humidifier) is installed and interconnected. The air conditioner outlet (cabin supply air) is connected to the existing cabin air ducting, which thereafter functions as the cabin supply air duct system; • The airflow control device 13 is installed and connected to the main ventilation duct and to the cabin ventilation and envelope ventilation supply ducts. • Return air control units 17 are installed in the existing return air plenums at the floor level of the cabin envelope 5. Care is required to ensure proper sealing around the housings of the return air control units 17 so as to minimize leakage; • Return air ducts 18 are installed on both sides of the aircraft and connected with the return air control units 17 and the existing outflow valve 19; • The system main control unit 22 is installed in the cockpit and connected to the airflow control device 13, return air control units 17, outflow valve 19 air conditioner 20 and fire suppression valve 39 in order to control the various elements of the system. In addition sensors for detecting temperature, humidity, smoke(fire) within the cabin and envelope and optionally an envelope/cabin pressure difference logger are installed at desired locations within the cabin and envelope and connected to the control unit 22 to provide information in respect of system operation; • If desired, heat exchanger units are installed in the lower lobe and interconnected with the return air ducts 18, and associated thermostats located in the cargo bay(s) 4, so that the cargo bay(s) 4 can be heated by warm return air 34. • Finally, the cabin liner 7 is reinstalled, with care being taken to close holes and gaps, so that desired pressures can be maintained within normal cabin ventilation air flow rates. In use, the above-described system can provide controlled ventilation of the upper lobe envelope 5 and within the cabin 3, in various ways, depending on the flight regime of the aircraft. In the following examples, four modes of operation of the system are described, with reference to Figures 3 to 7. Example 1, Normal Cruising Flight Under normal operation at cruising altitude, the system envelope flow 25 and cabin flow 26 are controlled such that the envelope pressure is slightly greater than that of the cabin. The envelope ventilation air 25 supplied to the envelope 5 through the shell-side nozzles 27 contacts the cold skin 5 and contaminants are removed at least in part by sorption (e.g., by the anti-corrosion/sorption treatment 41), condensation and filtration (e.g., by centrifugal and electrical forces), and then stored on the interior surface of the skin 5 and other cold surfaces within the envelope or as an aerosol. The extremely low relative humidity of the ventilation air 24, and thus the envelope ventilation air 25 (typically less than approx. 5% at cabin temperatures) means that no significant moisture condensation will accumulate within the envelope 5. The envelope ventilation air 25 then flows back through the insulation 10 (as shown by the arrows in Figure 3), and enters the cabin 3 by leakage through the seams 40 between panels of the cabin liner 7. For example, an envelope pressurization relative to the cabin 3 of between 0.5 and 5 Pa (preferably between approximately 1-2 Pa) and total envelope ventilation air 24 injection flows of less than the minimum cabin ventilation rate required for passenger transport aircraft of 10 c.f.m. per person can (at 8,000 ft. cabin pressure altitude) be maintained for a cabin liner 7 paneling leakage area of less than 440 cm 2 per six passenger row (for a 5 cfm injection flow, and a stack pressure of 2 Pa, the leakage area per row can be up to 100 cm ). For a leakage area of 440 cm , moisture diffusion from the cabin to the envelope through typical panel openings is less than 5 mg/s per row (crack length) at a cabin humidity of 60%. At this rate, a 30 row 180 passenger plane would accumulate a maximum of about 1 pound of moisture during a three hour flight. Actually, it will be negligible because convective transfer from the envelope to the cabin will offset upstream or back diffusion. To achieve the allowable leakage areas, the integrity of the cabin liner 7 paneling must be maintained throughout and any openings at the overhead compartment must be sealed. With this degree of sealing, during a sudden aircraft depressurization event (for example, if a cargo door opens in flight), one or more panels of the cabin liner 7 will "pop" to equalize the pressure difference between the cabin 3 and the envelope 5. Additionally, the damper 33 of the return air control units 17 can be designed so that both the envelope opening 31 and the cabin opening 32 will open automatically in a sudden depressurization event. When insulation continuity is maintained, envelope ventilation air 25 entering the cabin 3 from behind the insulation 10 will be warmed by dynamic insulation heat recovery as it passes through insulation 10 gaps. As shown in Figure 3, during normal flight at cruising altitude, envelope ventilation air 25 is injected behind and/or in front of the insulation 10, and the cabin recirculation system is operating (that is, cabin supply air 37 made up of cabin ventilation air 26 and recirculated air 36 are being supplied to the cabin 3 via the cabin air conditioner 20). The return air control units 17 are set so that return air 34 is drawn from the cabin 3. In this mode, the cabin air conditioner 20 can be operated to maintain cabin relative humidity levels in excess of 20% (preferably between 40 and 50%). Moisture condensation within the envelope 5 from humid cabin air is prevented by the relative pressurization of the envelope 5 and the envelope is kept dry. Furthermore, contaminant gases and particles within the ventilation air 25 are removed in part prior to entering the cabin 3 by sorption and condensation, and physical filtering as it passes back through the insulation 1, thereby improving cabin air quality over that typically encountered in conventional aircraft. Return air 34 is drawn from the cabin 3 through the return air control units 17 and the main return air duct 18. If desired, this return air 34 can be used to heat the lower lobe through the use of one or more heat exchangers (not shown). The outlet valve 19 operates to vent a portion of the return air 34 out of the aircraft as exhaust air 35, and supplies the remainder as recirculated air 36 to the cabin air conditioner 20. Example 2, Taxi and Ascent Figure 4 illustrates system operation during taxi and ascent to cruising altitude. Conventionally, the cabin 3 pressure is maintained to an altitude equivalent of approximately 8000 ft., which means that the cabin pressure during the cruise phase of flight will be approximately three-quarters of sea level pressure. Thus during the initial portion of ascent, the cabin depressurizes, and approximately one quarter of the air in the envelope 5 at take-off would normally tend to bleed into the cabin 3. During this period, the envelope 5 will be relatively warm in comparison to cruising altitude temperatures, and VOCs sorbed and condensed in the envelope may volatilize. The airflow control device 13 is operated to pressurize the cabin relative to the envelope. At the same time, the return air control units 17 are controlled to draw return air 34 from the envelope 5, and the outflow valve 19 vents all of the return air 34 out of the aircraft as exhaust air 35. This operation effectively purges VOC contaminants (chemical and microbial, if any) within the envelope 5, and prevents them from entering the cabin 3. In a conventional aircraft ventilation system, these contaminants would normally be drawn into the cabin during ascent. Example 3, Descent and Taxi Figure 5 illustrates system operation during descent from cruising altitude as the cabin pressurizes, and taxi after landing. During this period the envelope is comparatively cold relative to the outside temperatures, and injection of ventilation air into the envelope during this phase of flight would cause accumulation of moisture condensation. Accordingly, for descent and taxi, the airflow control device 13 operates to divert all ventilation air 24 into the cabin air conditioner 20 , and the return air control units 17 draw return air 34 from the cabin 3, thereby effectively isolating the envelope 5. The outflow valve 19 can be operated to vent all of the return air 34 as exhaust 35 or recycle some of the return air 34 back to the cabin air conditioner 20 as desired. Example 4, Ground Purging Operation of the environment control system of the invention during taxi and ascent (Example 2 above) is effective in purging VOCs from the envelope 5. However, in some cases it may be considered good practice to perform additional purging of the upper lobe envelope 5 as well as the lower lobe envelope 8 while the aircraft is parked (such as, for example, between flights). In this case, ventilation air 24 can be provided by a conventional ground conditioned air supply unit 42 connected to the two upper lobe ventilation air ducts 14 upstream of the airflow control device 13, as shown in Figure 6, and to the two lower lobe ducts 15. The airflow control device 13 directs ventilation air 24 into the envelope 5 via branch ducts 16 as envelope ventilation air 25, in order to volatilize VOCs adsorbing within the envelope 5 and to remove moisture. The ground conditioned air supply unit 42 is also connected to the lower lobe supply ducts 15 and branch ducts 16 to vent any moisture in this portion of the envelope. In order to accelerate this process, it may be desirable to operate the conditioned air supply unit 42 so as to heat the ventilation air 24. The return air control units 17 are set to draw return air 34 from the envelope 5, and the outflow valve 19 vents all of the return air 34 out of the aircraft as exhaust 35. This operation will remove moisture and air contaminant accumulation, if present, in the upper and lower lobe envelopes. Example 5, In-flight Fire and/or Pyrolysis Figure 7 illustrates the air handling system operation during an in-flight fire event in the envelope. When smoke (or combustion products) indicative of a fire is detected, the airflow control device 13is set to divert all ventilation air 24 to the cabin air conditioner 20. At the same time, the return air control units 17 are set to draw return air 34 from the envelope 5, and the outflow valve 19 operates to vent all of the (smoke-laden) return air 34 out of the aircraft as exhaust air 35. Diversion of the ventilation air 24 to the cabin air conditioner 20 (with the cabin air conditioner 20 on) allows the cabin 3 to be pressurized relative to the envelope 5, and thereby prevent infiltration of smoke and combustion products into the cabin 3 if the fire is in the envelope 5. At that stage, fire suppressant can be injected into the envelope (either the entire envelope 5 can be flooded with fire suppressant, or, alternatively, the fire suppressant may be directed into a selected quadrant of the envelope). Maintaining a positive cabin pressure relative to the envelope ensures that smoke, fire suppressant, and combustion products are substantially prevented from entering the cabin, thereby providing effective separation of passengers from noxious gases. If desired, however, the cabin air conditioner 20 can be turned off to stop the flow of ventilation air 24 into the cabin 3, after injection of fire suppressant into the envelope 5. This can be used to reduce the supply of oxygen available to the fire, but at the expense of allowing combustion products to leak into the cabin 3. Alternatively, if the fire is in the lower lobe envelope, then fire suppressant can be injected into that portion of the envelope using ducts 15 and 16. This system has the advantage over current fire suppression systems of not exposing animals, if present, to the health and safety hazards of fire suppressants and their combustion products in combination with fire and smoke. The above detailed description and examples define a preferred embodiment of the present invention, in which ventilation air may be independently supplied to each of four quadrants of the envelope 5; shell-side and cabin-side nozzles 27, 29 are respectively used to inject ventilation air behind and in front of the insulation blankets 10; envelope air flows due to stack effects are restricted by the use of flow blockers 28; chemical fire suppressants can be selectively injected into the envelope 5; and means are provided for on-the-ground purging the envelope 5 by the use of a ground conditioned air supply unit connected to the ventilation air inlet ducts. However, the skilled artisan will recognize that these features can be used in any desired combination, depending on the design and mission of the particular aircraft in question. For example, the skilled artisan will appreciate that the envelope 5 need not necessarily be divided into four quadrants, each of which are served by independent ventilation supply systems. It is not necessary to divide the envelope 5 into upper and lower lobes, if such a division is not desired by the aircraft designer. If desired, the envelope ventilation air stream 25, can be divided into upper and lower lobe supply streams, or alternatively both lobes of the envelope 5 can be ventilated using a common envelope ventilation air stream 25. Similarly, it is possible to utilize shell-side nozzles 27 alone; or cabin-side nozzles 29 alone; or shell-side nozzles 27 in one area of the envelope 5, and cabin-side nozzles 29 in another area of the envelope 5, all as deemed appropriate by the designer. Similarly, the skilled artisan will appreciate that the envelope 5 need not necessarily be divided into upper and lower, port and starboard quadrants. In practice, it is possible to divide the envelope 5 as required to provide a localized ventilation regime appropriate to a specific portion of the envelope 5. For example, it may be desirable to provide a ventilation regime in the crown portion of the envelope 5 (e.g. to eliminate "rain-in-the-plane" phenomenon) which differs from that provided in the sides of the envelope 5. Division of the envelope 5 in this manner can readily be accomplished by means of the present invention. Furthermore, the skilled artisan, will also recognize that, just as the envelope 5 can be divided radially into quadrants, it is also possible to divide the envelope 5 longitudinally into sections, such as, for example, by means of suitable flow barriers circumferentially disposed between the cabin liner 7 and the shell 6. Each longitudinal section may also be provided with independent envelope and cabin ventilation air steams 25, 26, and may also include its own set of return air control units 17, and return air ducts 34 etc. to thereby allow envelope ventilation control independent of other sections of the envelope 5. For example, it may be desirable to provide independently controllable envelope/cabin ventilation (e.g. in terms of air pressures and flow rates) in the cockpit and passenger cabin. Furthermore, within the passenger cabin, in may be desirable to have differing envelope ventilation regimes within passenger seating and food preparation areas. This can be accomplished by longitudinally dividing the envelope 5 into appropriate sections, and providing envelope and cabin ventilation air ducts 14, 21, appropriate cabin and/or shell-side nozzles 27, 29, and return air control units 17 etc. as required to provide the desired ventilation regime within each section. Longitudinal division of the envelope 5 also creates a further mode of operation of the system of the present invention during a fire or pyrolysis event. In particular, in a case of smoke in the cockpit, it would be possible to control ventilation regimes in all of the sections of the envelope 5 to deliver maximum air flow to the cockpit (perhaps with reduced ventilation air flow to the passenger cabin), and thereby more effectively purge smoke and combustion products from the cockpit area. In the illustrated embodiment, the return air control unit 17 and cabin air inlet 32 are located in the envelope space 5 near the floor 2 of the cabin. However, it will be appreciated that these components may equally be located elsewhere as deemed appropriate by the aircraft designer. Similarly, the locations or the envelope ventilation supply ducts 14, 15, the return air ducts 18 and the cabin ventilation supply duct 21 can be varied as deemed appropriate by the designer. The ability of the system of the invention to pressurize the cabin relative to the envelope, or vise- versa, is inherent to the present invention, and may be utilized to achieve any of the operating modes (in terms of envelope and cabin ventilation, and return air recirculation and venting) described in the above examples. However, it will be apparent that one or more of the operating modes may be omitted, if such mode of operation is unnecessary for the mission and/or design of any particular aircraft. For example, in some aircraft, it may be desirable or necessary to omit operating modes in which the cabin is pressurized relative to the envelope. In such circumstances, all return air may be drawn from the cabin exclusively, in which case the return air controller may be replaced by a simple fixed return air inlet in communication with the return air ducts. It is considered that the use of flow blockers will reduce natural convective (stack-effect) air flows within the envelope, and that this would likely have the effect of reducing moisture condensation within the envelope, even in the absence of envelope pressurization. The capability of the system of the present invention to pressurize the envelope with dry ventilation air will serve to virtually eliminate moisture condensation within the envelope, at least during the cruise portion of the flight cycle. The skilled artisan, will appreciate that flow blockers may be used independently of the other elements of the invention described herein. Thus the skilled artisan will recognize that flow blockers could be incorporated into an aircraft, even in the absence of an envelope ventilation system. Similarly, an envelope ventilation system may be used either in conjunction with, or without, flow blockers. Thus it will be appreciated that the above description of a preferred embodiment is intended to describe various elements, which may be used alone or in any desired combination as desired to achieve as appropriate to the particular circumstances. It will therefore be understood that the above-described preferred embodiment is intended to be illustrative, rather than limitative of the present invention, the scope of which is delimited solely by the appended claims. INDUSTRIAL APPLICABILITY The present invention is applicable to the aviation industry.

Claims: CLAIMS; 1. An environment control system for a body of an aircraft including at least a cabin and an envelope, the environment control system comprising: a) an air supply capable of providing a flow of dry ventilation air to the interior of the body; b) an airflow control device in communication with the air supply and capable of producing at least an envelope ventilation air stream; c) an envelope air distribution system capable of directing the envelope ventilation air stream into the envelope; d) a return air control unit capable of drawing a return air stream from the cabin. 2. An environment control system as defined in claim 1, wherein the airflow control device is further capable of dividing the flow of ventilation air into an envelope ventilation air stream and a cabin ventilation air stream. 3. An environment control system as defined in claim 2, further comprising a cabin air distribution system capable of directing the cabin ventilation air stream into the cabin. 4. An environment control system as defined in claim 3, wherein the return air control unit is capable of drawing a return air stream from a selected one of the cabin and the envelope. 5. An environment control system as defined in claim 1, wherein the envelope is divided into an upper lobe and a lower lobe, the envelope air distribution system being adapted to provide a respective upper lobe and lower lobe envelope ventilation air stream. 6. An environment control system as defined in claim 5, wherein each of the upper lobe and lower lobe are further divided into port and starboard sides, to define four quadrants of the envelope, the envelope air distribution system being adapted to provide a respective portion of the envelope ventilation air stream to each quadrant. 7. An environment control system as defined in claim 5, wherein the airflow control device is capable of controlling the upper and lower lobe envelope ventilation air streams to deliver respective different air flows to the upper lobe envelope and the lower lobe envelope. 8. An environment control system as defined in claim 1, wherein the air supply includes an air supply duct adapted to conduct bleed air from a compressor stage of an engine of the aircraft into the body of the aircraft. 9. An environment control system as defined in claim 1, wherein the envelope air distribution system comprises at least one envelope supply duct disposed longitudinally in the aircraft body, and at least one respective ventilation air branch line, whereby the envelope ventilation air can be distributed throughout the envelope in such a manner as to offset stack pressures. 10. An environment control system as defined in claim 9, wherein each ventilation air branch line comprises at least one nozzle for injecting ventilation air into the envelope. 11. An environment control system as defined in claim 10, wherein at least one nozzle is a shell-side nozzle capable of injecting envelope ventilation air between an insulation jacket and the exterior skin of the aircraft. 12. An environment control system as defined in claim 11, wherein two or more shell-side nozzles are provided in communication with each ventilation branch line, the shell-side nozzles being disposed at spaced intervals around a circumference of the envelope. 13. An environment control system as defined in claim 10, wherein at least one nozzle is a cabin-side nozzle capable of injecting envelope ventilation air between an insulation jacket and the cabin liner. 14. An environment control system as defined in claim 13, wherein two or more cabin-side nozzles are provided in communication with each ventilation branch line, the cabin-side nozzles being disposed at spaced intervals around the circumference of the envelope. 15. An environment control system as defined in claim 1, further comprising an anti-corrosion/VOC sorption treatment applied to an interior surface of the aircraft structure possibly exposed to condensation. 16. An environment control system as defined in claim 15, wherein the anti-corrosion/VOC sorption treatment is formulated to provide acceptable characteristics of: adhesion to metal surfaces; hydrophobic; low flammability; and low off-gassing at typical envelope temperatures during cruising flight. 17. An environment control system as defined in claim 16, wherein the anti-corrosion/VOC sorption treatment is formulated to: resist solidification within the aircraft envelope; sorb ventilation air VOCs at typical envelope temperatures during cruising flight and desorb said ventilation air VOCs at warmer temperatures substantially without hysteresis. 18. An environment control system as defined in claim 1, wherein the return air control unit is adapted to draw the return air stream from the cabin only. 19. An environment control system as defined in claim 1, wherein the return air control unit is adapted to selectively draw the return air stream from either one of the cabin and the envelope. 20. An environment control system as defined in claim 19, wherein the return air control unit comprises a housing, an envelope opening defined in the housing and in communication with the envelope, a cabin opening defined in the housing and in communication with the cabin, and a damper capable of selectively closing the envelope opening and the cabin opening. 21. An environment control system as defined in claim 1, further comprising at least one flow blocker disposed in the envelope and capable of at least partially blocking a circumferential flow of air within the envelope. 22. An environment control system as defined in claim 21, wherein there is at least one pair of flow blockers disposed within the envelope, members of each pair being symmetrically disposed on opposite sides of the body of the aircraft. 23. An environment control system as defined in claim 21, wherein the envelope air distribution system comprises at least one nozzle capable of injecting at least a portion of the envelope ventilation air stream into a portion of the envelope below each flow blocker, and at least one nozzle capable of injecting at least a portion of the envelope ventilation air stream into a portion of the envelope above each flow blocker. 24. An environment control system as defined in claim 21, wherein a flow blocker is disposed within the envelope at approximately mid-height of an upper lobe of the body of the aircraft. 25. An environment control system as defined in claim 1, wherein the cabin air distribution system comprises: an air conditioner communicating with the airflow control device for receiving at least a portion of the cabin ventilation air, and operative to condition the cabin ventilation air to create cabin supply air; and a cabin supply air duct capable of directing the cabin supply air into the cabin. 26. An environment control system as defined in claim 25, wherein the air conditioner is operative to control the relative humidity of the cabin supply air. 27. An environment control system as defined in claim 26, wherein the air conditioner is operative to maintain a relative humidity of the cabin supply air of at least 20%. 28. An environment control system as defined in claim 27, wherein the air conditioner is operative to maintain a relative humidity of the cabin supply air of between 20% and 80%. 29. An environment control system as defined in claim 28, wherein the air conditioner is operative to maintain a relative humidity of the cabin supply air of between 40% and 70%. 30. An environment control system as defined in claim 1, further comprising a return air duct in communication with the return air control unit, for conducting a flow of return air therefrom. 31. An environment control system as defined in claim 30, further comprising an outflow valve in communication with the return air duct, the outflow valve being capable of dividing the return air stream into an exhaust air stream and a recirculation air stream, the exhaust air stream being vented out of the aircraft, and the recirculation air stream being supplied back to the cabin 32. An environment control system as defined in claim 31, wherein the recirculation air stream is supplied to the cabin via an air conditioner. 33. An environment control system as defined in claim 1, wherein the airflow control device and the return air control unit are capable of operating cooperatively to maintain a predetermined pressure differential between the cabin and the envelope. 34. An environment control system as defined in claim 1, further comprising a fire suppression system in communication with the envelope air distribution system, the fire suppression system being capable releasing a flow of chemical fire suppressant into at least the envelope air distribution system when smoke or fire is detected in the envelope. 35. An environment control system as defined in claim 34, wherein the fire suppression system and the envelope air distribution system are adapted to cooperate to flood at least a portion of the envelope with the chemical fire suppressant. 36. An environment control system as defined in claim 34, wherein the fire suppression system comprises a container of chemical fire suppressant, a supply line in communication with the container and the envelope air distribution system for conducting the chemical fire suppressant there between, and a valve capable of controlling a flow of chemical fire suppressant from the container. 37. An environment control system as defined in claim 36, wherein the chemical fire suppressant is any one or more of Halon, carbon dioxide, nitrogen, and other fire suppressant agents, or mixtures of these. 38. A method of controlling the environment within an aircraft body including at least a cabin and an envelope, the method comprising: a) providing a flow of dry ventilation air; b) dividing the flow of ventilation air into an envelope ventilation air stream and a cabin ventilation air stream; c) supplying the envelope ventilation air to the envelope; d) supplying the cabin ventilation air to the cabin; e) drawing a return air stream from a selected one of the envelope and the cabin; and f) controlling the envelope ventilation air stream and the cabin ventilation air stream to maintain a predetermined pressure differential between the cabin and the envelope. 39. The method of claim 38, wherein, during a cruising portion of a flight cycle, the envelope ventilation air stream and the cabin ventilation air stream are controlled to maintain the envelope at a higher pressure than the cabin, and the return air stream is drawn from the cabin. 40. The method of claim 38, further comprising the step of injecting at least a portion of the envelope ventilation air into a space between an exterior skin of the aircraft body and an insulation jacket. 41. The method of claim 38, further comprising the step of injecting at least a portion of the envelope ventilation air into a space between an insulation jacket and a cabin liner. 42. The method of claim 38, further comprising the step of humidifying the cabin ventilation air prior to supplying same to the cabin. 43. The method of claim 39, further comprising the steps of venting a portion of the return air stream out of the aircraft and recirculating a remaining portion of the return air stream back into the cabin. 44. The method of claim 38, wherein during a taxi and ascent portion of a flight cycle, the envelope ventilation air stream and the cabin ventilation air stream are controlled to maintain the envelope at a slightly negative pressure relative to the cabin, and the return air stream is drawn from the envelope. 45. The method of claim 44, further comprising the step of venting substantially all of the return air stream out of the aircraft. 46. The method of claim 38, wherein during an in-flight fire and/or pyrolysis within the envelope or in the cabin: the envelope ventilation air stream and the cabin ventilation air stream are controlled to maintain the envelope at a lower pressure than the cabin, and the return air stream is drawn from the envelope. 47. The method of claim 46, further comprising the step of flooding at least a portion of the envelope with a chemical fire suppressant. 48. The method of claim 46, wherein the cabin ventilation air comprises substantially all of the total flow of ventilation air. 49. The method of claim 46, further comprising the step of venting substantially all of the return air stream out of the aircraft. 50. The method of claim 38, wherein during ground operations of the aircraft, the ventilation air stream is provided by a conventional ground conditioned air supply unit, and the return air stream is drawn from the envelope. 51. The method of claim 50, further comprising the step of venting substantially all of the return air stream from the upper lobe out of the aircraft. 52. The method of claim 51, further comprising the step of heating the ventilation air stream to accelerate volatilization of VOCs and any moisture within the envelope.

Inventors: MCNEIL CAMPBELL S L, MITALAS GINTAUTAS P, PRESTON KEITH F, WALKINSHAW DOUGLAS S, MCNEIL, CAMPBELL, S., L., MITALAS, GINTAUTAS, P., PRESTON, KEITH, F., WALKINSHAW, DOUGLAS, S.
Applicants: INDOOR AIR TECHNOLOGIES INC, MCNEIL CAMPBELL S L, MITALAS GINTAUTAS P, PRESTON KEITH F, WALKINSHAW DOUGLAS S, INDOOR AIR TECHNOLOGIES INC., MCNEIL, CAMPBELL, S., L., MITALAS, GINTAUTAS, P., PRESTON, KEITH, F., WALKINSHAW, DOUGLAS, S.
Application year: 1999
Main classification: B64D13
IPC codes: A62C3, B64C1, B64D13, B64D25