feet psi hPa in Hg °F °C Altitude Pressure Temperature 0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000 9,000 10,000 12,000 14,000 16,000 18,000 20,000 22,000 24,000 26,000 28,000 30,000 32,000 34,000 36,000 38,000 40,000 45,000 50,000 14.69 14.18 13.66 13.17 12.69 12.23 11.77 11.34 10.92 10.51 10.10 9.34 8.63 7.96 7.34 6.76 6.21 5.70 5.22 4.78 4.37 3.99 3.63 3.30 3.00 2.73 2.14 1.70 1013.2 977.2 942.1 908.1 875.1 843.1 812.0 781.8 752.6 724.3 696.8 644.4 595.2 549.2 506.0 465.6 427.9 392.7 359.9 329.3 300.9 274.5 250.0 227.3 206.5 187.5 147.5 116.0 29.92 28.86 27.82 26.82 25.84 24.90 23.98 23.09 22.23 21.39 20.58 19.03 17.58 16.22 14.94 13.75 12.64 11.60 10.63 9.72 8.89 8.11 7.38 6.71 6.10 5.54 4.35 3.42 59.0 55.4 51.9 48.3 44.7 41.2 37.6 34.0 30.5 26.9 23.3 16.2 9.1 1.9 –5.2 –12.3 –19.5 –26.6 –33.7 –40.9 –48.0 –55.1 –62.2 –69.4 –69.4 –69.4 15 13 11 9.1 7.1 5.1 3.1 1.1 –0.8 –2.8 –4.8 –8.8 –12.7 –16.7 –29.7 –24.6 –28.6 –32.5 –36.5 –40.5 –44.4 –48.4 –52.4 –56.3 –56.5 –56.5 Figure 16-40.  Cabin environmental systems establish conditions  quite different from these found outside the aircraft. Compressible seals around doors combine with various other  seals, grommets, and sealants to essentially establish an air  tight pressure vessel. This usually includes the cabin, flight  compartment, and the baggage compartments. Air is then  pumped into this area at a constant rate sufficient to raise  the pressure slightly above that which is needed. Control is  maintained by adjusting the rate at which the air is allowed  to flow out of the aircraft. A key factor in pressurization is the ability of the fuselage to  withstand the forces associated with the increase in pressure  inside the structure versus the ambient pressure outside. This  differential pressure can range from 3.5 psi for a single- engine reciprocating aircraft, to approximately 9 psi on  high performance jet aircraft.  [Figure 16-41]  If the weight  of the aircraft structure were of no concern, this would not  be a problem. Making an aircraft strong for pressurization,  yet also light, has been an engineering challenge met over  numerous years beginning in the 1930s. The development of  jet aircraft and their ability to exploit low drag flight at higher  altitude made the problem even more pronounced. Today,  the proliferation of composite materials in aircraft structure  continues this engineering challenge.  In addition to being strong enough to withstand the pressure  differential between the air inside and the air outside  the cabin, metal fatigue from repeated pressurization  and depressurization weakens the airframe. Some early  pressurized aircraft structures failed due to this and resulted  in fatal accidents. The FAA’s aging aircraft program was  instituted to increase inspection scrutiny of older airframes  that may show signs of fatigue due to the pressurization cycle. Aircraft of any size may be pressurized. Weight  considerations when making the fuselage strong enough  to endure pressurization usually limit pressurization to  high performance light aircraft and larger aircraft. A few  pressurized single-engine reciprocating aircraft exist, as well  as many pressurized single-engine turboprop aircraft.  The degree of pressurization and the operating altitude of  any aircraft are limited by critical design factors. A cabin  pressurization system must accomplish several functions  if it is to ensure adequate passenger comfort and safety. It  must be capable of maintaining a cabin pressure altitude of  approximately 8,000 feet or lower regardless of the cruising  altitude of the aircraft. This is to ensure that passengers and  crew have enough oxygen present at sufficient pressure to  facilitate full blood saturation. A pressurization system must  also be designed to prevent rapid changes of cabin pressure,  which can be uncomfortable or injurious to passengers and  crew. Additionally, a pressurization system should circulate  air from inside the cabin to the outside at a rate that quickly  eliminates odors and to remove stale air. Cabin air must also  be heated or cooled on pressurized aircraft. Typically, these  functions are incorporated into the pressurization source.  To pressurize, a portion of the aircraft designed to contain  air at a pressure higher than outside atmospheric pressure  must be sealed. A wide variety of materials facilitate this.  The source of air to pressurize an aircraft varies mainly  with engine type. Reciprocating aircraft have pressurization  sources different from those of turbine-powered aircraft.  Note that the compression of air raises its temperature. A  means for keeping pressurization air cool enough is built into  most pressurization systems. It may be in the form of a heat  exchanger, using cold ambient air to modify the temperature  of the air from the pressurization source. A full air cycle air  conditioning system with expansion turbine may also be used.  The latter provides the advantage of temperature control on  the ground and at low altitudes where ambient air temperature  may be higher than comfortable for the passengers and crew. 16-24 Sources of Pressurized Air