Aircraft Systems

in a corresponding increase in cabin altitude. Differential Decompression is defined as the inability of the aircraft’s control is used to prevent the maximum differential pressure, pressurization system to maintain its designed pressure for which the fuselage was designed, from being exceeded. differential. This can be caused by a malfunction in the This differential pressure is determined by the structural pressurization system or structural damage to the aircraft. strength of the cabin and often by the relationship of the cabin size to the probable areas of rupture, such as window Physiologically, decompressions fall into the following two areas and doors. categories: • Explosive decompression—a change in cabin pressure The cabin air pressure safety valve is a combination faster than the lungs can decompress, possibly pressure relief, vacuum relief, and dump valve. The pressure resulting in lung damage. Normally, the time required relief valve prevents cabin pressure from exceeding a to release air from the lungs without restrictions, such predetermined differential pressure above ambient pressure. as masks, is 0.2 seconds. Most authorities consider any The vacuum relief prevents ambient pressure from exceeding decompression that occurs in less than 0.5 seconds to cabin pressure by allowing external air to enter the cabin be explosive and potentially dangerous. when ambient pressure exceeds cabin pressure. The flight • Rapid decompression—a change in cabin pressure in deck control switch actuates the dump valve. When this which the lungs decompress faster than the cabin. switch is positioned to ram, a solenoid valve opens, causing the valve to dump cabin air into the atmosphere. During an explosive decompression, there may be noise, and one may feel dazed for a moment. The cabin air fills The degree of pressurization and the operating altitude of with fog, dust, or flying debris. Fog occurs due to the rapid the aircraft are limited by several critical design factors. drop in temperature and the change of relative humidity. Primarily, the fuselage is designed to withstand a particular Normally, the ears clear automatically. Air rushes from the maximum cabin differential pressure. mouth and nose due to the escape of air from the lungs and may be noticed by some individuals. Several instruments are used in conjunction with the pressurization controller. The cabin differential pressure gauge Rapid decompression decreases the period of useful indicates the difference between inside and outside pressure. consciousness because oxygen in the lungs is exhaled rapidly, This gauge should be monitored to assure that the cabin does reducing pressure on the body. This decreases the partial not exceed the maximum allowable differential pressure. A pressure of oxygen in the blood and reduces the pilot’s cabin altimeter is also provided as a check on the performance effective performance time by one-third to one-fourth its of the system. In some cases, these two instruments are normal time. For this reason, an oxygen mask should be combined into one. A third instrument indicates the cabin rate worn when flying at very high altitudes (35,000 feet or of climb or descent. A cabin rate-of-climb instrument and a higher). It is recommended that the crewmembers select the cabin altimeter are illustrated in Figure 7-42. 100 percent oxygen setting on the oxygen regulator at high altitude if the aircraft is equipped with a demand or pressure demand oxygen system. Cabin differential 2 4 CA 1 B 0 I 0 N 0 A F L e T et 0 p (p r o es u s n u d re s p in e d r i s c q a u to a r re I 35 0 inch differential) .5 THO C U A SA B N IN D C FT L I P M ER B MIN 30 P D R I E F S F S 1 5 0 6 6 2 Cabin pressure PSI .5 25 5 altitude indicator I 3 10 4 4 (thousands of feet) 2 20 15 Maximum cabin differential pressure limit Cabin rate-of-climb indicator Cabin/differential pressure indicator Figure 7-42. Cabin pressurization instruments. 7-36