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