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Effect of Nonstandard Pressure and Temperature

Effect of Nonstandard Pressure and Temperature


  • It is easy to maintain a consistent height above ground if the barometric pressure and temperature remain constant, but this is rarely the case.
  •  The pressure-temperature can change between takeoff and landing even on a local flight. If these changes are not taken into consideration, fl ight becomes dangerous.
  • If altimeters could not be adjusted for nonstandard pressure, a hazardous situation could occur.
  • For example, if an aircraft is fl own from a high pressure area to a low pressure area without adjusting the altimeter, a constant altitude will be displayed, but the actual height of the aircraft above the ground would be lower then the indicated altitude.
  • There is an old aviation axiom: “GOING FROM A HIGH TO A LOW, LOOK OUT BELOW.” Conversely, if an aircraft is fl own from a low pressure area to a high pressure area without an adjustment of the altimeter, the actual altitude of the aircraft is higher than the indicated altitude.
  • Once in fl ight, it is important to frequently obtain current altimeter settings en route to ensure terrain and obstruction clearance.
  • Many altimeters do not have an accurate means of being adjusted for barometric pressures in excess of 31.00 inches of mercury ("Hg).
  • When the altimeter cannot be set to the higher pressure setting, the aircraft actual altitude will be higher than the altimeter indicates. When low barometric pressure conditions occur (below 28.00), fl ight operations by aircraft unable to set the actual altimeter setting are not recommended.
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