Altitude generally refers to the vertical distance above a fixed reference point, such as mean sea level. For aviation, a more specific measurement is necessary to standardize flight conditions and performance calculations. This theoretical measurement is known as Pressure Altitude (PA), derived purely from the ambient atmospheric pressure. It provides a standardized reference point independent of actual physical height or changing local weather conditions. PA is foundational for understanding aircraft behavior and ensuring safe vertical separation worldwide.
Defining Pressure Altitude
Pressure Altitude is formally defined as the height above the Standard Datum Plane (SDP). The SDP is a hypothetical level where the pressure is exactly 29.92 inches of mercury (inHg) or 1013.2 millibars (mb). This establishes a fixed baseline for all atmospheric calculations in aviation, independent of local weather conditions. PA represents the theoretical altitude corresponding to the measured atmospheric pressure.
The importance of PA lies in its constancy across different weather systems, providing a consistent reference. If the actual atmospheric pressure is higher than 29.92 inHg, the Pressure Altitude will be negative. Conversely, a lower-than-standard pressure results in a positive PA.
PA serves as a fundamental benchmark for pilots and air traffic controllers. Standardizing this measurement allows for accurate comparison of aircraft performance worldwide. It ensures flight planning is based on a consistent, global atmospheric model.
The Necessity of the Standard Atmosphere Model
The Standard Datum Plane (SDP) is established through the International Standard Atmosphere (ISA) model. The ISA is a theoretical construct defining how pressure, temperature, and density change with altitude. It assumes specific conditions at mean sea level: 15° Celsius (59° Fahrenheit) and 29.92 inHg (1013.2 mb). The ISA provides a stable baseline necessary for flight planning calculations.
The ISA model defines standardized lapse rates, describing the predictable rate at which temperature and pressure decrease. The standard temperature lapse rate is approximately 2° Celsius for every 1,000 feet gained. These standardized rates allow for consistent, mathematical calculation of Pressure Altitude.
Actual atmospheric conditions rarely match the ISA model. Aircraft performance must be calculated using a fixed reference point to maintain regulatory consistency and safety. The ISA provides this standardized baseline, ensuring all aircraft and air traffic management systems work from the same atmospheric framework.
Determining Pressure Altitude in Practice
Pilots determine Pressure Altitude by manipulating the aircraft’s altimeter. To read PA directly, the pilot adjusts the pressure setting knob (Kollsman window) to the standard pressure of 29.92 inHg. The instrument then displays the current Pressure Altitude corresponding to the ambient air pressure.
At lower altitudes, pilots use the local atmospheric pressure setting to read their actual height above sea level for terrain clearance. Regulatory requirements mandate the use of Pressure Altitude above 18,000 feet in the United States, marking the start of Flight Levels (FL).
When flying at or above FL180, all aircraft must set their altimeters to the standard 29.92 inHg. This uniform setting ensures every plane references the same theoretical pressure datum for safe vertical separation in high-altitude airspace.
For planning purposes, Pressure Altitude can be calculated mathematically. The calculation is approximately: \(PA = \text{Elevation} + 1000 \times (29.92 – \text{Current Altimeter Setting})\). A low local pressure results in a high Pressure Altitude, indicating reduced performance.
The Critical Link to Aircraft Performance
Pressure Altitude is the initial component in determining Density Altitude (DA). While PA accounts for pressure variations, DA incorporates the effect of temperature deviations from the ISA model. Density Altitude is the Pressure Altitude corrected for non-standard temperature, representing the theoretical altitude where air density matches the actual density.
Air density is the primary determinant of aircraft performance, influencing both lift and engine power. Less dense air, resulting from high temperatures or low pressure, causes a significant reduction in capability. High Pressure Altitude immediately suggests a reduced density air mass.
High Density Altitude severely compromises flight efficiency. High DA means air molecules are dispersed, causing the wings to generate less lift and requiring a higher true airspeed for takeoff. Engine performance also degrades because engines draw in less mass of air for combustion.
The practical consequence of high Density Altitude includes longer takeoff and landing distances and a reduction in maximum allowable weight. For instance, an airport at 5,000 feet above sea level might have a DA of 8,000 feet on a hot day. This condition could necessitate a 50% longer takeoff roll compared to a standard day.