Pressure altitude (PA) is a fundamental concept in aviation that provides a standardized way to measure altitude. It is defined as the height of an aircraft above the Standard Datum Plane (SDP), a theoretical level where the barometric pressure equals 29.92 inches of mercury (inHg). This measurement standardizes atmospheric conditions, allowing pilots and engineers to use a consistent baseline for flight planning. This standardized altitude is applied directly to aircraft performance calculations and for ensuring vertical separation between aircraft flying at high altitudes.
Understanding Pressure Altitude
Pressure altitude creates a common reference point for all flight operations, regardless of the actual, real-time weather conditions. The Standard Datum Plane is established by the International Standard Atmosphere (ISA) model, which defines a theoretical day at sea level with a temperature of 15 degrees Celsius and a barometric pressure of 29.92 inHg. An aircraft’s altimeter measures the ambient air pressure outside the aircraft and converts this pressure into a corresponding altitude based on the standardized rate pressure decreases with height. When the local atmospheric pressure is exactly 29.92 inHg, the pressure altitude is identical to the indicated altitude.
The primary practical use of pressure altitude is in determining an aircraft’s performance. Aircraft performance, including lift, engine power, and takeoff/landing distances, is directly affected by air density. Performance charts are standardized and published based on the assumption that the aircraft is operating at a specific pressure altitude. If the actual pressure is lower than 29.92 inHg, the air is less dense, and the aircraft performs as if it were at a higher elevation.
Calculating Pressure Altitude Manually
The manual calculation of pressure altitude involves a simple formula that corrects the indicated altitude for non-standard pressure. This calculation uses the difference between the standard sea level pressure of 29.92 inHg and the current local altimeter setting (QNH). This local setting is typically obtained from weather reports like the METAR or ATIS.
The approximate formula is: Pressure Altitude = Indicated Altitude + [(29.92 – Current Altimeter Setting) x 1,000]. The correction factor of 1,000 feet is used because, in the lower atmosphere, the barometric pressure changes by approximately one inch of mercury for every 1,000 feet of altitude change.
To illustrate this, consider an aircraft at an indicated altitude of 5,000 feet, where the current altimeter setting is 30.12 inHg. The calculation begins by finding the pressure difference: 29.92 minus 30.12 equals -0.20. Multiplying this by 1,000 feet yields a correction of -200 feet, resulting in a pressure altitude of 4,800 feet. In this high-pressure scenario, the aircraft is operating in denser air, which effectively lowers its pressure altitude.
In a low-pressure scenario, where the altimeter setting is 29.42 inHg, the difference is +0.50, giving a correction of +500 feet. For the same indicated altitude of 5,000 feet, the pressure altitude becomes 5,500 feet (5,000 + 500). The lower-than-standard pressure results in less dense air, meaning the aircraft performs as if it were 500 feet higher than its actual indicated altitude.
Determining Pressure Altitude Using the Altimeter
Pilots can determine the pressure altitude directly from the instrument using the barometric setting window on the altimeter, often called the Kollsman window. This small window displays the pressure value that the altimeter is currently using as its sea-level reference point.
The procedure is straightforward: the pilot adjusts the altimeter setting knob until the standard sea level pressure of 29.92 inHg is displayed in the Kollsman window. Once this specific setting is dialed in, the reading shown on the altimeter dial is the pressure altitude. This instrumental method works because the altimeter is mechanically performing the exact same correction as the manual formula, referencing the Standard Datum Plane.
This simple adjustment is the method mandated for all aircraft operating above 18,000 feet in the United States and Canada, which is known as the “flight levels.” By having all aircraft set their altimeters to 29.92 inHg at these high altitudes, every plane uses the same pressure reference. This ensures uniform vertical separation and simplifies air traffic control procedures. Below 18,000 feet, pilots typically use the local altimeter setting to display indicated altitude.