Pressure altitude is defined as the altitude corrected for variations in atmospheric pressure that are not standard for a given height. This measurement represents the altitude where the atmospheric pressure is equal to the pressure in the International Standard Atmosphere (ISA) at that same altitude. It serves as the common baseline for calculating aircraft performance factors, such as takeoff distance, climb rate, and engine horsepower output. Pressure altitude determination is used universally for safe and efficient flight operations across varying weather conditions and geographical locations.
Understanding the Standard Datum Plane
The concept of pressure altitude relies on an imaginary reference point called the Standard Datum Plane (SDP). This theoretical plane represents the level in the atmosphere where the pressure is exactly 29.92 inches of mercury (inHg) or 1013.25 hectopascals (hPa). These specific values are internationally agreed-upon standards for pressure at mean sea level under ideal atmospheric conditions. The altimeters in aircraft are calibrated based on this ISA model.
Since atmospheric pressure constantly changes with weather fronts and local conditions, the physical location of the Standard Datum Plane shifts daily. On a high-pressure day, the SDP effectively sinks below mean sea level, while on a low-pressure day, it rises above it. Pressure altitude is the vertical distance, measured in feet, between the aircraft and this fluctuating theoretical plane. This value standardizes performance calculations regardless of the actual elevation or current local barometric reading.
Finding Pressure Altitude Using an Altimeter
The most direct method for determining pressure altitude is by using an aircraft’s barometric altimeter. This instrument displays altitude based on the pressure it senses. To obtain the pressure altitude, the pilot must first locate the small adjustable setting window on the altimeter face, often called the Kollsman window.
The pilot then rotates the adjustment knob until the standard sea level pressure of 29.92 inHg is displayed. Once this specific pressure value is set, the altitude indicated by the altimeter needles is the aircraft’s current pressure altitude. This procedure effectively removes the local atmospheric pressure variation from the altimeter reading, forcing the instrument to reference the universal Standard Datum Plane.
This method is important for aircraft flying at high altitudes, typically above 18,000 feet in the United States, known as the transition altitude. At this height, all aircraft are required to set their altimeters to the standard 29.92 inHg setting, and their reported altitudes are referred to as Flight Levels. Using this standardized reference ensures uniform vertical separation and safety managed by Air Traffic Control.
Calculating Pressure Altitude Using a Formula
When an altimeter is unavailable, or for pre-flight planning on the ground, pressure altitude can be calculated mathematically using a formula that incorporates the local atmospheric conditions. This calculation relies on the relationship that for every 1,000 feet of change in altitude, the atmospheric pressure changes by approximately one inch of mercury. The formula converts the difference between the standard pressure and the local pressure into a corresponding altitude change.
The common “rule of thumb” formula used for this calculation is: Pressure Altitude = Field Elevation + [(29.92 – Local Altimeter Setting) x 1,000]. The first step involves finding the difference by subtracting the current Local Altimeter Setting, often referred to as QNH, from the standard pressure of 29.92 inHg. This difference, which may be a positive or negative number, is then multiplied by 1,000 to convert the pressure difference into a correction in feet.
For example, consider an airport with a Field Elevation of 500 feet and a Local Altimeter Setting of 30.12 inHg. The calculation begins by finding the pressure difference: 29.92 minus 30.12 equals -0.20. This result is then multiplied by 1,000, yielding a correction factor of -200 feet. Finally, this correction is added to the Field Elevation: 500 feet plus -200 feet results in a Pressure Altitude of 300 feet.
A negative correction value indicates that the local pressure is higher than the standard pressure, effectively pushing the Standard Datum Plane lower and reducing the pressure altitude. Conversely, a positive correction factor, which would occur with a local setting lower than 29.92 inHg, means the Standard Datum Plane is higher, increasing the calculated pressure altitude.