The average person probably knows that cruising altitude for a commercial aircraft is 35,000 feet - or at least in that region. But what they might not know is that there are five types of altitude when it comes to aviation. True altitude, indicated altitude, pressure altitude, density altitude, and absolute altitude are all representations of altitude that differ based on a number of factors and guidelines. This blog will explain each type of altitude as well as the tools used to measure them.
Before looking at the five types of altitude, let’s first look at how altitude is measured. The most common tool for measuring altitude is an altimeter. An altimeter is a switch pressure altitude part that uses an aneroid barometer to measure the atmospheric pressure via a static port on the outside of the aircraft. The aneroid altimeter is calibrated to display the atmospheric pressure directly as an altitude above mean sea level. These tools, called pressure altimeters, contain a hollow box that expands as pressure falls and contracts as pressure rises. When the box changes in size, a system of levers and gears moves a dial to display the measurement.
The other type of altimeter, while more advanced, is actually much simpler in terms of function. This type of altimeter, a radar altimeter, sends radio waves down to sea level where they are reflected off a surface and return to the aircraft. The altimeter times the radio wave’s journey and uses that information to calculate altitude. These altimeters are faster and more precise, making them very popular in high-speed aircraft and aircraft that fly at low altitudes. Now that you have a better understanding of how altitude is calculated, let’s look at its five types.
The first type of altitude is called true altitude. This represents the height of an aircraft above mean sea level (MSL). Mean sea level is the average sea level, because actual sea level is variable. Outside the context of aviation, true altitude is what people would call elevation. Most personal aircraft are not equipped to measure true altitude, so it is not used commonly. The next type, indicated altitude, is the altitude indicated by the altimeter. It is used to gauge a plane’s distance from ground obstacles, terrain, and the vertical distance between other aircraft occupying the same airspace. This distance is called vertical separation and, while indicated altitude is relatively accurate means of identifying vertical separation, it is only used below 18,000 feet.
Pressure altitude is the altitude above the standard datum plane, an imaginary line set by the altimeter at 29.92 inches of mercury - the standard measurement of a pressure altimeter. Pressure altitude is critical when computing the performance data of an aircraft such as takeoff and landing distances. Pressure altitude is also the altitude pilots use when flying at 18,000 feet or above. Density altitude is similar to pressure altitude, but differs in that it is corrected for the constantly changing temperature. Rather than being an indication of altitude above ground or sea level, density altitude represents the air density in a given location at the current temperature. Depending on air density, aircraft can have more or less lift, as well as more or less engine power. Because of this, the pilot needs to know the density altitude in order to adjust factors like air speed, takeoff and landing distance, and more. The final type of altitude is absolute altitude. Very simply, absolute altitude is the exact height of an aircraft above ground level, meaning it is constantly changing during flight. This is measured by a radar altimeter.
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