Types of Altitudes in Aviation
When it comes to flying airplanes, there are numerous types of altitudes you, as a pilot, must be aware of to ensure flight safety. If you're new to flying, think of piloting as being equivalent to what baking is to cooking. A chef can play around with different recipes for his or her Bolognese sauce, but a pastry chef (much like a chemist) must follow exact instructions to bake a souffle, or else it will fall. There are five common methods for measuring altitude in an aircraft, and each has its own applications and limitations.
True altitude is the height of the airplane above Mean Sea Level (MSL), a value that represents the average sea level (because actual sea level is variable). True altitude is similar to what you might call elevation in non-aviation contexts.
Most personal aircraft is not equipped to measure true altitude, so it is not used to indicate a plane's altitude. However, area forecasts (FAs) report cloud height in MSL, or true altitude. Also, airport elevations, terrain, and obstacle clearance altitudes listed on the visual flight rules (VFR) sectional charts are often given in MSL.
Indicated altitude is what is indicated on the altimeter in your airplane. It is an approximation of true altitude as measured by the altimeter. The altimeter is a basic flight instrument that measures the atmospheric pressure at the airplane's flight altitude and compares it to a preset pressure value.
The preset pressure value typically is based on the nearest weather reporting site. However, because the weather site is on the ground (and doesn't move with the plane), the pressure reported at the site may differ from the pressure at the plane's actual location, affecting the accuracy of the altimeter reading.
Indicated altitude is used to gauge a plane's distance from ground obstacles and terrain as well as vertical distances to other planes in the area, known as vertical separation. Using indicated altitude to gauge vertical separation is a relatively accurate (assuming all of the planes in a given area are set to the same weather station), but this practice is used only at altitudes under 18,000 feet.
Pressure altitude is the altitude above the standard datum plane, a theoretical level indicated by an altimeter set to 29.92" Hg, the standard pressure setting. Pressure altitude is measured with barometric pressure, and a plane's altimeter is essentially a fine-tuned barometer.
Pressure altitude is important when it comes to computing aircraft performance data, including such things as takeoff and landing distances. It's also the altitude that operators use while flying above 18,000 feet or in Class D airspace, which requires everyone in flight to set their altimeters to 29.92" Hg in order to standardize the indicated altitudes. You can actually determine the pressure of the air by calculating the difference between the pressure altitude and the current altimeter setting.
Density altitude is important for determining the performance of an aircraft, or how the aircraft will behave under certain conditions. Density altitude is pressure altitude corrected for nonstandard temperature. Because the temperature constantly changes (and is therefore nonstandard), it's very important for pilots to know the density altitude.
Density altitude is not an indication of altitude above the ground or above sea level. Rather, it is a measure of the air density in a given location at the present temperature. Air density decreases with altitude; there's less air to breath at 5,000 feet than at sea level. Cold air is denser than warm air. In denser air, airplane wings have more lift, and airplane engines are more powerful because there is more oxygen to burn. As air density decreases (density altitude increases), pilot must compensate their air speed, take off and landing distances, and other factors to maintain safety.
At sea level, the standard temperature for air density calculations is 15 C. The surface temperature decreases, on average, about 2 degrees per 1,000 feet increase in elevation. For example, an airport in Colorado at 5,000 feet in elevation will have a standard temperature of 5 C. However, if the actual temperature at that airport is above the standard temperature, the density altitude will be higher than normal, and airplanes might perform as though they're at, for example, 7,000 feet instead of 5,000 feet.
Absolute altitude (AGL) is the exact height above ground level, or the actual height above the earth's surface. It is measured by a radar altimeter, which uses radar signals to measure actual distance from the ground to the aircraft. METARs and TAFs report cloud cover in AGL. Absolute altitude also is used to help land large aircraft equipped with radar altimeters. Most small aircraft do not have radar altimeters and must substitute with indicated altitude and charts for instrument (IMC) flying and other operations.