Altitude is the vertical distance above a specific reference point. While you may be familiar with the term, you may not know that there are five types of altitude. There are many factors that determine altitude including the vertical distance above mean sea level and above the ground surface, as well as pressure and density. In this blog, we will be providing an overview of five different types of altitude, thus giving you a better understanding of their distinguishing features and importance.
Beginning with indicated altitude, this type can be read directly off the altimeter within an aircraft. The altimeter is either mounted on an aircraft’s instrumental panel or can be worn on a person’s wrist. If situated on the instrumental panel, it is typically enclosed in a case that is affixed to the exterior of the aircraft by an air pressure inlet at the back-end of the housing. Alternatively, you can also derive altitude data from the Global Positioning System (GPS), which provides altitude as a part of the area’s location by receiving signals from different satellites.
Similarly, pressure altitude information is derived from an altimeter that has been set to 29.92” (inHg). This setting can be described as standard pressure altitude wherein the aircraft is above the standard datum plane. The latter is the theoretical location in which at 15 degrees Celsius, the altimeter setting will equal 29.92 inches of mercury. Pressure altitude serves a particularly important role as it is the basis for determining aircraft performance as well as for aircraft flying above 18,000 feet Mean Sea Level (MSL). As such, all aircraft flying at similar flight levels will have the same altimeter setting.
Density altitude is pressure altitude that has been adjusted for non-standard temperatures. It is especially important for calculating aircraft performance data. Density altitude is the altitude the aircraft will be performing at regardless of its actual altitude. With increased temperatures, your airplane does not perform as well. For example, your takeoff distance may be longer, you may experience vapor lock, and you may not climb as fast. The hot temperatures will cause your density to increase, thus your aircraft will feel like it is flying at a higher altitude.
Next, true altitude is defined as the vertical distance of your aircraft above sea level. The units used to express this altitude is Mean Sea Level (MSL). Aeronautical charts often use MSL for airspace altitudes, terrain figures, airways, and more. It is important not to confuse true altitude with the height of the aircraft above ground level as these are different.
Lastly, absolute altitude is the aircraft’s height above the ground below. As absolute altitude is constantly changing, hills, valleys, and mountainous terrain can transform the absolute altitude accordingly. Typically expressed in feet above ground level (AGL), a radar altimeter, or radio altimeter, measures altitude above the terrain that is presently beneath an aircraft by determining the time it takes a beam of radio waves to reflect from the ground and bounce back to the aircraft. It is important to note that radar altimeters can provide readings up to 2,500 feet AGL.
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