Introduction to Altitude: An Overview

Altitude, in the simplest of definitions, refers to the vertical distance between a point in the air and a reference point on the ground. In the world of aviation, this concept is of utmost importance. Pilots, air traffic controllers, and flight engineers alike must have a deep understanding of altitude and its many intricacies.

There are different ways to measure and interpret altitude, each serving a unique purpose in aviation. Understanding these various types is critical, not just for those flying the aircraft, but also for those who design and maintain these flying machines. It is also vital for air traffic controllers who manage the airspace to ensure safe and efficient flight operations.

To the untrained eye, it may seem like a straightforward concept. But in reality, it is a complex notion with various layers. To better understand it and its role in aviation, it is important to delve into the details. This article aims to provide an in-depth look into the five types of altitude used in aviation.

Importance of Altitude in Aviation

Altitude plays a crucial role in the operation of aircraft. It affects everything from the aircraft’s performance to its safety. Aircraft performance, including speed, fuel efficiency, and engine operation, all change with it. High altitudes can lead to thinner air, which can impact engine performance and fuel consumption.

Furthermore, it is integral to air traffic control. In order to maintain safe distances between aircraft, air traffic controllers use it to separate planes. This is why flights are often assigned specific flight levels to ensure that they do not come into conflict with each other.

In addition, it is also crucial for navigation. Pilots use its readings to determine their position and to follow designated flight paths. Understanding the different types and how they are used is, therefore, essential for anyone involved in aviation.

Understanding the Five Types

There are five types in aviation: absolute, true, indicated, pressure, and density altitude. Each of these has a specific purpose and is used under different circumstances.

Absolute type refers to the actual height of an aircraft above the Earth’s surface. True type is the elevation above mean sea level. Indicated altitude is what is displayed on an aircraft’s altimeter. Pressure type is used to calculate the aircraft’s performance, and density type is used to determine how the aircraft will perform in different weather conditions.

Each type provides specific information that pilots, and air traffic controllers need to ensure safe and efficient flight operations. The following sections will delve deeper into each of these types.

Absolute Altitude

Absolute type refers to the vertical distance of an aircraft above the ground level. This measurement is particularly relevant when an aircraft is flying at low levels, for example, during take-off, landing, or when flying over mountainous terrain.

To measure it, aircraft use an instrument called a radar altimeter. This device sends a radio wave down to the ground and then measures the time it takes for the wave to bounce back. By calculating this time, the radar altimeter can determine the exact distance between the aircraft and the ground.

Understanding the absolute type is crucial for pilots, especially when flying in low visibility conditions. It helps them avoid obstacles and ensures a safe landing by providing the exact height above the ground.

True Type

This type is the vertical distance of an aircraft above mean sea level (AMSL). Unlike the absolute type, which changes with the terrain, the true type remains constant and is unaffected by the topography underneath the aircraft.

True altitude is particularly important for navigation purposes. It is used in aeronautical charts and maps, which depict flight paths and airspace in terms of AMSL. Without knowing it, pilots would be unable to accurately determine their position or follow their intended flight path.

True altitude is usually determined by using an altimeter, a barometric instrument that measures altitude based on atmospheric pressure. However, because atmospheric pressure changes with weather conditions, altimeters need to be regularly calibrated to ensure accurate readings.

Indicated Altitude

This type refers to the altitude reading displayed on an aircraft’s altimeter. This value is derived from the atmospheric pressure at the aircraft’s current altitude. As the aircraft ascends or descends, the atmospheric pressure changes, causing the altimeter to display a different indication.

However, it is not always the same as true altitude. This is because the altimeter is calibrated to standard atmospheric conditions, which assume a certain temperature and pressure at sea level. If the actual conditions deviate from these standard conditions, the indicated type will differ from the true type.

Despite this, indicated altitude is still useful in aviation. It is primarily used by air traffic controllers to separate aircraft vertically. By ensuring that all aircraft use altimeters calibrated to the same standard conditions, air traffic controllers can maintain safe distances between them.

Pressure Type

This type is the height above a standard datum plane (SDP), a theoretical level where the atmospheric pressure is 29.92 inches of mercury (Hg). This type is used in aircraft performance calculations and high-altitude flight planning.

To determine it, pilots set their altimeter to 29.92 inches of mercury, regardless of the actual atmospheric pressure. The altimeter then displays the altitude, which indicates the aircraft’s level in relation to the SDP.

Although pressure type does not provide a true measure of the aircraft’s height above the ground or sea level, it is crucial for understanding how the aircraft will perform under different atmospheric conditions. It is particularly important for high-altitude flight, where the atmospheric pressure significantly deviates from standard conditions.

Density Type

This type is a theoretical it represents the aircraft’s performance in different weather conditions. It is determined by correcting pressure altitude for non-standard temperature variations.

In aviation, an aircraft’s performance is significantly affected by the air density, which changes with altitude, temperature, and humidity. As the air density decreases, the aircraft’s lift and engine performance also decrease, making it harder to take off and climb.

By calculating the density type, pilots can anticipate how their aircraft will perform under the current atmospheric conditions. This is particularly important during the hot summer months or in high-altitude airports, where the air density can be significantly lower than standard conditions.

How Pilots Use Different Types

Pilots use different the types for various purposes throughout a flight. During takeoff and landing, they rely on the absolute type to know their exact height above the ground. For navigation and to follow air traffic control instructions, they use true and indicated types. And for flight planning and to anticipate their aircraft’s performance, they use pressure and density type.

Understanding the different types and how to use them is a crucial part of a pilot’s training. It helps them ensure the safety and efficiency of their flights, and it allows them to adapt to changing atmospheric conditions.

Its Role in Aviation Safety

Altitude plays a crucial role in aviation safety. By understanding and correctly using the different types, pilots can avoid obstacles, follow their intended flight paths, and maintain safe distances from other aircraft.

Air traffic controllers also rely heavily on them to manage the airspace. By assigning different flight levels to different aircraft, they can prevent mid-air collisions and ensure efficient air traffic flow.

Furthermore, understanding it is also crucial for aircraft design and maintenance. Engineers need to consider the effects on different aircraft systems, such as the engines, pressurization system, and aerodynamic surfaces. This ensures that the aircraft can safely and efficiently operate at different altitudes.


Altitude is a complex concept in aviation with different layers. It affects every aspect of flight operations, from aircraft performance and navigation to air traffic control and safety. By understanding the different types – absolute, true, indicated, pressure, and density altitude – pilots, air traffic controllers, and engineers can ensure safe and efficient flight operations.

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