Pilots use several different altitude measurements in aviation, each with its own significance and purpose. Here's an explanation of the differences between pressure altitude, density altitude, absolute altitude, indicated altitude, true altitude, and absolute altitude:
Pressure altitude is the altitude above a standard reference point, which is typically set to 29.92 inches of mercury (Hg) at sea level. Pilots use this reference point for altitude calculations, as it allows them to compare altitudes accurately, regardless of the current atmospheric pressure. Pressure altitude is mainly used for flights above 18,000 feet MSL, flight planning, and air traffic control.
Density altitude is a measure of air density. It is calculated by taking pressure altitude and adjusting it for the current atmospheric conditions, including temperature and humidity. High density altitude indicates less dense air, which can affect aircraft performance, such as engine power and lift. Pilots use density altitude to determine how an aircraft will perform under specific weather conditions.
Absolute altitude is the actual height above the ground or a specific reference point (e.g., terrain, obstacles, or ground level). This altitude is vital for avoiding obstacles during flight. It can be measured using radar altimeters or by visually assessing the distance between the aircraft and the ground.
Indicated altitude is the altitude displayed on the aircraft's altimeter, which is based on the atmospheric pressure at sea level. It's the primary altitude reference for the pilot during most phases of flight. However, it doesn't account for variations in atmospheric pressure, so pilots need to adjust it when flying at different locations or altitudes.
True altitude is the actual height above mean sea level (MSL). It is calculated by adjusting indicated altitude for local atmospheric pressure conditions. True altitude is used for accurate terrain clearance and to determine the aircraft's position on aeronautical charts. It is essential for ensuring safe obstacle clearance during flight.
Radar altitude is determined by the aircraft's radar altimeter, which measures the distance between the aircraft and the ground directly below it. This is critical for low-level flying and landing approaches, especially in adverse weather conditions. Radar altimeters are typically accurate at low altitudes and are used for precise altitude information during landing.
In summary, each type of altitude serves a specific purpose in aviation. Pressure altitude is a reference point for calculations, density altitude is used to assess aircraft performance, absolute altitude indicates the actual height above the ground, indicated altitude is what the altimeter reads, true altitude provides the MSL reference, and radar altitude is vital for low-level flying and landing. Pilots must understand and use these altitude measurements appropriately for safe and efficient flight operations.