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The Different Types of Altitude in Aviation Explained

Altitude is one of the most fundamental concepts in aviation, but it’s also one of the most misunderstood by new pilots. To the untrained eye, altitude may simply mean “how high the airplane is flying.” However, in aviation, there are several different types of altitude, each with its own meaning, purpose, and use in flight operations. Understanding these distinctions is critical for safe flying, accurate navigation, and effective performance calculations.


Let’s break down the major types of altitude: indicated, true, absolute, pressure, and density altitude.



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Indicated Altitude

Definition: Indicated altitude is the altitude shown directly on the altimeter when it is set to the current local altimeter setting.

Key Points:

  • It’s what the pilot sees in the cockpit without correction.

  • It is accurate only when the altimeter is correctly set to the local barometric pressure at sea level.

  • Indicated altitude is the most commonly referenced altitude during flight operations.

When it matters:

  • Used for maintaining altitude assignments from Air Traffic Control (ATC).

  • Provides a practical reference for pilots during flight.


True Altitude

Definition: True altitude is the aircraft’s actual height above mean sea level (MSL).

Key Points:

  • Charts, terrain elevations, and obstacles are all referenced to mean sea level, so true altitude is what keeps airplanes clear of mountains and towers.

  • While indicated altitude can approximate true altitude, they may differ if atmospheric conditions (temperature and pressure) are not standard.

When it matters:

  • Important for navigation and terrain clearance.

  • Used in instrument approaches, where published altitudes always reference MSL.


Absolute Altitude

Definition: Absolute altitude is the aircraft’s actual height above the terrain directly beneath it, measured above ground level (AGL).

Key Points:

  • Unlike true altitude (referenced to sea level), absolute altitude changes with the elevation of the terrain below.

  • Typically measured with a radar altimeter in larger aircraft, but not available in most small general aviation planes.

When it matters:

  • Crucial during low-level operations like takeoff, landing, or military training flights.

  • Helps determine clearance over terrain and obstacles.


Pressure Altitude

Definition: Pressure altitude is the altitude indicated when the altimeter is set to the standard pressure setting of 29.92 inHg (1013 mb).

Key Points:

  • Pressure altitude is not used for navigation but for aircraft performance calculations.

  • Above the transition altitude (in the U.S., 18,000 feet), all aircraft set their altimeters to 29.92 inHg, so everyone is using pressure altitude as a reference.

When it matters:

  • Used in flight planning to calculate performance data such as takeoff distance, rate of climb, and cruise performance.

  • Standardized reference above transition altitude ensures separation between aircraft.


Density Altitude

Definition: Density altitude is pressure altitude corrected for non-standard temperature. It represents the altitude at which the aircraft “feels” like it is flying, based on air density.

Key Points:

  • High temperature, high humidity, and high elevation all reduce air density, increasing density altitude.

  • A higher density altitude reduces engine power output, propeller efficiency, and wing lift.

When it matters:

  • Critical for takeoff and landing performance.

  • On hot summer days at high-elevation airports, density altitude can be thousands of feet higher than the actual field elevation, significantly degrading aircraft performance.


Putting It All Together

  • Indicated Altitude → What you read on the altimeter.

  • True Altitude → Height above mean sea level (used on charts).

  • Absolute Altitude → Height above ground level.

  • Pressure Altitude → Altitude relative to standard pressure (29.92 inHg).

  • Density Altitude → Pressure altitude adjusted for temperature, affecting performance.


Final Thoughts

Altitude is more than just a number—it’s a concept that can mean different things depending on the situation. Pilots must understand the type of altitude they’re dealing with to navigate safely, clear terrain, communicate effectively with ATC, and calculate accurate performance.


Whether you’re a student pilot or a seasoned aviator, mastering these altitude definitions ensures you not only know how high you’re flying, but also what that altitude really means.



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