Barometric Altitude – Deep Glossary for Aviation

Definition

Barometric altitude is a pressure-derived indication of an aircraft’s height above a reference datum, typically mean sea level (MSL), based on atmospheric pressure measurement. In aviation, this value is critical for navigation, maintaining safe separation, and avoiding terrain and obstacles. The measurement is obtained using the aircraft’s altimeter, which senses static air pressure and, referencing the International Standard Atmosphere (ISA), translates it into an altitude readout.

Barometric altitude does not represent the exact vertical distance above the terrain—rather, it is a theoretical value assuming standard atmospheric conditions (ISA: 15°C at sea level, 1013.25 hPa pressure). Differences between actual and standard conditions can introduce errors if not corrected, but the primary role of barometric altitude is to provide a universal reference for all aircraft, ensuring safe vertical separation managed by air traffic control.

Above the transition altitude, aircraft set their altimeters to the standard pressure (1013.25 hPa or 29.92 inHg), creating a consistent system for flight levels worldwide. This practice is mandated by ICAO standards and is foundational to global airspace structure.

How Barometric Altitude is Measured

Barometric altitude is measured by an altimeter, a specialized, highly calibrated aneroid barometer. The altimeter receives static air pressure from a static port on the aircraft’s fuselage. Inside, a stack of flexible aneroid wafers expands or contracts as pressure changes, moving mechanical or digital indicators to display the altitude.

The altimeter is calibrated to display altitude under standard atmospheric conditions. Pilots can enter a reference pressure (QNH, QFE, or standard) in the Kollsman window—a small display on the instrument. Setting the correct pressure ensures the altimeter indicates the aircraft’s altitude above the desired reference (e.g., mean sea level for QNH).

Key factors influencing measurement accuracy:

• Instrument calibration
• Static port placement
• Atmospheric deviations (temperature and pressure)
• Mechanical lag or error

Modern aircraft may also use digital or electronic altimeters, integrating data for improved accuracy. Advanced systems can fuse barometric, GPS, and radar altitude information for precision approaches or automated flight controls.

Key Types of Altitude in Aviation

Aviation uses several distinct definitions of altitude, each serving different operational and safety purposes.

Indicated Altitude

Indicated altitude is the direct reading from the altimeter when set to the local QNH (mean sea level pressure). It shows the aircraft’s height above mean sea level, used for terrain clearance and compliance with most ATC instructions below the transition altitude.

Accuracy depends on correct QNH setting and standard atmospheric conditions. If not updated, errors can accrue, especially during changing weather.

Pressure Altitude (Barometric Altitude)

Pressure altitude is the reading from the altimeter when set to the standard pressure of 1013.25 hPa (29.92 inHg). This is the key reference above the transition altitude, enabling uniform assignment of flight levels worldwide.

Pressure altitude is fundamental for air traffic separation, flight level assignment, and aircraft performance calculations. All aircraft above transition altitude use this common datum, eliminating local pressure discrepancies.

True Altitude

True altitude is the actual vertical distance above mean sea level, accounting for deviations from ISA (mostly temperature and pressure). It’s the value used for precise terrain and obstacle clearance.

Pilots may need to correct the indicated altitude for temperature deviations, especially in cold weather, as the altimeter can overestimate the aircraft’s height above ground.

Absolute Altitude

Absolute altitude is the height above the terrain directly beneath the aircraft (Above Ground Level, AGL). It is typically measured by a radar altimeter, especially during low-level flight, approach, and landing.

Absolute altitude is vital for automated landing systems, terrain awareness, and low-level operations, but not for en-route navigation or ATC separation.

Density Altitude

Density altitude is pressure altitude corrected for non-standard temperature (and, to a lesser extent, humidity). It represents the altitude in the standard atmosphere with the same air density as the current conditions.

High density altitude (caused by high elevation, temperature, and humidity) means reduced aircraft performance—longer takeoff runs, lower climb rates, etc. Proper calculation is critical for safe operations at high or hot airports.

Altimeter Setting and Use

Kollsman Window

The Kollsman window is an adjustable display on the altimeter where pilots set the reference pressure. Setting the correct value calibrates the altimeter for current atmospheric conditions and desired reference (QNH, QFE, or standard).

How to Set the Altimeter

1. Obtain the current pressure setting:
   From ATIS, AWOS, or ATC (QNH or QFE).
2. Set the value in the Kollsman window.
3. Cross-check with known field elevation on the ground.
4. Update regularly in flight as directed by ATC or published procedures.
5. Switch to standard pressure when climbing through transition altitude; revert to QNH when descending through transition level.

These steps ensure accuracy for safe flight and compliance with ATC instructions.

Altimeter Setting Procedures

QNH is standard globally below transition altitude. QFE is used in some countries for approaches/landings. Standard pressure is mandatory above transition altitude for flight levels.

Reference Standards and Pressure Settings

International Standard Atmosphere (ISA)

The ISA is the reference model for average atmospheric pressure, temperature, and density at various altitudes. It provides the baseline for altimeter calibration and aircraft performance calculations.

Deviations from ISA require correction for accurate altitude readings and performance planning.

QNH, QFE, and Standard Pressure

• QNH: Altimeter reads altitude above mean sea level (MSL).
• QFE: Altimeter reads height above airfield (zero at reference).
• Standard Pressure: 1013.25 hPa (29.92 inHg); used for flight levels above the transition altitude.

Correct use of these settings is essential for safe flight, especially during transitions between airspace segments and for legal compliance.

Conclusion

Barometric altitude remains the cornerstone of vertical navigation and airspace safety in aviation. Through the use of calibrated altimeters, standardized pressure settings, and adherence to global procedures, pilots and controllers achieve safe separation, terrain clearance, and efficient flight operations. Understanding the nuances of barometric, indicated, pressure, true, absolute, and density altitude—and the correct use of QNH, QFE, and standard pressure—is vital for every aviator.

For further guidance, consult ICAO Annex 2, Annex 6, Annex 11, ICAO Doc 8168, and regional Aeronautical Information Publications (AIPs).

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