under what condition is indicated altitude the same as true altitude?

Championisme authors

2 min read

·

20-10-2024

32

0

Share

When Does Indicated Altitude Equal True Altitude? Understanding the Differences in Aviation

For pilots and aviation enthusiasts alike, understanding the difference between indicated altitude and true altitude is crucial. While they may seem interchangeable, they represent distinct measurements and can have significant implications for flight safety. But under what conditions do these two measurements align, and when should we be particularly aware of their discrepancies?

What are Indicated Altitude and True Altitude?

• Indicated Altitude (IA): This is the altitude reading displayed on the aircraft's altimeter. It is based on the pressure difference between the local atmospheric pressure and the pressure set on the altimeter, typically the standard sea level pressure (29.92 inches of mercury).
• True Altitude (TA): This refers to the actual height of the aircraft above mean sea level (MSL). It considers the actual atmospheric pressure at the aircraft's location and accounts for variations in air density and temperature.

Why the Difference?

The discrepancies between IA and TA stem from variations in atmospheric pressure and temperature. Here's why:

• Non-Standard Atmospheric Conditions: The standard atmosphere model assumes a constant pressure and temperature lapse rate. However, real-world conditions can deviate significantly, leading to discrepancies in pressure readings.
• Temperature Variations: Warmer air is less dense than cold air. This means the aircraft's altimeter, which measures pressure, will read higher in warm air than it would in cold air at the same true altitude.
• Wind and Terrain: Wind can push the aircraft up or down, affecting the perceived altitude. Similarly, mountainous terrain can create local variations in atmospheric pressure, influencing the altimeter reading.

When Does IA = TA?

Ideal Conditions: Indicated altitude will equal true altitude under the following ideal conditions:

• Standard Atmosphere: When the atmospheric pressure and temperature match the standard atmosphere model.
• Zero Wind: When the aircraft is not influenced by wind.
• Flat Terrain: When the aircraft is flying over flat terrain.

Practical Implications:

In reality, these ideal conditions rarely exist, making it essential for pilots to understand the potential for discrepancies.

• High-Altitude Flying: At higher altitudes, the effects of temperature variations become more pronounced.
• Hot Days: On hot days, the altimeter will tend to read higher than the true altitude.
• Cold Days: On cold days, the altimeter will tend to read lower than the true altitude.

How Pilots Account for Differences:

• Altimeter Settings: Pilots constantly adjust the altimeter to compensate for changing pressure conditions.
• Flight Planning: Pilots carefully consider factors like temperature and wind conditions during flight planning.
• Instrument Accuracy: Pilots rely on various instruments, including the altimeter, to ensure accurate navigation and safe flight operations.

Conclusion:

While the ideal scenario sees indicated altitude equaling true altitude, it's crucial to understand the factors that can influence these measurements. By understanding the potential discrepancies and taking appropriate measures, pilots can ensure safe and efficient flights.

Source:

• "Atmospheric pressure and temperature effects on altimeter readings." Journal of Aircraft, Volume 39, Issue 6 (2002), Pages 997-1002 - Authors: T.J. Davis, D.R. Soderman, A.P. Kurdila

Additional Notes:

• Flight Levels: In controlled airspace, pilots fly at specific flight levels based on their indicated altitude, which is then converted to true altitude by air traffic control.
• GPS Technology: Modern aircraft often incorporate GPS systems, which provide more accurate and direct measurements of the aircraft's position and altitude.

This article aims to provide a simplified explanation of the concepts of indicated altitude and true altitude. For comprehensive information, refer to aviation textbooks and regulatory documents.