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What is pressure altitude in aviation?

What is Pressure Altitude in Aviation?

In aviation, pressure altitude is a critical concept for pilots to understand, particularly when navigating at high elevations. It is often misunderstood or overlooked, despite its importance in ensuring accurate altitude readings and safe navigation. In this article, we will delve into what pressure altitude is, why it is crucial, and how pilots can accurately navigate using this important metric.

What is Pressure Altitude?

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Pressure altitude is the aircraft’s altitude above sea level, as calculated by barometric instruments, based on the current barometric pressure. In other words, it is the altitude equivalent to the aircraft’s altimeter setting, which must be adjusted for changes in atmospheric pressure.

Confusion Alert!

Altitude is often confused with indicated altitude (the altitude shown by the aircraft’s altimeter), true altitude, and flight level altitude. While these three terms are related to aircraft altitude, they are different and distinct concepts.

Pressure Altitude > Indicated Altitude >

Indicated altitude is the altitude shown by the altimeter, affected by air density and deviations from standard atmospheric conditions (e.g., temperature deviations). Pressure altitude is a calculated value representing the altitude equivalent to the indicated altitude, taking into account changes in atmospheric pressure.

Why is Pressure Altitude Important?

Pressure altitude is crucial because it helps pilots:

• Correctly set instruments: Pilots must manually set the altimeter before takeoff to ensure their instruments show the correct indication of altitude.
• Maintain accurate altitude: An accurate pressure altitude reading can prevent accidental deviations from aircraft altitude, reducing the risk of collision with terrain.
• Make informed climb and descent decisions: Flight planning and decision-making, such as setting cruise altitudes and navigating during descent and climb, rely on precision pressure altitude calculations.
• Calculate critical altitudes: Pilots must track pressure altitude to determine critical moments, like crossing _decision altitude (the initial point for descending below Instrument Flight Rules (IFR) minima) for instrument landing system (ILS) approaches.

Pressurization vs. Ground Level Pressure

What is Ground Level Pressure (GMP)?

Ground Level Pressure > GMP >

GMP is atmospheric pressure measured at sea level, approximately 1013.25 millibars (mbar) when the aircraft is on the ground.

What’s the Connection?

pressure altitude calculation requires knowledge of both Ground Level Pressure (GMP) (the starting point) and the actual atmospheric pressure at the aircraft’s current altitude.

Mathematical Calculation

to calculate pressure altitude, apply the following formula:

Pressure Altitude (PA) = Indicated Altimeter Reading ( IA) Variation (Δ)

Pressure Altitude (PA)= IA(IA)Δ p** (atmosphere pressure)

Where:

Variation (Δ) reflects the difference between the desired pressure (1013.25 mbar at sea level) and the actual reading.

Variation at Different Altitudes

Example of Variation calculations at different altitudes-

Altitude (ft)variation (mbar)
10,000 feet+150 mbar
20,000 feet+380 mbar
30,000 feet+660 mbar
40,000 feet+1.02 bar

Adjusting Altimeter settings

Key takeaways:>

  • **Pilots must manually set the altimeter before takeoff at the standard atmospheric pressure altitude (1013.25 mbar) for QNH/QFE settings.”
  • Later, pilots adjust the QNH/QFE setting periodically to reflect changes in Barometric pressure during flight
  • Conclusion

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