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Questions and Answers
What is the pressure altitude measured from?
What is the purpose of setting the altimeter to 29.92 Hg?
Above what altitude is pressure altitude used as the standard reference altitude in the US and Canada?
What is the relationship between pressure altitude and true altitude?
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Why is pressure altitude important in aviation?
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What is the primary function of a flight computer such as ASA's CX3 or Sporty's Electronic E6B?
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What is the unit of measurement for air pressure in the context of pressure altitude?
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What is required above 18,000 feet in terms of altimeter settings?
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What is the primary purpose of using a standard reference altitude like pressure altitude?
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What is transmitted by the Mode C transponder?
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What is the air pressure at the Standard Datum Plane (SDP)?
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What is pressure altitude used for in flight planning?
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What is the purpose of setting the altimeter to a standard setting above 18,000 feet?
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What is the primary benefit of understanding pressure altitude?
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How does ATC determine an aircraft's true altitude?
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What is the primary difference between pressure altitude and true altitude?
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When do true altitude and pressure altitude match?
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What is the altimeter setting for true altitude on the ISA day?
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What is density altitude?
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When are pressure altitude and density altitude the same?
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How do you calculate pressure altitude using the altimeter?
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What is the rule of thumb formula for pressure altitude?
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What is the purpose of the detailed pressure altitude formula?
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What is the Standard Datum Plane (SDP)?
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What happens to the density altitude when the temperature is higher than the standard atmosphere?
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What is the effect of non-standard temperature on aircraft performance?
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Study Notes
Pressure Altitude Basics
- Pressure altitude is the altitude read on an altimeter when set to 29.92 Hg.
- It is the height above a Standard Datum Plane (SDP), a theoretical level where air pressure equals 29.92 inches of mercury (Hg).
- Think of it as a standard "reference altitude."
Practical Application
- Pressure altitude is used as a "baseline" for many other types of altitude, such as density altitude.
- It is used above 18,000 ft in the US and Canada to ensure all aircraft fly according to the same reference.
Pressure Altitude vs. True Altitude
- True altitude is the altitude above Mean Sea Level (MSL).
- Pressure altitude and true altitude are the same when standard atmospheric conditions exist.
Pressure Altitude vs. Density Altitude
- Density altitude is pressure altitude corrected for non-standard temperature.
- Density altitude is the "effective pressure altitude" an aircraft "feels" for a given temperature.
Calculating Pressure Altitude
- There are three primary ways to determine pressure altitude: using an altimeter, the pressure altitude formula, or a flight computer.
- The rule of thumb formula for pressure altitude is: Pressure Altitude = (29.92 − Altimeter Setting) x 1000 + Elevation.
- The detailed formula for pressure altitude is a more accurate but more complex calculation.
Using a Flight Computer
- A flight computer, such as ASA's CX3 or Sporty's Electronic E6B, can be used to calculate pressure altitude easily and accurately.
How to Use Pressure Altitude
- Pressure altitude is used in flight planning and aircraft performance charts.
- It is used as the baseline for other calculations, such as density altitude.
- Above 18,000 ft, pilots are required to set their altimeters to the standard 29.92 inches of mercury.
- Mode C transponders transmit pressure altitude data.
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