True altitude and pressure altitude are both ways of measuring the height of an aircraft, but they use different reference points. True altitude is the actual height above mean sea level (MSL). Pressure altitude, on the other hand, is the altitude indicated on an altimeter when it is set to a standard atmospheric pressure of 29.92 inches of mercury (inHg).
Here's a breakdown of the differences:
True Altitude
- Definition: True altitude is the vertical distance of an aircraft above mean sea level. It's the actual height above the average level of the ocean.
- Measurement: True altitude is measured by using a precise reference point, that being the average sea level.
- Stability: True altitude provides a stable measurement of an aircraft's vertical distance with the Earth's surface.
Pressure Altitude
- Definition: Pressure altitude is the altitude read on an altimeter when the altimeter setting is adjusted to 29.92 inHg. It's essentially a standardized measurement of atmospheric pressure expressed as an altitude.
- Measurement: Pressure altitude is not directly referenced to mean sea level but instead to a standard atmospheric pressure.
- Use: It's used in aviation for performance calculations, such as determining aircraft climb rate and engine power.
- Variations: Pressure altitude will only equal true altitude on a day when standard pressure (29.92 inHg) matches the actual pressure at sea level. This means that in the majority of situations, they will not be the same value.
Key Differences Summarized
| Feature | True Altitude | Pressure Altitude |
|---|---|---|
| Reference Point | Mean Sea Level (MSL) | Standard Atmospheric Pressure (29.92 inHg) |
| Meaning | Actual height above sea level | Standardized pressure measured as an altitude |
| Equality | Does not fluctuate with changes in pressure | Only equal to true altitude on standard atmospheric days |
| Primary Use | Provides a direct measurement of height above MSL | Used for aircraft performance calculations |
How They Relate
According to the reference: If it's a standard day and sea level pressure is 29.92 inHg, then true altitude will equal pressure altitude. However, if the actual sea level pressure is higher or lower than 29.92 inHg, the true altitude and the pressure altitude will be different. An altimeter set to 29.92 inHg will show pressure altitude. For it to display true altitude, it must be adjusted with the current local altimeter setting.
Why the Difference Matters
- Aviation: Pilots must understand the difference between true and pressure altitude for accurate navigation, performance calculations, and safe operations.
- Safety: This difference becomes crucial when flying in non-standard atmospheric conditions or when conducting precision approaches.
Practical Example
Imagine an airport with an elevation of 1,000 feet MSL (true altitude).
- On a standard day (29.92 inHg), the pressure altitude at the airport will also be approximately 1,000 feet.
- However, if the atmospheric pressure at sea level is higher than standard (e.g. 30.12 inHg), the pressure altitude at the airport would be lower than the true altitude. This is because the pressure at 1,000 feet will correspond to an altitude less than 1000 feet when set to 29.92 inHg. Conversely, when the atmospheric pressure is lower, the pressure altitude will be higher.
