![Pressure Altitude - [What it is, How to calculate Pressure Altitude and why is it important??]](/img/logo.svg){kind=logo}
How to calculate the Pressure Altitude, what is it and why is it important??
The Pressure Altitude (PA) is the height of aircraft above the Standard Pressure 1013.25Hpa or 29.92Hg, and it is important to understand what it is because the Aircraft Performances are based on the PA.
In a standard Atmosphere the pressure at the mean sea level, also called QNH, is equal 1013.25Hpa.
So, in Standard Atmosphere, an aircraft that is on the threshold of a runway with 0 feet of elevation has a Pressure Altitude of 0 feet, and an aircraft on a runway that is at 1000feet of elevation has a PA of 1000feet.
It is important to understand two factors:
1. The pressure at the mean sea level (QNH) is not always 1013.25 Hpa, it can change rapidly due to the pressure system of the Atmosphere.
2. The Aircraft doesn't care about the physical elevation of the runway, it cares about the pressure that is around it.
So, if we take the example that I made before but this time we change the QNH to 1000hpa what will happen is the following:
The real Aircraft Altitude stays constant however, the Aircraft, will feel that is at a higher Altitude because the pressure is lower than 1013hpa.
With the lower pressure, the Air is less dense so the aircraft performance will be impaired.
If we compare the two examples we will see the Aircraft in the second case (QNH 1000hpa) will need a longer Take Off Run for the take-off.
In order to calculate the Pressure Altitude, we need to know that in a standard atmosphere for every hpa there is a difference in altitude of 27feet.
The difference between 1013hpa and 1000hpa is 13hpa, if we multiply the 13 by 27 we will come up with the Pressure Altitude of the Aircraft, which is 351feet.
When you are preparing for takeoff you need to know your QNH and correct your Allowed Takeoff Weight.
If you want to know more about Altimeter Settings watch the following RU-vid video:
WHAT IS QNH, QNE, QFE: • QNH, QFE and QNE - [A...
ALTITUDE vs HEIGHT vs FLIGHT LEVEL: • QNH and QNE Altimeter ...
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Disclaimer:
THIS VIDEO IS FOR ENTERTAINMENT PURPOSES ONLY.
NOT FOR REAL-LIFE OPERATIONS.
PLEASE REFER TO THE OFFICIAL MANUAL AND DOCUMENTATION.
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9 сен 2024
