True Altitude VS Pressure altitude with E6B

 

Disclaimer: 

▪︎This content is Not an official document and does not represent the views of Airbus or any other aviation authority.

▪︎The information provided may be incorrect or misinterpreted and should not be relied upon for decision-making. 

▪︎Always refer to official documents and consult with a qualified aviation professional before making any decisions based on the information provided in this blog post.

▪︎The information provided in this blog post is based on personal study and review.

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True altitude. 

True altitude is the height above mean sea level (MSL). 

This means that the vertical distance between your aircraft and the average sea level is what is measured. 

 

Pressure altitude

 In simple terms, pressure altitude is the altitude read on the altimeter when it's set to 29.92 inches of mercury (1013 hPa), known as the QNE setting. This setting provides us with the current pressure altitude, which is the reference point for flight levels.

(Note: QNH represents the barometric pressure as reported by a specific station.)

If we set the altimeter to QNE on the ground and then ascend in the aircraft, the pressure altitude will increase just like it does with the QNE setting.

 In this scenario, there will be a difference in altitude between the QNH setting provided by the ground station and the QNE setting selected in the aircraft.

If the QNH setting is lower than the QNE setting, it means that the lower pressure setting of 1013 hPa on the altimeter reflects the airport's elevation as if it were at a lower pressure. This, in turn, indicates that when the altimeter is set to 1013 hPa in an area with lower atmospheric pressure, the pressure altitude will read higher than usual.

(if you turn the altimeter setting to higher value, the indicated altitude goes up)

It's to remember that flying at higher altitudes can lead to reduced aircraft performance due to the lower air density and pressure."

 

 

 

Finding True altitude with E6B

To find true altitude using an E6B flight computer, We will need to find out pressure altitude first and the temperature to compensate. 

 

Thanks to the altimeter on the aircraft, we can determine pressure altitude by setting the altimeter to 29.92.
After obtaining this pressure altitude reading, refer to the altitude computation window, and align the temperature scale directly over the pressure altitude.

 

 

temp -20 celcius and the pressure altitude 10000ft.

 

 

 Once we have the pressure altitude on the inner scale as indicated by the aircraft altimeter, we can then read the true altitude on the outer scale.

 

 

QNE setting and altimeter. 

But we need to know one more thing to determine the true altitude apart from pressure altitude and temperature. 
Since local pressure varies every day, in order to ascertain the true altitude, we should be aware of the QNE setting and the indicated altitude. 

This is because true altitude changes with the local pressure. Just knowing the pressure altitude doesn't mean we can determine the true altitude.

Imagine we are on the ground with the aircraft set to the QNE setting. The altitude always displays the same number, which is airport's elevation. However, the actual QNE setting varies from day to day.

So, if we set the aircraft to QNE and refer to it on the inner circle to read the true altitude on the outer scale, 

for example, if the altitude displays 10,500 feet with 30.42 inches and the true altitude reads 9,900 feet, 

in this case, the true altitude is lower by about 600 feet.

(-20degree celcius, and the pressure altitude is 10000ft)

 

altimeter reads 10500ft, with 30.42inches then the true altitude reads 9900ft

 

 

 

when it's cold, look out below. 

 

So, let's consider a scenario where the ground temperature is -10 degrees Celsius, and the pressure altitude is 0 feet.
If we apply the same calculation, when the aircraft altimeter reads 1000 feet with whatever QNE setting, the true altitude will be about 910 feet. which is less than 1000ft.