Types of altitudes

Types of altitudes



•  Indicated. It is read directly from the altimeter. If it is set with the QNH, the indicated altitude will be approximately equal to the altitude of the airplane above mean sea level (MSL).


         QNH : It is the preassure at see level Altimeter, flight instrument, QNH, PA


Altimetry, differences of QNE, QNH and QFE

• Real Altitude , or actual altitude, is the actual altitude above sea level. The altitude of airports, mountains, obstacles, etc. The altitude shown on charts are given in true altitude.

• Pressure Altitude (PA) . Altitude read when the altimeter is set to  QNE (  the altimeter scale is 1013).


pressure altitude, true altitude, absolute altitude

in other words we have ISA atmosphere, which was created by engereers ( check the post International Stantandard atmosphere ), that atmosphere has some predetermined values shown in the picture below 

ISA values of Preassure and altitude

So when the atmospheric pressure is 22.22 in. Hg the pressure altitude is 8000 ft , so each time we have an outside pressure of 22.22 in.Hg,  our PA is 8000 ft . Which only will be the same of our true altitude in  case of the values of the atmosphere are completaly the same as I.S.A values ( very difficult to happen).

Imagine this situation you are about to take off in a 2000 ft height field, and your atmospheric pressure it is 20.57 in. hg. So in this case our true altitude would be 2000 ft , however our PA would be 10.000 ft.

To conclude the PA is the atmospheric pressure expressed in terms of altitude ( ISA Altitude)

  Why is PA so important ?

Because it is used to calculate  the next altitude, called density altitude. It is a way to find out how an aircraft behaves against an event , for example for calculating the runway lenght you need for taking off, as you can see in the image bellow.

performances take off chart
• Density Altitude (DA). It is the Pressure altitude Corrected by non-standard temperature deviation. As you can see it is the first part of the performance chart shown above.

take off performance chart


The concept is the same as PA in the way that dependending on the external density our DA changes for example :

International Standard atmosphere table


Imagine we are about to take off from a 2000 ft height field which density is 0.9428 our DA would be 2000 ft , because the density of our true altitude  is the same as the I.S.A density ( check the table above). If the density would have been  0.7620, our true altitude is still 2000ft , however our DA is 9000ft .

Why it is so important the DA ?

As we know DA is related to our exterior density, and the density is related to the mass of the fluids ( density = mass/volume). So if we have a big air density, we have big mass of air. If we have a big mass of air the engine works better  , the flight controls works better ….. . Due to is possible to move more amount of air .

To conclude : the bigger is the atmospheric pressure and density the lower the PA and DA are ( check the tables above), so the aircraft behaviour is better.




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