hectopascals

[Guest skyfox49]

This is my first post here. I have a question that i cannot grasp properly. I will struggle to type this..:;)6: I have asked my instructor and he explains, but i cant get my head around it.

 

This would be on the topic of pressure or hectopascals. Every flight, we hear the QNH on the ATIS. We set this on the subscale of the altimeter. I know pressure changes all the time! but how, does an altitude change above sea level, as in a 'height'? I also know that for every 30' feet there is one hectopascal added. Im aware of a standard 1013.2 Hpa.

 

Thanks in advance

 

 

[Guest pelorus32]

Good question and welcome.

 

I suspect that your confusion lies in the difference between QNH and QFE. Also do not confuse altitude with elevation. Rather than explain from scratch, here are a few links:

 

http://en.wikipedia.org/wiki/QNH

 

http://www.auf.asn.au/groundschool/umodule3.html

 

http://en.wikipedia.org/wiki/Atmospheric_pressure#Mean_sea_level_pressure

 

Please come back if these don't answer your question and ideed let us know if they do.

 

Kind regards

 

Mike

 

 

[Guest TOSGcentral]

OK, I will have a crack at answering this one in very (probably painfully) simple terms.

 

A key to your confusion may reside in your understanding that you ADD one h/p per 30’ of altitude. The 30’ bit is right but you SUBTRACT one h/p per 30’ as air pressure decreases with altitude – hence why high flying aircraft are pressurised to maintain them at an acceptable pressure for the occupants and spare their ears the discomfort of the pressure change by climbing and descending.

 

The situation may be clearer if we go back to old terminology where instead of hectopascals we used millibars (which are exactly the same thing but just another name to make all the text books obsolete overnight J) One ‘bar’ representing a standard atmosphere at sea level and a millibar being one thousandth of a bar.

 

No visualize a stack of bricks each 30’ thick. The bottom brick (at sea level) has (say) 1000 bricks stacked on top of it so is subject to the pressure of all those bricks bearing down on it. That is the sea level air pressure.

 

However, if we climb to 9000’ (9000 divided by 30) we have the pressure of 300 less bricks bearing down on us so that is the lower air pressure at that altitude.

 

The altimeter measures the pressure via a sealed container that is capable of expanding and contracting in response to outside air pressure changing. The expansion and contraction is measured via a linkage that drives needles around on the instrument dial – expressing them in terms useful to the pilot.

 

As sea level air pressure changes from place to place, day to day and even during the course of the day, then there is another, adjustable, over-ride linkage hooked into the instrument which is the altimeter sub scale.

 

A very simple way of expressing this is that it gives the pilot the means of setting how many bricks are currently present.

 

So instead of air traffic control telling you that outside the met office at Goanna Gully there is a stack of 1005 bricks instead the will come back with “Goanna Gully QNH 1005).

 

To make things a bit more even for pilots transiting the Goanna Gully area then various pressure readings from around the area are averaged and expressed as “Area QNHâ€Â.

 

As I said – painfully simplified, but does that give you a better idea of what is going on?

 

Aye

 

Tony