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QNH, Local QNH And Area QNH  
User currently offlinemawingho From Hong Kong, joined Jul 2012, 41 posts, RR: 0
Posted (2 years 4 months 2 weeks 4 days 5 hours ago) and read 6910 times:

"When departing your airfield you will need to set the area QNH for your region. This can be up to 5 hPa (150ft) different from the local aerodrome QNH."

What are the differences between QNH, local QNH and area QNH?

7 replies: All unread, jump to last
 
User currently offlinerfields5421 From United States of America, joined Jul 2007, 7607 posts, RR: 32
Reply 1, posted (2 years 4 months 2 weeks 4 days 5 hours ago) and read 6900 times:

I can only speak from my limited GA experience - but at times the setting for the airport from which I am taking off is different from the setting which the area ATC control is using for separation purposes. So after takeoff and upon contacting the departure ATC, I have to adjust the altimeter.

The arrival and departure setting is usually what is the pressure at DFW, not what might be at my departure or arrival airport some 40-60 miles away.

It doesn't happen often, but I've seen it a couple times.

[Edited 2012-08-02 06:30:04]

User currently onlineglen From Switzerland, joined Jun 2005, 225 posts, RR: 2
Reply 2, posted (2 years 4 months 2 weeks 4 days 3 hours ago) and read 6866 times:

General: QNH is the actual pressure reduced to sea level. From actual pressure at a location (=QFE) QNH can be derived by knowing the actual elevation and assuming a temperature and pressure gradient according ISA standard-atmosphere.

An altimeter set to QFE indicates zero. When set to (local) QNH it indicates the elevation (true altitude) as long the aircraft is on ground. In the air, with QNH setting, the altimeter indicates altitude but including an error for non standard temperature (indicated altitude).
As long as you stay local and every aircraft has local QNH-setting, altimeter readings are comparable and therefore separation is guaranteed (as long everyone flies the correct altitudes of course), as every altimeter has the same error, based on non-standard temperature.
If you leave local, other traffic will be present which departed from airports with probably different local QNH. To have comparable altimeter readings you need to define an area QNH, which is derived from a specific point within this area.

Going one step further: For flying higher (above specified altitudes) you will meet aircraft originating from very different areas. In order to have again comparable indications and to avoid resetting the altimeter every time you enter a new area, the altimeters are set to standard sea level pressure QNE (1013 hPa, 29.92in). Altimeter indicates now Flight Level. This setting has no relation to any actual pressure. So besides the error for temperature you have also an error for pressure not being standard in your altitude indication. Again every aircraft in this airspace has the same error in his indication and therefore the altimeter readings are comparable again.



"The horizon of many people is a circle with zero radius which they call their point of view." - Albert Einstein
User currently offlinezeke From Hong Kong, joined Dec 2006, 9228 posts, RR: 76
Reply 3, posted (2 years 4 months 2 weeks 2 days 20 hours ago) and read 6752 times:

Quoting mawingho (Thread starter):

Which AIP did you get that from so I can give you the appropriate answer.



We are addicted to our thoughts. We cannot change anything if we cannot change our thinking – Santosh Kalwar
User currently offlinemawingho From Hong Kong, joined Jul 2012, 41 posts, RR: 0
Reply 4, posted (2 years 4 months 2 weeks 1 day ago) and read 6594 times:

if you're referring the material (Aeronautical Information Publication), I am reading the BAK published by Aviation Theory Center written by David Robson.

User currently offlinemawingho From Hong Kong, joined Jul 2012, 41 posts, RR: 0
Reply 5, posted (2 years 4 months 2 weeks 1 day ago) and read 6589 times:

Quoting glen (Reply 2):
glen From Switzerland, joined Jun 2005, 179 posts, RR: 2Reply 2, posted s_lt(1343920301, 'l F j Y H:i:s');Thu Aug 2 2012 23:11:41 your local timeThu Aug 2 2012 08:11:41 UTC (3 days 2 hours 38 minutes ago) and read 272 times:

General: QNH is the actual pressure reduced to sea level. From actual pressure at a location (=QFE) QNH can be derived by knowing the actual elevation and assuming a temperature and pressure gradient according ISA standard-atmosphere.

Thank you for your reply. For QNH, if we know the QFE (from Airport to the Flight) and the elevation (From Mean Sea Level to the Airport), then we can get QNH. Is that what you mean?

So, if we fly within local, then set local QNH
if we fly within an area, we need to set the area QNH, but before we arrive to the destination, should we set back to local QNH?

If we fly over flight level, we set the standard QNH.

If we fly from one country to another country, is that the step we need to set the altimeter?
Local QNH -> Area QNH -> Standard QNH
-> Area QNH -> Local QNH


Quoting glen (Reply 2):
Altimeter indicates now Flight Level. This setting has no relation to any actual pressure. So besides the error for temperature you have also an error for pressure not being standard in your altitude indication. Again every aircraft in this airspace has the same error in his indication and therefore the altimeter readings are comparable again.


By the way, why you said it's no relation to any actual pressure? Isn't the altimeter use the static pressure change to determine the altitude?

Why the temperature could make an error to the altimeter? Because of icing or fog? Or the temperature can cause the change to the air molecule thus lead to change to the pressure?

Thank you for your help in advance.


User currently offlineCosmicCruiser From United States of America, joined Feb 2005, 2255 posts, RR: 15
Reply 6, posted (2 years 4 months 2 weeks 1 day ago) and read 6587 times:

I may be a dummy but of all my low alt VFR flying I did yrs ago I never heard of "area" QNH. As you flew along a course you could obtain progressive alt settings from other stations along the route. Back when I hung out at the local ATCC they had all the area station's alt settings projected on a screen for all to see . sometime I think the difference between what they give and approach control is theirs is not up to date.

User currently onlineglen From Switzerland, joined Jun 2005, 225 posts, RR: 2
Reply 7, posted (2 years 4 months 2 weeks 4 hours ago) and read 6517 times:

Quoting mawingho (Reply 5):
Thank you for your reply. For QNH, if we know the QFE (from Airport to the Flight) and the elevation (From Mean Sea Level to the Airport), then we can get QNH. Is that what you mean?

Yes, but this part of the answer was meant as a general, theoretical part. Usually you receive and set QNH, then you can read your altitude. Or on ground you just set the actual elevation on the altimeter and you can read your QNH on the altimeter.
The way you describe it, it still works in states where they use QFE for normal operation, e.g. Russia. You receive the QFE for a specific runway and then (because we operate on QNH) we have a table which states how much hPa to add to the QFE to receive actual QNH.

Quoting mawingho (Reply 5):
If we fly from one country to another country, is that the step we need to set the altimeter?
Local QNH -> Area QNH -> Standard QNH
-> Area QNH -> Local QNH

Theoretically it could work like this. Normally it depends about your cruising altitude. If you stay below transition altitude (the altitude to switch from QNH to standard setting) you will receive an the area QNH once you contact e.g. flight information services. Every time you enter a new area you receive a new area QNH and change finally to local QNH at the end of the trip. If you fly above transition altitude you will change directly from local QNH to standard setting and back to the actual local QNH when descending to your destination.

Quoting mawingho (Reply 5):
By the way, why you said it's no relation to any actual pressure? Isn't the altimeter use the static pressure change to determine the altitude?

I want to say it has no relation to any actual pressure measured on ground. Of course the altimeter needs static pressure to have its indication, but with standard setting it displays its altitude above the altitude where there is actually a pressure of 1013hPa. But the displayed altitude has therefore no relation to sea level or to a height above field which would be the case with QNH or QFE setting.

Quoting mawingho (Reply 5):
Why the temperature could make an error to the altimeter? Because of icing or fog? Or the temperature can cause the change to the air molecule thus lead to change to the pressure?

An altimeter is measuring only actual pressure and every pressure is related to a certain altitude for display. As warmer air is expanding, also the distance between pressure levels expands in warm air. For example: With the same pressure on ground, the level with a pressure of 500 hPa is higher above ground in warm air due to this expansion. Measuring the same pressure, the aircraft is therefor actually higher than in cold air, where the air is contracted. As a rule of thumb the error is 4% for every 10°C deviation from standard temperature.



"The horizon of many people is a circle with zero radius which they call their point of view." - Albert Einstein
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