QNH is used when you want to adjust your altimeter to a pressure level.
If you take your Airport diagram\ chart, you will see that there a field elevation. Field elevation is in relation to the the sea level. If your chart says the field elevation is 980 ft ..you are actually at 980 feet above sea level. This makes sense, since you can't be at 980 ft above ground ...while being on the ground ..lool.
Now with the QNH, if on your Altimeter you put in the right altimeter setting you will have 980 feet indicated on your altimeter. Which corresponds to your altimeter setting.
Now when you are flying, when you get above 18 000 ft .... your altitude is no longer reported as altitude but as Flight Level. FL 200 corresponds to 20 000 ft.
When you fly above 18 000 ft , you change your altimeter setting to 29.92 .... why ?
Because 29.92 has been stated as being the pressure in the standard atmosphere, which says that at Sea Level in the standard atmosphere the pressure is of 29,92 Hg and the temperature is 15 degrees Celsius and the air cools down at 2degrees Celsius per 1000 ft.
Thing you have to remember is that whatever QNH you give in, it gives you your height above sea level and not ground !! this is why when you select Radar altimeter to be indicated on your HUD you will have a difference.
Now, why do we set an altimeter setting ? because the standard atmosphere that is used as a reference ... almost never exists. It is very rare actually that it will be 15 celsius at sea level with a pressure of 29.92.
Talking about pressure i mean here the force exerted by a column of air, this gives you the pressure that the air exercess above the ground or the altitude at which you are.
Cold Air is denser than Warm air. For a same quantity of Air, Cold air is heavier than warm air, it also means that cold air exercise more pressure than warm air.
This also means that the pressure of the air that you are going to have in warm air at 10 000 feet is not the same as the one you are going to have in cold air at 10 000 ft.
This means that for two different air Masses ... 10 000ft is not equal to the same QNH on your altimeter.
This is why you have to reset altimeter setting, before takeoff, during navigation below 18 000 ft, during descent\before landing ..to make sure we are not too low or too high.
Remember, above 18 000 ft, everybody has to set 29.92 as the QNH on his altimeter. You are going to tell me that it is not correct, but ... since everybody flies with the exact same "error" ... we are safe.
So again QNH = height above Sea level known as ASL.
NOw the QFE ... if you set that in in your altimeter ... it will indicate your altitude above the airport, so on the ground you will be at 0ft. But it is only useful for the airport ..for landing or taking of, and it only gives you your altitude above the airport ..not above the surrounding terrain .. like mountains. Because if you set in the QFE ..and fly towards a mountain ...even if you fly very low above it ... it will never get near to 0 ft ...this is again not your altitude above Ground, but above airport only ... and this is also why it is almost not used anymore.
I myself never used QFE, but it depends from the countries you fly in. In France they do not use it anymore, they do use QNH.
QNH is in Hg ... and QNE is the same but in hPa ( hectopascals) and since 1 hPa = 1 millibar ...you cans say both.
And 29.92 Hg is equivalent to 1013.2 hPa.
In Northern America we use Hg and in Europe they do use hPa. ....
So to resume it all:
QNH/QNE is the pressure level at which you fly that gives you an altitude. QNH/QNE varies with the Weather, ( low pressure, high pressure ).
QFE .... is not used anymore in the cicvil ..don't know about the military, but i don't think so ..because it is only useful above the airport.
In BMS4, you need to hit the T key twice for a menu to request QNH/QNE at the beginning of you approach.
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发表于 2011-10-29 14:38:31
楼主
发表于 2011-10-29 15:07:32
