Hi i wonder what QNH is????

Hi,the QNH is one of the old "Q-code" and is an altimeter setting.When crossing downward the transition level on arrival or on the tarmac before your departure, you have to set in your altimeter to the local pressure (reduced to sea level): this is the QNH (or altimeter setting, press B in FS!). Your altimeter will read on the ground the airfield elevation. All instrument procedures below TL/TA are drawn with altitudes in QNH. In the US, you'll use the QNH whenever below 18000 ft.Other old "Q-code" still used are:QFE: if set on QFE your altimeter will read zero on the ground on the airfield for which you set the QFE (in mnay approach plates, eg Jeppesen, the "runway elevation" is also given in hPa to subtract from QNH to obtain QFE)QNE: the standar (29.92 inHg or 1013.25 hPa) setting, to be used above TL/TA to fly in the "flight levels" area.Hope this helps.

Got this picture from the Airforce manual. I think it's excellent in displaying the various settings.http://forums.avsim.net/user_files/95843.jpg

Is QNH the same as hPa - the PMdg setting is in hPaBernie Lonnenhttp://www)precisionmanuals.com/images/forum/ng_driver.jpg

Think of QNH as a measurement and hPa as the unit(ie as in distance and nautical miles)I believe QNH is metric and therefore measured in hPa (same as millibars)The phrase "altimeter setting" is used in the states and has thus an imperial unit - PSI (pounds per square inch)Regards,Mark

Thanks for the explanation MarkBernieprecisionmanuals.com/images/forum/ng_driver.jpg

There are many 'Q' codes and they are simply abbreviations for various things. QNH is a 'Q' Code used for the actual Atmospheric Pressure at Mean Sea Level at a particular time and place. The sea level pressure fluctuates with weather patterns and so is updated several times a day for each area of operations.Altimeters actually measure air pressure. Previously all pressures related to a U shaped tube, sealed at one end with a near perfect vaccuum in it and the other end open to the atmosphere and containing a column of Mercury or water. This is called a BAROMETER. Quote fromhttp://inventors.about.com/gi/dynamic/offs...5/CE004667.htmlbarometer , instrument for measuring atmospheric pressure. It was invented in 1643 by the Italian scientist Evangelista Torricelli, who used a column of water in a tube 34 ft (10.4 m) long. This inconvenient water column was soon replaced by mercury, which is denser than water and requires a tube about 3 ft (0.9 m) long. The mercurial barometer consists of a glass tube, sealed at one end and filled with pure mercury. After being heated to expel the air, it is inverted in a small cup of mercury called the cistern. The mercury in the tube sinks slightly, creating above it a vacuum (the Torricellian vacuum). Atmospheric pressure on the surface of the mercury in the cistern supports the column in the tube, which varies in height with variations in atmospheric pressure and hence with changes in elevation, generally decreasing with increases in height above sea level. Standard sea-level pressure is 14.7 lb per sq in. (1,030 grams per sq cm), which is equivalent to a column of mercury 29.92 in. (760 mm) in height; the decrease with elevation is approximately 1 in. (2.5 cm) for every 900 ft (270 m) of ascent. UnquoteA basic Altimeter is simply a Barometer which uses an evacuated metal diaphragm {hard to fly with an Hg column in the cockpit and besides.. where would I put my coffee? ;)} working against atmospheric pressure, moves a needle and so indicates variations of relative pressure.The static (still) air pressure drops about 1 millibar (mB) or 1 HectoPascal (HPa) for every 30 feet we ascend. mB is an old fashioned measurement wheras HPa is an SI unit. The Yanks as usual have to be different and so they use a standard pressure of 29.92 inches of Mercury written as 29.92"Hg. This is just another measuremennt of the height of that same old mercurcy column at sea level at 15 degrees C.As we climb so the static (still) air pressure decreases and we see that indicated on the altimeter (barometer) but instead of marking it in mB or HPa we mark it is feet. Easy huh?

>Think of QNH as a measurement and hPa as the unit>>(ie as in distance and nautical miles)>>I believe QNH is metric and therefore measured in hPa (same as>millibars)>>The phrase "altimeter setting" is used in the states and has>thus an imperial unit - PSI (pounds per square inch)>TRY INCHES OF MERCURY>Regards,>>Mark

Of couse, you're right!My mistakePressure in the States is read in inHG, inches of mercurcy.Thanks,Mark