Published: 2008-10-12 13:25:29
Updated: 2008-10-12 13:25:29
Posted October 12, 2008
By Khalid Anwar
MIKE MOSS SAYS: Khalid, I'm not sure I entirely follow the question as written, but an aircraft sitting on the ground at an airport with field elevation 2000 feet above mean sea level will have a pressure altimeter that should read roughly zero if the altimeter is set according to the airfield's current station pressure (that is called the QFE altimeter setting and is intended to show the height of an aircraft above ground in the vicinity of an airfield). The QNH setting should ideally give a height of 2000 feet when the aircraft is sitting on the ground, since the QNH altimeter reading is intended to give the aircraft elevation above sea level. QFF is not typically used directly in an aircraft altimeter, and is a code that refers to what a meteorologist would typically call MSLP or Mean Sea Level Pressure, which can vary a bit from the altimeter setting calculated for QNH purposes due to some differing assumptions used in "reducing" the measured station pressure to sea level.
Finally, there is another altimiter setting standard that you did not mention here, called QNE, in which the altimeter is fixed at a setting of 1013.35 millibars (or 29.92 inches Hg), in which case the indicated altitude may not reflect reality relative to the ground or to sea level, but is at least standardized from one aircraft to another and used to maintain safe vertical separation during cruising at levels where the exact altitude above ground isn't critical. When using the QNE setting, the altitude is referred to as "flight level."