QFE is the isobaric surface pressure at the reference point. At other altitudes the altimeter will give an indication of the height above that reference point. With the aerodrome QFE set in the subscale, your altimeter will read zero on the highest point on the runway and at other altitudes will read the height above the airfield elevation.
For precision approach runways or for instrument runways when the threshold is 7 ft or more below aerodrome elevation, the QFE may be based on the threshold elevation [ICAO Doc , 4. With the runway threshold QFE set in the subscale, your altimeter will read zero on the runway threshold. QNE is different to the other altimetry Q codes in that it is an altitude not a pressure although it is commonly incorrectly described as such.
With Standard Pressure Pressure to height conversions in altimetry are based on ISA. In non-ISA conditions, altimeter readout may be significantly different than the true altitude of said altimeter. Since cold air is denser than warm air, isobaric surfaces are vertically more constrained towards the ground. It is for flight in colder-than-ISA that particular attention must be paid to true altitude. The altimeter readout, being an underestimate of the actual altitude, may lead crews to think they are higher than they actually are, and can lead to serious incidents if not accidents.
Since all altimeters are based on ISA, notwithstanding instrument error, altitude readouts between aircraft will be similar. We tend to grow up doing in one way and then half to learn the others depending on where we fly. Everything here is from the references shown below, with a few comments in an alternate color. Are they the altimeter setting that you dial in or are they the resulting altitude? Read on. Flight level FL. A surface of constant atmospheric pressure which is related to a specific pressure datum, Note 2.
There is no mention of QNE at all. It reads zero when you are on the runway and gives your height above it when you are airborne. FAA reference material. It reads runway elevation when you are on the runway and is based on an altimeter setting adjusted until the station's correct elevation above sea level is read.
The term does not appear to be used by the ICAO, though the concept itself is used to produce flight levels. QNE is explicitly defined in U.
FAA sources. The terms "altimeter setting" and "barometric pressure" can be confusing but should not be. They are the same thing. You input barometric pressure into your altimeter and it produces altitudes. Click photo for a larger image. There isn't a lot of source material on what you are setting in what is known as the "barometric scale" by some, but is more properly called the "Kollsman Window," in honor of Paul Kollsman the person who invented the first sensitive barometer.
There are three choices, each of which is based on how high a column of mercury can be pushed up by atmospheric pressure in a vacuum tube. See: Altimetry Theory for a more indepth explanation. Most of us in the United States are accustomed to using inches for that scale. Setting A hectopascal is equal to a millibar, which is one-thousandth of a "standard atmosphere" which is equal to 1, "dynes" which comes to about 0.
All that really matters to you is that on a standard day you set hPa and on less than standard days is varies from there. A competing system that has pretty much died out in all but a few countries is millimeters of mercury, or mm.
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