Figure: QNE / QNH / QFE, from Eddie's notes.
Definitions (QNE, QNH, and QFE)
Getting a clearly stated, unambiguous definition for QNE, QNH, and QFE is harder than you might think. Are they the altimeter setting that you dial in or are they the resulting altitude? Read on . . .
ICAO Doc 8168 Procedures for Air Navigation Services
Flight level (FL). A surface of constant atmospheric pressure which is related to a specific pressure datum, 1013.2 hectopascals (hPa), and is separated from other such surfaces by specific pressure intervals.
Note 1.— A pressure type altimeter calibrated in accordance with the Standard Atmosphere:
a) when set to a QNH altimeter setting, will indicate altitude;
b) when set to a QFE altimeter setting, will indicate height above the QFE reference datum; and
c) when set to a pressure of 1013.2 hPa, may be used to indicate flight levels.
Note 2.— The terms “height” and “altitude”, used in Note 1 above, indicate altimetric rather than geometric heights and altitudes.
[ICAO Document 8168, Vol 1, §1, Ch 2]
QFE — Atmospheric pressure at aerodrome elevation (or at runway threshold)
QNH — Altimeter sub-scale setting to obtain elevation when on the ground
So QNH and QFE are the pressure settings you put into the altimeter. The "QFE reference datum" isn't given in any ICAO document but appears to be runway elevation at the threshold. There is no mention of QNE at all.
U.S. Aeronautical Information Manual
[AIM, Pilot Controller Glossary]
QNE− The barometric pressure used for the standard altimeter setting (29.92 inches Hg.).
QNH− The barometric pressure as reported by a particular station.
QFE (“Field Elevation”) - QFE is a pressure setting you dial into your altimeter to produce the height above the runway. It reads zero when you are on the runway and gives your height above it when you are airborne. This appears to be consistent between ICAO and U.S. FAA reference material.
QNH (“Height Above Sea Level”) - QNH is a pressure setting you dial into your altimeter to produce the height above sea level. 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. This appears to be consistent between ICAO and U.S. FAA reference material.
QNE ("En Route") - QNE is a pressure setting of 29.92 inches or 1013 hPa that will produce a standard atmosphere altitude and provides the basis for flight levels. 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.S. 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.
Definitions (Transition Layers, Altitudes, Transitions)
Figure: Transition Altitudes, from Eddie's notes.
ICAO Doc 8168 Procedures for Air Navigation Services
[ICAO Document 8168, Vol 1, §1, Ch 1]
1.1 These procedures describe the method for providing adequate vertical separation between aircraft and for providing adequate terrain clearance during all phases of a flight. This method is based on the following basic principles:
a) States may specify a fixed altitude known as the transition altitude. In flight, when an aircraft is at or below the transition altitude, its vertical position is expressed in terms of altitude, which is determined from an altimeter set to sea level pressure (QNH).
b) In flight above the transition altitude, the vertical position of an aircraft is expressed in terms of flight levels, which are surfaces of constant atmospheric pressure based on an altimeter setting of 1 013.2 hPa.
c) The change in reference from altitude to flight levels, and vice versa, is made:
1) at the transition altitude, when climbing; and
2) at the transition level, when descending.
d) The transition level may be nearly coincident with the transition altitude to maximize the number of flight levels available. Alternatively, the transition level may be located 300 m (110 ft) above the transition altitude to permit the transition altitude and the transition level to be used concurrently in cruising flight, with vertical separation ensured. The airspace between the transition level and the transition altitude is called the transition layer. Where no transition altitude has been established for the area, aircraft in the en-route phase shall be flown at a flight level.
f) The adequacy of terrain clearance during any phase of a flight may be maintained in any of several ways, depending upon the facilities available in a particular area. The recommended methods in the order of preference are:
1) the use of current QNH reports from an adequate network of QNH reporting stations;
2) the use of such QNH reports as are available, combined with other meteorological information such as forecast lowest mean sea level pressure for the route or portions thereof; and
3) where relevant current information is not available, the use of values of the lowest altitudes or flight levels, derived from climatological data.
[ICAO Document 4444, Ch 1] The altitude at or below which the vertical position of an aircraft is controlled by reference to altitudes.
[ICAO Document 4444, Ch 1] The airspace between the transition altitude and the transition level.
[ICAO Document 4444, Ch 1] The lowest flight level available for use above the transition altitude.
More about this: Transition Altitude / Layer / Level.
How does it work?
Figure Sensitive Altimeter Components, from Instrument Flying Handbook, Figure 3-3.
[Instrument Flying Handbook, pg. 3-3] The sensitive element in a sensitive altimeter is a stack of evacuated, corrugated bronze aneroid capsules like those shown in figure 3-3. The air pressure acting on these aneroids tries to compress them against their natural springiness, which tries to expand them. The result is that their thickness changes as the air pressure changes. Stacking several aneroids increases the dimension change as the pressure varies over the usable range of the instrument.
Portions of this page can be found in the book International Flight Operations, Part VIII, Chapter 3.
FAA Alternate Airport Flight Planning Using GPS and WAAS Policy Statement, Effective 4/04/13 to 5/01/13
FAA-H-8083-15, Instrument Flying Handbook, U.S. Department of Transportation, Flight Standards Service, 2001.
FAA-H-8261-1, Instrument Procedures Handbook, U.S. Department of Transportation, Flight Standards Branch, 2004
ICAO Doc 4444 - Air Traffic Management, 16th Edition, Procedures for Air Navigation Services, International Civil Aviation Organization, October 2016
ICAO Doc 8168 - Aircraft Operations - Vol I - Flight Procedures, Procedures for Air Navigation Services, International Civil Aviation Organization, 2006