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Altimetry

Eddie sez:

There are three basic ways to set your altimeter in terms of the aircraft relative to the ground and mean sea level: QFE, QNH, and QNE. There are also three ways to measure the altimeter setting: inches, hectopascals, and millimeters. We tend to grow up doing in one way and then half to learn the others depending on where we fly.

Easy enough you say? Not so fast. Make sure you also take a look at:

Everything here is from the references shown below, with a few comments in an alternate color.

Last revision:

2020-12-26


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

[ICAO Document 8168, Vol 1, §1, Ch 1] and [ICAO Document 4444, Ch 1] and others

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.

Conclusions

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.

images

Figure: QNE / QNH / QFE, from Eddie's notes.
Click photo for a larger image

Altimeter Setting Procedures

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.

Inches (in) of mercury

Most of us in the United States are accustomed to using inches for that scale. Setting 29.92 means the mercury is pushed 29.92 inches.

Hectopascals (hPa)

The ICAO standard when it comes to setting your altimeter is the hectopascal, abbreviated hPa. A hectopascal is equal to a millibar, which is one-thousandth of a "standard atmosphere" which is equal to 1,000 "dynes" which comes to about 0.0145 pounds per square inch. All that really matters to you is that on a standard day you set 1013 hPa and on less than standard days is varies from there.

Millimeters (mm) of mercury

A competing system that has pretty much died out in all but a few countries is millimeters of mercury, or mm.

Because inches and hPa are so common, most aircraft designed for international travel will have a way of setting either. To set millimeters you may need a table to do the conversion:

MM+0+1+2+3+4+5+6+7+8+9
Inches
70027.5627.6027.6427.6827.7227.7627.8027.8327.8727.91
71027.9527.9928.0328.0728.1128.1528.1928.2328.2728.31
72028.3528.3928.4328.4628.5028.5428.5828.6228.6628.70
73028.7428.7828.8228.8628.9028.9428.9829.0229.0629.09
74029.1329.1729.2129.2529.2929.3329.3729.4129.4529.49
75029.5329.5729.6129.6529.6829.7229.7629.8029.8429.88
76029.9229.9630.0030.0430.0830.1230.1630.2030.2430.28
77030.3130.3530.3930.4330.4730.5130.5530.5930.6330.67
78030.7130.7530.7930.8330.8730.9130.9430.9831.0231.06
79031.1031.1431.1831.2231.2631.3031.3431.3831.4231.46

If, for example, you are told the altimeter setting is 765 millimeters, you would set 30.12 inches.

images

Figure: Mercury barometer, from Eddie's drawings
Click photo for a larger image


See Also:

Aeronautical Information Manual

FAA-H-8083-15B, Instrument Flying Handbook, U.S. Department of Transportation, Flight Standards Service, 2012.

ICAO Doc 4444 - Air Traffic Management, 16th Edition, Procedures for Air Navigation Services, International Civil Aviation Organization, October 2016

ICAO Doc 4444, Amendment 9 to the PANS-ATM, 15 June 2020

ICAO Doc 8168 - Aircraft Operations - Vol I - Flight Procedures, Procedures for Air Navigation Services, International Civil Aviation Organization, Sixth Edition, 2018