QNH
Encyclopedia
QNH is one of the many Q codes. It is defined as, "barometric pressure adjusted to sea level." It is a pressure setting used by pilots
, air traffic control
(ATC), and low frequency
weather beacon
s to refer to the barometric setting which, when set on an aircraft's altimeter, will cause the altimeter to read altitude above mean sea level
within a certain defined region. Within United Kingdom airspace, these are known as Altimeter Setting Regions, (ASRs) These regions may be large areas, or apply only to the airfield for which the QNH was given. An airfield QNH will cause the altimeter to show airfield altitude, that is, the altitude of the centre point of the main runway above sea level on landing, irrespective of the temperature.
In the United Kingdom the lowest forecast value of QNH for an altimeter setting region is called the "Regional Pressure Setting" and may be used to ensure safe terrain separation when cruising at lower altitudes. In some parts of the world a similar procedure is adopted and this is known as "Regional QNH" however this name has been modified to the above in the UK to avoid ambiguity.
The abbreviation QNH originates from the days when voice modulated radio was often difficult to receive, and communication was done by Morse Code. To avoid the need for long Morse transmissions, many of the most commonly used communications were incorporated into a Q code
. To ask for atmospheric pressure at sea-level (i.e., at zero altitude) the letters 'QNH' would be transmitted. A common mnemonic for QNH is "No Height", (whereas the mnemonic often used for QFE is "Field Elevation"). Q codes are sometimes mistakenly quoted as being only questions but there are Q codes for statements, QKC is a statement about sea states and QAU, I am about to jettison fuel.
, which refers to the altimeter setting that will cause the altimeter to read the height above a specific aerodrome
or ground level, and therefore read zero on landing. While using QFE is convenient while flying in the traffic circuit of an airfield, the most common procedure when flying 'cross country' is to set the altimeter to either the local QNH or the standard pressure setting – 1013.25 hPa (29.92 inHg). When 1013 hPa (hectoPascal/millibar) is set on an altimeter subscale the aircraft's vertical position (in feet, divided by one hundred) is referred to as a Flight level
instead of an altitude.
Air Traffic Control will pass the QNH to pilots on clearing them to descend below the transition level, as part of air traffic control
clearance, on request of the pilot or when the QNH changes.
A typical radio conversation might be:-
Here, the pilot requests the regional air pressure, which is given as 1016 millibars for the Cotswold
Altimeter Setting Region ("ASR") (one of twenty ASRs into which UK Lower Airspace is divided). The pilot is required to read back the safety critical part of the transmission (in this case, the QNH value).
In most parts of the world, QNH is given in hectopascals (in the past in millibars) . In North America, QNH is given in hundredths of inches of mercury
(in the example, ATC would say ", altimeter three zero zero one" meaning 30.01 inches of mercury).
Aviator
An aviator is a person who flies an aircraft. The first recorded use of the term was in 1887, as a variation of 'aviation', from the Latin avis , coined in 1863 by G. de la Landelle in Aviation Ou Navigation Aérienne...
, air traffic control
Air traffic control
Air traffic control is a service provided by ground-based controllers who direct aircraft on the ground and in the air. The primary purpose of ATC systems worldwide is to separate aircraft to prevent collisions, to organize and expedite the flow of traffic, and to provide information and other...
(ATC), and low frequency
Low frequency
Low frequency or low freq or LF refers to radio frequencies in the range of 30 kHz–300 kHz. In Europe, and parts of Northern Africa and of Asia, part of the LF spectrum is used for AM broadcasting as the longwave band. In the western hemisphere, its main use is for aircraft beacon,...
weather beacon
Weather beacon
A weather beacon is a beacon that indicates the local weather forecast in a code of colored or flashing lights. Often, a short poem or jingle accompanies the code to make it easier to remember....
s to refer to the barometric setting which, when set on an aircraft's altimeter, will cause the altimeter to read altitude above mean sea level
Above mean sea level
The term above mean sea level refers to the elevation or altitude of any object, relative to the average sea level datum. AMSL is used extensively in radio by engineers to determine the coverage area a station will be able to reach...
within a certain defined region. Within United Kingdom airspace, these are known as Altimeter Setting Regions, (ASRs) These regions may be large areas, or apply only to the airfield for which the QNH was given. An airfield QNH will cause the altimeter to show airfield altitude, that is, the altitude of the centre point of the main runway above sea level on landing, irrespective of the temperature.
In the United Kingdom the lowest forecast value of QNH for an altimeter setting region is called the "Regional Pressure Setting" and may be used to ensure safe terrain separation when cruising at lower altitudes. In some parts of the world a similar procedure is adopted and this is known as "Regional QNH" however this name has been modified to the above in the UK to avoid ambiguity.
History and origin of the term
Student pilots sometimes remember QNH as "Query Newlyn Harbour". Newlyn Harbour in Cornwall, UK is home to the National Tidal and Sea Level Facility which is a reference for mean sea level. Another way to remember is "Q - Not Here" meaning it refers to the pressure setting that applies away from the airfield. This is to distinguish it from QFE, which novices sometimes confuse.The abbreviation QNH originates from the days when voice modulated radio was often difficult to receive, and communication was done by Morse Code. To avoid the need for long Morse transmissions, many of the most commonly used communications were incorporated into a Q code
Q code
The Q code is a standardized collection of three-letter message encodings, also known as a brevity code, all of which start with the letter "Q", initially developed for commercial radiotelegraph communication, and later adopted by other radio services, especially amateur radio...
. To ask for atmospheric pressure at sea-level (i.e., at zero altitude) the letters 'QNH' would be transmitted. A common mnemonic for QNH is "No Height", (whereas the mnemonic often used for QFE is "Field Elevation"). Q codes are sometimes mistakenly quoted as being only questions but there are Q codes for statements, QKC is a statement about sea states and QAU, I am about to jettison fuel.
Altitude above mean sea level
QNH differs from QFEQFE
QFE is a three letter acronym which can have meanings in aviation, in software development, and in network usage. It can refer to*QFE, a Q code used by pilots and air traffic control that refers to atmospheric pressure and altimeter settings...
, which refers to the altimeter setting that will cause the altimeter to read the height above a specific aerodrome
Aerodrome
An aerodrome, airdrome or airfield is a term for any location from which aircraft flight operations take place, regardless of whether they involve cargo, passengers or neither...
or ground level, and therefore read zero on landing. While using QFE is convenient while flying in the traffic circuit of an airfield, the most common procedure when flying 'cross country' is to set the altimeter to either the local QNH or the standard pressure setting – 1013.25 hPa (29.92 inHg). When 1013 hPa (hectoPascal/millibar) is set on an altimeter subscale the aircraft's vertical position (in feet, divided by one hundred) is referred to as a Flight level
Flight level
A Flight Level is a standard nominal altitude of an aircraft, in hundreds of feet. This altitude is calculated from the International standard pressure datum of 1013.25 hPa , the average sea-level pressure, and therefore is not necessarily the same as the aircraft's true altitude either...
instead of an altitude.
Air Traffic Control will pass the QNH to pilots on clearing them to descend below the transition level, as part of air traffic control
Air traffic control
Air traffic control is a service provided by ground-based controllers who direct aircraft on the ground and in the air. The primary purpose of ATC systems worldwide is to separate aircraft to prevent collisions, to organize and expedite the flow of traffic, and to provide information and other...
clearance, on request of the pilot or when the QNH changes.
A typical radio conversation might be:-
- Pilot:
, request Cotswold QNH - ATC:
, Cotswold QNH 1016 - Pilot: QNH 1016,
Here, the pilot requests the regional air pressure, which is given as 1016 millibars for the Cotswold
Cotswolds
The Cotswolds are a range of hills in west-central England, sometimes called the Heart of England, an area across and long. The area has been designated as the Cotswold Area of Outstanding Natural Beauty...
Altimeter Setting Region ("ASR") (one of twenty ASRs into which UK Lower Airspace is divided). The pilot is required to read back the safety critical part of the transmission (in this case, the QNH value).
In most parts of the world, QNH is given in hectopascals (in the past in millibars) . In North America, QNH is given in hundredths of inches of mercury
Inch of mercury
Inches of mercury, ' is a unit of measurement for pressure. It is still widely used for barometric pressure in weather reports, refrigeration and aviation in the United States, but is seldom used elsewhere....
(in the example, ATC would say "
Meteorological model
The letters QNH may also refer to the Quasi-Non-Hydrostatic meteorological model.See also
- Flight levelFlight levelA Flight Level is a standard nominal altitude of an aircraft, in hundreds of feet. This altitude is calculated from the International standard pressure datum of 1013.25 hPa , the average sea-level pressure, and therefore is not necessarily the same as the aircraft's true altitude either...
- QFE
- QFFQffQFF is a Q code. It is the MSL pressure derived from the barometric pressure at the station location by calculating the weight of an imaginary air column, extending from the location to sea level, assuming the temperature and relative humidity at the location are the long term monthly mean, the...
- Pressure altitudePressure altitudeIn aviation, pressure altitude is the indicated altitude when an altimeter is set to an agreed baseline pressure setting. The baseline pressure is 1013.25 hPa, equivalent to 1013.25 millibar, or 29.92 inches of mercury. This setting is equivalent to the air pressure at mean sea level in the...