Barometric altitude measurement in aviation

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The barometric altitude measurement in aviation is based on the approximate determination of altitude by measuring the air pressure . The method is imprecise for determining the absolute altitude (above sea ​​level ), but it generally fulfills the requirement that the flight altitude is so reliable relative to other aircraft that the flight altitude or flight level assigned by air traffic control allows any close encounter with other aircraft excludes.

Various parameters and processes are used in aviation to measure air pressure, some of which differ from those in meteorology .

Measuring principle

Altimeters are barometers that show an altitude corresponding to the air pressure under normal conditions .

In order to be able to correct local air pressure deviations, which constantly occur due to high and low pressure areas in the atmosphere, the zero mark of the barometric altimeter must be able to be changed by the user. The setting of this zero mark and also the basic calibration of the barometer takes place on the basis of a standard atmosphere , which is defined worldwide by a technical regulation.

The altimeter needs a take-off point for the static air pressure , the static port . This can be located at special points on the fuselage or integrated into a pitot tube . In aircraft without a pressurized cabin , a reserve tapping point can be located in the cabin.

Analog display devices

Altimeter. With this device, the air pressure is set in units of inHg ( inches of mercury )

Analog display devices (called “clocks” in jargon) show the pressure and thus the estimated change in altitude mechanically directly via pointers and a standardized scale. Depending on the unit of measurement used, one revolution of the “minute” hand results in 1000 m or 1000 feet, with a scale of 20 m or 20 feet per graduation mark. A second "hour" hand or a dial, comparable to the date display on a wristwatch, displays the 1000 increments. For particularly high altitudes, the 10,000 foot steps are also displayed by means of a pointer attached to the outer edge of the scale.

The picture on the right shows an altimeter with three pointers. The little window at the 3 o'clock position of the display shows the set air pressure value , which corresponds to QFE or QNH etc. (see below) and must be set with the button at the bottom left. It is called the " Kollsman Window".

In commercial air traffic with instrument flight , altimeters with counter / pointer display or equivalent display are required. That means classical instruments with three hands are no longer allowed in this case. A digital display is now mostly used. The barometric altitude is shown in the primary flight display and a replacement instrument.

The rate of descent and climb of the aircraft, i.e. the change in altitude over time, is indicated by so-called variometers .

Digital display devices

Altimeter. With this device, the air pressure is set in the unit hPa ( hectopascal )

Digital display devices show the pressure change and thus the estimated change in altitude on an electronic display. In the example shown on the right, metric units of measurement are used (altimeter = altitude in meters and variometer = rise / fall in meters per second).

The picture shows a combined altimeter with variometer, as used by microlight and paraglider pilots. On the left you can see the altimeter in the QNH setting mode , which has been set to the value 1011 (hPa). On the right you can see, in altimeter mode, the height of 122 m resulting from the QNH value.


The accuracy of a barometric altimeter is a few decameters (dam) and decreases with increasing altitude . To estimate the absolute altitude ( true altitude , Eng. True altitude) have the local air pressure changes are regularly taken into account by adjusting the zero mark due to meteorological changes (weather). In addition to the altitude, the pressure depends on the temperature and humidity distribution .

Correction values ​​are determined by aviation weather stations on the ground and communicated to the pilots in the relevant area. Depending on the intended use, there are different standardized air pressure specifications, which are identified by Q groups .

Note on this: "In winter the mountains are higher". Reason: in a column of cold air, the density of the air on the ground is higher. As a result, the air pressure in a column of cold air decreases faster with altitude than in a warm column. The displayed flight altitude is displayed higher than the true altitude due to the lower air pressure. In pilot circles it is also said: "From high to low it usually goes wrong". Reason: If the air pressure set at the airport of departure is maintained when entering a low pressure area, the altimeter will show a higher altitude than the one actually flown due to the lower air pressure. In particular with flights under Instrument Meteorological Conditions (IMC) there is then the risk of a controlled flight into terrain (CFIT).

Q key

→ See main article: Q keys

The Q keys were arbitrarily defined in Morse time to speed up traffic.

Fun z. For example, if a pilot goes to the airfield “Please QNH”, this means: “Please tell me how I have to adjust my altimeter so that it shows the exact height of the airfield after landing.” The answer from the airfield is then e.g. B. "QNH 1010", which means: "If you set 1010 hPa on the setting scale of your altimeter, it will show the exact altitude after landing."

In aviation in Europe, the Q keys for altitude information (QNH, QNE, QFE) are in use.


QFE is the measured air pressure on the ground ( Atmospheric pressure at airfield elevation , Merkhilfe: Engl. Field Elevation ). The unit of measurement is the hectopascal hPa (in the USA inch Hg).

If the QFE is set on the altimeter, it will show an altitude of 0 m or 0 ft on the ground in an aircraft. In flight it shows approximately the altitude above the (nearby) airfield whose QFE value was set.

In aerobatics, the altimeter is generally set to QFE so that the altitude above the ground is displayed immediately. It is also used in gliding on the field . Outside of the above-mentioned applications, which are characteristic of short flights at low altitudes to and from the same airfield according to VFR , the altimeter setting on QFE is only common in a few areas.


The QNH pressure is the estimated pressure at sea ​​level at the location of the airfield. If it is set as ground pressure on the altimeter, the altimeter shows the approximate altitude of the aircraft above sea level. On the runway of the airfield z. B. its height above sea level.

It is weather-dependent, although its calculation is based in part on the standard atmosphere , which is why it is only an approximate value: A flat-rate (weather-independent) amount is deducted from the current, actual and weather-dependent pressure of the site for the height of the site above the sea. This flat rate is based on a pressure drop of 1.22 hPa every 10 m.

With this combination of real pressure and standard pressure drop, the QNH pressure is easy to calculate, since the same value can always be deducted from the burst pressure, and yet it is close to reality.

The pressure at sea level calculated as precisely as possible on the basis of the actual current weather (the actual pressure drop) is called QFF.

Although the QNH pressure is a good estimate of the altitude, an altimeter calibrated with it - like any pressure-based altimeter - does not show the real altitude to the meter due to the weather. However, since this deviation is the same for all aircraft, it does not matter. (This is why it is not allowed to fly according to the GPS altitude. Only if the current atmosphere coincidentally coincides with the standard atmosphere, all displayed altitudes are geometrically correct.)

The QNH is location-dependent. Therefore, the altimeter must not only be set to the local QNH at the take-off and landing location, but also to the QNH values ​​of the nearest airfields with air traffic control during the entire flight. Above the transition altitude (or "Transition Altitude"; in Germany generally 5,000 feet MSL or 2,000 feet AGL), the display must be switched to standard pressure (1013.25 hPa); conversely, if the transition area is not reached (or "transition level"; in Germany usually at least FL60), the reference pressure QNH must be set again. The current value of the transition area can be queried via the respective ATIS .

The uniform pressure references ensure that (especially in IFR cruise above the transition altitude) all aircraft have the same altimeter setting, which is essential for the flight level (FL).


QNE is not a Q-code defined by the International Civil Aviation Organization ICAO for use in aviation. However, there are national aviation authorities or weather services (e.g. German Weather Service) that determine this, e.g. B. the Civil Aviation Authority of the United Kingdom (CAA) in the Radiotelephony Manual of 2016. However, these national definitions or definitions that only apply to parts of aviation differ from one another.

In Germany, however, this Q-Code is not mentioned in the aviation manual. The use of the (generally unofficial) code QNE is therefore problematic due to the lack of a global definition and the existence of local, sometimes different specifications.


QFF refers to the air pressure, converted to sea ​​level , taking into account the measuring location and the current atmospheric conditions, in particular the vertical air pressure gradient and the temperature. The QFF value corresponds to the isobar curves in the ground weather map. QFF is of no importance for altitude measurement in airplanes, but it is for meteorology : it is used to compare barometric pressure values ​​around the world.

See also

Tow cone

Individual evidence

  1. REGULATION (EC) No. 859/200 OPS 1.652
  2. Transition Altitude (TA), Transition Level (TL), Transition Layer. (PDF) In: Retrieved June 2, 2019 .
  3. ICAO DOC 8400 "ICAO Abbreviations and Codes"
  4. CAP 413 Radiotelephony Manual Edition 22
  5. Luftfahrtthandbuch Deutschland, GEN 2.2, Abbreviations used in AIS publications