How does a height knife work


The Kollsman Window

In aviation, knowing the exact air pressure is one of the most important bases for determining the altitude of the aircraft. Due to the measurement in the aircraft itself, the displayed altitude depends on the current air pressure at the location where the aircraft is currently located. 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 changeable by the user. The setting of this zero mark and also the basic calibration of the barometer is carried out using a standard atmosphere, which is specified worldwide by a technical regulation. The altimeters therefore offer the option of calibrating the displayed altitude using the current air pressure. The window at the 12 o'clock position of the display in the picture on the right, the so-called "Kollsman window", shows the set air pressure value, which corresponds to the relevant value (QFE or QNH etc.) and must be set with the button at the bottom left . Setting this reference pressure value defines the reference plane from which the height is measured. This is shown in the altimeter setting diagram below. This value is determined by aviation weather stations on the ground and communicated to the pilots in the relevant area via the flight information service (FIS) or the local flight control.

Depending on the intended use, there are differently standardized air pressure specifications, which are identified by so-called Q groups. More on that below.


The scale ring

For cross-country flights, the altimeter is often set to the height of the take-off site before take-off. The air pressure reduced to sea level (QNH value) then appears on the scale of the setting window. During the flight the altimeter shows the altitude above sea level or MSL. In addition to the height above sea level, the relative height above a certain point in the terrain is often of interest. For example, in glider flight, the height above the landing site must be known in order to calculate the target approach. To do this, the arrow on the rotatable outer scale ring is adjusted to the height of the target landing area. The height available up to the landing point can then be read on the height graduation on the ring (reading corresponds to the QFE setting). Other examples are the safe flying over obstacles in the ground (mountain ranges) or control zones, whereby the difference in height can also be read off immediately after setting the ring accordingly.


The height"

The reference heights for the altimeter

Now it's time to clarify the meanings of the term "altitude": an aircraft altimeter only measures air pressure ... and nothing else. If the air pressure changes, e.g. due to a change in temperature or humidity, the altimeter display will of course change accordingly. However, the altimeter simply measures the air pressure. It is thus also clear that the altimeter does not actually "measure" the altitude, but rather it measures the air pressure and, due to its construction, shows the altitude corresponding to this pressure according to the specifications of the standard atmosphere. The term "pressure altitude" actually refers to any altitude indicated by the altimeter.

As can be seen from the diagram at the bottom right for the altimeter setting, under the conditions of the standard atmosphere, a height of 100 m is always 100 m. What is different, however, is the reference plane from which this height is measured. The altitude of the aircraft displayed by the altimeter is therefore always the height above the reference plane set in the altimeter. In addition, the altitude could also be measured from an external point of view or by a system that is independent of the air pressure, e.g. via the GPS.

It is therefore important for pilots to know that an aircraft altimeter only measures air pressure and not altitude. This is particularly important in view of the ever increasing use of GPS devices. In an airplane flying at a fixed pressure altitude indicated by the altimeter, the altitude may be different on the GPS, which measures the true distance to sea level. The difference can be small, but it can also be large enough to cause an airborne collision if a pilot is flying at a GPS altitude rather than the assigned pressure altitude. To solve this problem, some GPS devices now have their own air pressure sensor so that they can also display the pressure altitude.

Based on this, various terms have become common in aviation to denote the heights based on different reference levels:

Reference levels and terms


Altitude AMSL (Above Mean Sea Level) = altitude above sea level


Height of one point above MSL


Height above a reference point, e.g. above the slope = absolute height, or above ground (GND) or MSL, such as a building or an obstacle

Flight level
Flight level (FL)

Height above the standard level 1013.25 hPa in Hectofeet, e.g. FL 50 = flight level 50 = height 5000 ft above the pressure surface 1013.25 hPa (QNE)

In the same way, the heights based on the height measurement are named differently:

  • QNH height
    The QNH height is the height above the pressure surface, which results from the air pressure calculated back to MSL according to the values ​​of the standard atmosphere (theoretical air pressure).
  • displayed height (indicated altitude)
    altitude displayed on the altimeter
  • calibrated height (calibrated altitude)
    the "displayed height" is corrected for the instrument error (Instrument Error IE) and the position error (Position Error PE), i.e.
    calibrated height = displayed height - (IE and PE)
    • IE (Instrument Error):
      Inaccuracy of the instrument (altimeter). The instrument error is supplied by the manufacturer with the instrument using a calibration table. This calibration table is in the technical files of the aircraft and has nothing to do with the deviation table, which relates to the compass. The pilot usually does not know the instrument error.
    • PE (position error, installation error):
      The position error must be taken into account for IFR flights. It arises as a result of an imprecise decrease in the static pressure on the aircraft.
  • true height (true altitude - TA -)
    is the "true" height above sea level. This altitude corresponds to the QNH altitude corrected for the deviation at non-standard temperature. The true altitude is therefore the temperature-corrected QNH altitude.
    Rule of thumb for determining the temperature-related altitude error: A temperature deviation of 1 ° C compared to the standard value corresponds to an altitude error of 0.4%
    “True height” = QNH height ± 0.4% per K temperature deviation from the ISA standard
    Displayed Altitude: 8,000 ft
    Temperature: -11 ° C
    Deviation from the standard: -10 K (standard temperature in 8,000 ft: -1 ° C)
    true height ": 7,680 ft [8,000 - (10 x 0.4% = 4% = 320 ft)]
  • absolute height (absolute altitude)
    is the actual altitude above the overflown area (as it would for example be indicated by an echo sounder).
  • Print height (pressure altitude)
    is basically the height above the print area set in the Kollsman window. Usually, however, this means the height above the standard pressure area of ​​1013.25 hPa. The pressure altitude is required to calculate the density altitude, the true altitude and the true airspeed. The pressure height is calculated by adding or subtracting a correction factor to the respective height. This value is calculated from the difference between the current QNH value and the standard pressure of 1013.25 hPa, multiplied by the barometric altitude level (30 ft).
    Print height = QNH height + (1013 - QNH) x 30
  • Density height (density altitude
    is the temperature corrected pressure altitude. More on this in the "Density Height" chapter.

You can find more information on this in the written statements on height measurement. Further details on the height can be found in the chapter "Density height".


Q codes

The Q codes, which are still partly used in aviation today, were defined in Morse times in order to accelerate radio traffic. For this purpose, a number of standard phrases, which were repeated frequently, were each assigned a code. The Q codes are neither abbreviations nor acronyms, so the letter sequences have no "literal sense", they mean nothing. Rather, the list of phrases used was simply "numbered" with the first letter being a Q to indicate the Q code. The following two letters were given arbitrarily.

If, for example, a pilot sends to the airfield: "Please QNH", this means: "Please give me the air pressure value at the airfield so that I can set my altimeter so that it shows the exact height of the airfield for landing." The answer of the airfield is then e.g. "QNH 1010", which means: "If you set 1010hPa on the setting scale of your altimeter, it will show the exact altitude after landing."

Q codes are rarely used today: For example, in aviation for altitude information (QNH, QNE, QFE) and for navigation support, in France also for indicating the active runway (QFU).

The Q codes listed below relate to information for setting the altimeter and thus defining the reference surface for height measurement.

QFE (absolute air pressure) is the air pressure on the ground measured at the meteorological station or at the airfield. It is reduced to the airport reference point. Since the measuring barometer is seldom at the height of the airport reference point, this value is reduced to it in accordance with the specifications of the ICAO standard atmosphere. So the QFE value is the space pressure. Because of the dependence on the height of the measuring location (airfield), the QFE values ​​of different airfields cannot be compared with one another.

If the QFE value is set on the pressure scale of the barometric altimeter, the height above the runway is displayed on the main scale. If the aircraft is on the runway, the altimeter shows the altitude 0 m or 0 ft. The QFE value is always smaller than the QNH value when the airfield is above MSL. If the runway of an airport is exactly at sea level, QFE and QNH are the same size, if it is below MSL (e.g. in Amsterdam), the QFE value is greater than the QNH value. This is shown in the picture below on the right.

Memorandum: QFE is the height of the runner (QFE = Ku-F-E) on the square (0 m or 0 ft).
(Instead of a landing gear, the early aircraft only had a skid on which to take off and land.)

QFF (relative air pressure) denotes the currently measured air pressure at the measuring location, reduced to sea level, taking into account the actual temperature conditions at the station (not the ideal value of the ISA). This is the air pressure information as it is, for example, in the newspaper or on television for a weather report; is used. When calculating this value, the local air pressure, the temperature, the air humidity or the vapor pressure, an altitude factor and the local altitude above sea level are included in the calculation of the air pressure at sea level.

The QFF value is important in meteorology because the isobars drawn in the ground weather map relate to the QFF. In this respect, reference is made to the "Weather map" chapter. Otherwise the QFF is not used in aviation.

The abbreviation QNH stands for the air pressure at the measuring location which is reduced to sea level according to the temperature values ​​of the ISA standard atmosphere (this is the QFE). In contrast to the QFF, the current measurement is not used as the temperature value for the reduction, but the ISA temperature corresponding to the altitude. If the actual atmosphere deviates from the temperature of the standard atmosphere, these two values ​​are consequently different. The QNH is therefore usually somewhat less precise than the QFF value, but is sufficient for barometric altitude measurement.

To determine the QNH, the meteorologist first determines the current air pressure (QFE). In order to obtain comparable values ​​worldwide, the QNH is always based on a temperature of 15 ° C and the respective altitude above sea level for the reduction, regardless of how warm or cold it actually is. The air humidity is not taken into account. This value can be used to read in a table of the standard atmosphere, which level the measured air pressure corresponds there. The elevation is deducted from this height, i.e. reduced to sea level. For the resulting altitude, he again takes the corresponding pressure from the table of the standard atmosphere - this is now the current QNH of the airfield. This QNH is communicated in the METAR, in the ATIS reports as well as the announcements from FIS or flight control. If the sub-scale of the altimeter is set to this QNH, it shows the QNH altitude at which the aircraft is located. On the ground, the altimeter shows the height of the airfield (or the aircraft's position). Incidentally, you can also (approximately) determine the QNH yourself as long as the aircraft is on the ground: If the airport elevation is set on the altimeter, the QNH can be read on the secondary scale.

If the altimeter is set to the QNH value, the flight altitude (which is falsified by the influence of temperature) is displayed above MSL. On the ground, the altimeter therefore shows the local altitude above mean sea level. The flight altitude is displayed in the air. The most important thing in air traffic is that all aircraft in an area fly with the same altimeter setting, i.e. the same QNH. The calculated QNH value may also be inaccurate meteorologically, but in air traffic it is guaranteed that this does not have a negative effect, since the error is then the same for all air traffic participants.

The QNH value depends on the local, current air pressure (QFE), which means that this is lower the closer you are to a low pressure area. Such air pressure-related deviations in the altimeter can be considerable. For this reason, an aircraft must fly below the so-called Transition height (Transition Altitude; in Germany: 5,000 ft) always set the current QNH value of the airport closest to the flight path. The local airfield QNH is set for landing, provided that it is authorized to issue it. The pilot usually fetches the QNH from the nearest commercial airport via radio. The aircraft then lands at space above sea level (and not at altitude 0).

In flight practice, the altimeter is therefore set to the QNH value for cross-country flights in visual flight below the transition altitude. The transition height is announced in the ATIS messages.

The incorrect readings of the altimeter due to deviating air pressure ratios are shown below.

QNE is the height of a place above the standard pressure level of 1013.25 hPa. The unit of measurement is feet (ft).

This standard pressure area of ​​1013.25 hPa is also used for flying to flight levels. The altimeter is set up to display flight-level altitudes by setting the reference barometric pressure to the standard barometric pressure of 1013.25 hPa (hectopascals). The transition height in Germany is 5,000 ft MSL or 2,000 ft GND - whichever is higher. From this height, the air traffic is staggered vertically in flight areas according to the semicircular rule. Below the transition height, the respective QNH is set as the reference air pressure. With the changeover of the altimeter setting to the value of the standard pressure area of ​​1013.25 hPa, all aircraft affected are exposed to the same meteorological fluctuations in the atmosphere, so that all altimeters then indicate incorrectly by an identical value. This is necessary for air traffic control reasons for vertical staggering of air traffic. Flight levels are not given in ft, but without a unit, e.g. 6000 ft = FL 60.

Because an altimeter can only be set within certain limits (mostly between 950 and 1050 hPa), the QNH can no longer be used when the air pressure is very low, as can occur in the tropics. In this case, the pilot must use the QNE. The QNE of an airfield is the altitude shown by the altimeter, which is set to the standard pressure of 1013.25 hPa, of an aircraft standing on the field; it is thus equal to the pressure height of the square.

The QNE - i.e. the pressure height of the place - can be determined quite easily from the QNH. The following formula applies:

QNE = (1013 hPa - QNH) x 27 ft / hPa + (height of the place above sea level)

27 ft / hPa is the corresponding barometric altitude level, i.e. the difference in altitude between two pressure surfaces at sea level that have a pressure difference of 1 hPa.

QNH 960 hPa, airfield altitude 500 ft, altimeter setting 1013.
QNE = 1013 - 960 = 53 × 27 = 1431 + 500 = 1931 ft (altimeter reading on landing).


Altimeter adjustment

In practice there are therefore 3 values ​​for the altimeter setting:


Altimeter shows the height of the place

Value comes from ATC


Altimeter shows 0

Value comes from ATC


Altimeter shows airport altitude at standard air pressure

Standard 1013.2 hPa

The QNH setting is particularly common in practice.


Incorrect readings from the altimeter

The altimeter is calibrated according to the standard atmosphere, i.e. it shows the altitude that is assigned to this air pressure in the standard atmosphere. The display is only correct if the atmosphere in which the aircraft is flying corresponds to the standard atmosphere. But that is almost never the case. The relationship between altitude and pressure, which the barometric altimeter uses to display, is influenced by high and low pressure areas and by the air temperature. When measuring altitude with a barometric altimeter one must therefore always be aware that the displayed altitude is only accurate if the conditions of the standard atmosphere are currently met. In practice, however, the real atmosphere deviates more or less from this standard. Furthermore, one must always bear in mind that in flight with a constant altitude display, in reality only the pressure is constant. If the pressure surface that is being flown on is inclined, the aircraft gains altitude when the pressure surface increases or it loses height when the pressure surface drops. Due to the calibration of the altimeter based on the conditions of the standard atmosphere, any deviation from these conditions inevitably leads to incorrect displays.

As already mentioned in the chapter "Air density", warm air expands, cold air contracts. In a warm air mass, the 500 hPa pressure surface, i.e. the height at which there is an air pressure of 500 hPa, is at a higher altitude than the normal 5,600 m. As the air mass cools down or when it approaches lower pressure, the 500 ha- Pressure area down, by the way, like any other pressure area, and may even fall below the normal height of 5,600 m.
This is shown in the diagram on the right.

Further information can be found in the "Printing areas" chapter.

With the help of the barometric altimeter in our aircraft, we fly according to pressure areas, namely according to the pressure altitude or pressure area, the value of which is set in the Kollsman window. This is the reason why the sub-scale of the altimeter is set to the current QNH during take-off and landing as well as below a certain reference altitude.

If the air pressure on the ground drops, the altimeter shows too high; if the air pressure on the ground rises, the altimeter shows too low. When flying from high to low, you should therefore be particularly careful, as the altitude displayed on the altimeter is greater than the real one! However, if the setting of the altimeter on the flight path is not adapted to the changed pressure conditions, the aircraft will also lose altitude with the sinking pressure surface. It is therefore necessary to always set the altimeter to the QNH of the nearest air traffic control point.

This is the background for the old aviation wisdom:
"From high to low goes wrong."

If the difference between the standard pressure and the actually measured air pressure is known, the error can easily be estimated with the help of the barometric altitude level.

Since the mean temperature of the air layer is also included in the barometric altitude determination and the values ​​of the ICAO standard atmosphere are also used for this, another error can occur in the altitude determination due to temperature. If the current temperature is lower than that of the standard atmosphere, the aircraft flies lower than the altimeter indicates; if it is higher, the altimeter indicates an altitude that is too low.



If it is warmer, the altimeter shows less. Is it colder, more.
An old Flying rule summarizes the problem in the following rule of thumb:

"You don't get old from warm to cold"
"In winter the mountains are higher."

For deviations from the temperature curve of the standard atmosphere, the false indication is about 2% per 5 ° C temperature deviation.

In addition, the air can be significantly accelerated when flowing over orographic obstacles. In the mountains, when approaching passes and ridges, pronounced zones of lower air pressure can arise due to the Venturi effect on the leeward side. Accordingly, the altimeter shows that the altitude is too low
or like that Flying rule says:
"From high to low goes wrong."