The Barometric altitude measurement in aviation is based on the height estimation by measuring the air pressure. For determining the absolute level of the method is inaccurate. However, it is sufficient to demand that the altitude relative to other aircraft to determine and define the flight surfaces.
To measure the air pressure in the aviation various parameters and methods are used, which partly differ from those of Meteorology.
Principle of measurement
Altimeters are barometers that indicate the air pressure under normal conditions appropriate height.
To local air pressure variations that occur constantly in the atmosphere by high - and low-pressure areas, correct, the zero mark of the barometric altimeter by the user must be able to be changed. Setting this zero mark and also the Basiseichung the barometer establish the standard atmosphere, which is defined by a technical regulation worldwide.
The altimeter needs a pick-up point for the static pressure, the static port. This may be due to specific points of the hull or be integrated into a pitot tube. For airplanes without pressurized cabin a reserve pick-up point can be in the cabin.
Analog display devices
Analog display devices ( called in the jargon of " clocks" ) show the pressure and thus the estimated altitude change to mechanically directly via pointers and a normalized scale. Depending on the unit of measurement used results in a rotation of the " minutes " pointer 1000 m or 1000 feet, with a scale of 20 meters or 20 feet per tick mark. The display of the run through 1000 steps takes a second " h " pointer or a number plate, comparable to the date of a wristwatch. At very high altitudes in addition the 10,000 -foot steps by means of an attached on the outer edge of the scale pointer are displayed.
In the picture, an altimeter is shown with three pointers. The window at the 3- position of the clock display shows the current air pressure value at which QFE or QNH etc. (see below) and must be set equal to the bottom left of the button. It is called " Kollsman Window".
The settling and rate of climb of the aircraft, that is the change in altitude per unit of time, to show so-called variometer.
Digital Display Units
Digital display device showing the pressure and so that the estimated amount of change on an electronic display. In the example shown right metric units are used ( altimeter = height in meters and variometer = climb / descent in meters per second).
In the picture is a combined altimeter with vertical speed, as it is used by ultralight and glider pilots represented. Left is the QNH altimeter in setting mode, the value of 1011 ( hPa) was set to see. On the right is, in the Altimeter mode, resulting from the QNH value height of 122 m.
The accuracy of a barometric altimeter is a few decameter (dam ) and decreases with higher altitude. To estimate the absolute altitude ( true altitude, Eng. True altitude ), the local air pressure must be accounted for on a regular basis by adjusting the zero mark due to changes of meteorological (weather). In addition to the amount of pressure on the temperature and humidity distribution depends.
Correction values are determined by meteorological stations on the ground and communicated to the pilots in the corresponding region. Depending on the application, there are different standardized air pressure data, which are characterized by Q- groups.
Watch this sentence: "In the winter, the mountains are higher," because at colder temperature, the indicated altitude is greater than the true height. In pilot circles it also says: "From high to low most of you go wrong." Reason: If the set at the departure airport air pressure during test flight in a low pressure area is retained, the altimeter shows there through the lower air pressure a height greater than that actually flown on. Especially when flying under instrument Meteorological Conditions (IMC ) then there is a danger of a Controlled flight into terrain ( CFIT ).
→ Main article: Q- key
Q- key have been arbitrarily defined in the Morse time to speed up the traffic.
Funct such as a pilot to the airfield " Solicit QNH ", it means: "Please tell me how I need to adjust my altimeter so that it just displays the space height after landing. " The answer of the airfield is then eg. " QNH 1010", which means " when you start with 1010 hPa on the adjustment of your altimeter, it will display exactly after landing the space height. "
In aviation in Europe, the Q keys are for the altitude (QNH, QNE, QFE) in use.
QFE is the measured pressure at ground level ( engl. Atmospheric pressure at airfield elevation, hint: engl FieldElevation. ). The unit is hPa hPa (in the U.S. in. Hg ).
If the QFE set on the altimeter, as he shows in an aircraft on the ground at a height of 0 m or 0 ft.
When aerobatic flying, the altimeter is generally set to QFE to get any time the amount displayed directly above the floor. Outside of the art flight, the altimeter setting on QFE is common only in a few areas.
QNH is the virtual pressure at sea - level, assuming the standard atmosphere at measuring the pressure present at the level of the station. ( Hint: engl Nautical Height. ). The unit is hPa hPa (in the U.S. in. Hg ).
If the actual current meteorological pressure distribution is used instead of the standard atmosphere is obtained QFF.
If altimeter set to QNH value, the real altitude is displayed above sea level on the ground. Other heights are not displayed correctly in general. Since this deviation but for all aircraft is the same, it does not matter. (This is why can not be flown to the GPS altitude. Only if the actual atmosphere coincidentally coincides with the standard atmosphere, all heights are geometrically displayed correctly.)
The QNH of a place changes with the weather. Therefore, the altimeter is set only for takeoff and landing on QNH, above a certain altitude ( the transition altitude ) is changed the display to standard pressure. This ensures that in cruise flight, all aircraft have the same altimeter setting, which for Graduation ( FL) is essential.
QNE is the height above the standard pressure of 1013.25 hPa = 760 Torr ( mm Hg ) = 29.92 inHg. The unit is typically feet (ft).
The term QNE is now used rarely in aviation. All altitudes above the transition altitude ( transition altitude, TA) be measured with this standard pressure setting. The QNE - height corresponds to the pressure altitude (pressure altitude ). Likewise (FL flight level, ) each flight level is a QNE - height.
The QNE an airfield is the height, which shows the set to the standard pressure of 1013.25 hPa altimeter of a standing in the square aircraft; This amount differs from the "real" height of the square from, it is the current pressure altitude of the place and changes with the weather.
The QNE - so the pressure altitude of the place - you can quite easily determine from the QFE. The following formula applies:
27 ft / hPa is an approximate value for the barometric altitude level, the height difference between two pressure surfaces at sea level, which have a pressure difference of 1 hPa.
QNE is often mistakenly misunderstood to refer to the altimeter setting to standard pressure.
QFF means the air pressure, converted to sea level taking into account the measurement location and the current atmospheric conditions. This pressure corresponds to the Isobarics in the surface weather map. For the height measurement in aircraft QFF has no meaning. The QFF is used for global comparison of air pressures.