Theorie Basiskurs

General Information About the Barometric Altimeter

The Barometric Altimeter is based on setting an air pressure. Since the air pressure is not the same at every location, depending on the weather conditions (high and low pressure areas), it must be set for the current location. The value set should corresponds to altitude zero.
The barometric altimeter thus determines the current altitude via the measured air pressure (which decreases with increasing altitude).

In aviation, the two most important values for setting the altimeter are QNH and QFE. These so-called Q-groups originate from the time of the Morse codes.

• QNH: The air pressure relative to the sea level at the current location. I.e. the pilot sets the QNH, he shows the altitude above sea level at the current location. As a mnemonic, you can remember "Nautical Height."
• QFE: The air pressure at the current location. If the QFE pressure is set, then zero altitude is indicated for the current location. "Field Elevation" helps to remember.
• QNE: Not used by the International Civil Aviation Organization (ICAO) and not mentioned in the German Aeronautical Information Publication. In some countries it is defined differently. It corresponds analogously to the ISA (see below). Since the ISA is defined globally, it should be used preferentially. You can use the mnemonic "Not Equal", because the value (unlike the ISA) is defined differently in different countries.

As a rule, the QNH is always used in civil aviation.

Units of measurement

Europe is Hectopascal (hPa), USA it is Inches of Mercury (inHg, mercury), Russia it is millimetres of Mercury (mm, mercury).
1 inHg = 33.86 hPa

Standard Pressure

The ISA (International Standard Atmosphere) is an atmospheric model defined by the ICAO (International Civil Aviation Organization) with fixed values:

Pressure at sea level:	1013.25 hPa or 29.92 inHg or 760 mmHg
Temperature at sea level:	15°C
Air density at sea level:	1.226 kg/m³
Relative Humidity:	0%
Temperature decrease:	2°C per 1,000 feet

The ISA represents a mean value that all aircrew must use above a certain altitude. This uniform setting greatly simplifies the coordination between controllers and pilots and minimizes the risk of collision.
If pilots were to fly permanently with their QNH assigned by the airport, it would be possible for the altimeters of two aircraft to show different values even though they are at the same altitude.

Transition Altitude, TA (transition altitude)

The Transition Altitude (when climbing) is the point to set ISA 29.92 inHg (or 1013 hPa / 760 mmHg) but is defined differently in each country. Here are some examples:

Germany	5,000 feet MSL
Austrian range	10,000 feet MSL
USA	18,000 feet MSL
England	3,000 feet MSL

In Germany, the standard pressure must be set to 29.92 inHg or 1013 hPa from 5,000 ft MSL upwards.
If 2,000 ft AGL is higher than 5,000 ft AMSL, 2,000 ft AGL is used as the TA.
Below TA height is refered to as "Altitude" and above TA it as a "Flight Level" or in the military range of Angels.

The flight level is always indicated in 100 ft.
Example: FL 300 corresponds to 30,000 feet. FL 255 corresponds to 25,500 feet. FL 253 is invalid, because it is out is the 500 steps.
Whereas Angels will indicate in 1,000 feet.
Example: Angels 30 corresponds to 30,000 feet

Transition Level, TL

The opposite of the transition height is the transition level. This applies to aircraft in descent. The TL is always at least 1,000 feet higher than the TA. The space in between is called the Transition Layer.

The TL is variable and depends on the prevailing air pressure, but as already mentioned, at least 1,000 feet higher than the TA.
Normally the TL is announced by the ATIS (Automatic Terminal Information Service).

Why must the TL now be higher than the TA?

Assume an aircraft sinks from high altitude at standard pressure from 29.92 inHg to 5,000 feet and now changes to the actual air pressure of QNH 28.32 inHg. The altimeter now shows 3,500 feet instead of 5,000 feet. This means that with ascending and descending aircraft, the TA overlaps. To prevent this, the TRL is always higher than TA and the lower the actual QNH at the airfield, the higher the TRL. In our example, the TRL would have to be 7,000 feet so that you are still over 5,000 feet when switching to the Airfield QNH.

Transition Layer

The transition layer between TA and TRL is called the transition layer. Logically, this layer is always at least 1,000 feet in size.

More information:

• MSL AGL QFE QNH | Altitudes For Aviators | Aviation Explained auf YouTube