Flight level

A flight level ( English flight level , FL ) referred to in the aviation an area equal air pressure in the atmosphere. An aircraft following a flight level shows - since the flight altitude is measured via the ambient pressure - a constant altimeter display. Flight levels are the most important method of displaying flight altitude in commercial aviation. By assigning flight areas, air traffic control stacks airplanes vertically, i. H. Safety distances ensured.

definition

Schematic representation of two flight areas. The actual altitude depends on the meteorological conditions. (Not to scale.)

The flight level indicates the altitude in 100 feet (ft) under standard conditions (air pressure at sea level 1013.25 hPa; air temperature at sea level 15 ° C). All aircraft that express their flight altitude as a flight level have therefore set the reference air pressure of their altimeter to the normal value 1013.25 hPa; its display value in feet divided by 100 gives the flight area. Example: An aircraft is at flight level 85 when its altimeter, set to normal pressure, shows an altitude of 8,500 feet.

That the real current situation of the local Earth's atmosphere significantly from the average and idealized ICAO - international standard atmosphere can differ, it therefore is of no consequence because the deviation for all participating aircraft is the same, so that the correct vertical separation from each other is ensured.

The actual altitude at which a flight level is located is not a fixed altitude, but primarily depends on the current air pressure : one and the same flight level in a high pressure area is higher above sea level than in a low pressure area . However, this does not interfere in the airspace. When cruising, it does not depend on the actual altitude, but only on the height graduation of the aircraft among each other.

Examples

• If the air pressure on the ground is 1013.25 hPa, flight level 100 is 10,000 feet (3048 m) above the ground (assuming standard conditions).
• If the air pressure on the ground is only 995 hPa (for example in a low pressure area), the (imaginary) normal pressure area is 500 feet below the ground. Flight level 100 is 10,000 feet above the ground under standard conditions, in this case 9500 feet (~ 2900 m) above the ground.

Since the extent of the altitude-related decrease in air pressure also depends slightly on the air temperature, the actual distance between flight level 100 and normal pressure area can deviate by a few percent from the standard value of 10,000 ft. However, this error affects all altimeters in the same way and is therefore negligible.

Transition from actual altitude to flight levels

In the airspace close to the ground, the actual altitude is flown. To do this, the altimeter of each aircraft must be set to the current meteorological air pressure of the imaginary sea level. This information, known for short as QNH , is obtained from the nearest controlled airfield. The altimeter is always set to QNH before take-off; it may be necessary to readjust it for cross-country flights (entry into different weather and thus different pressure conditions).

Climb

The conversion of the altimeter of the actual height (about QNH) on plane surfaces (above atmospheric pressure) is carried out during the climb upon reaching the transition altitude (engl. Altitude transition , TA ). The transition height in Germany is 5000 feet MSL , but at least 2000 feet AGL . From here is air traffic vertically the semicircular flight control (engl. Semicircular rule ) staggered in flight levels.

descent

In descent, the reference air pressure must be changed again from 1013.25 hPa to QNH when the transition level is reached ( TRL ; German unused transition area , see also transition height). The TRL is determined in Germany based on the current QNH:

• QNH greater than 1013 hPA → TRL 60
• QNH from 978 to 1013 hPA → TRL 70
• QNH less than or equal to 977 → TRL 80

Designation of the flight areas

IFR flight areas

Flight levels are specified as the height in feet above the standard isobar of 1013.25 hPa. For the sake of simplicity, the information is given in hectofoot. The flight level FL 60 (as it would be indicated on the radio ) corresponds to an altitude of 6000 feet above the standard isobar.

• In Germany, instrument pilots (IFR) always use the flight areas that can be divided by ten (FL (40), (50), (60), 70 to 350). Depending on the real pressure, there are no FL 40, 50, 60 and 70 (e.g. from 977 hPa).
• Visual pilots (VFR) use the flight levels ending on 5 (FL (45), (55), 65, 75, 85 and 95), whereby with magnetic courses above ground from 000 ° - 179 ° odd (FL 55, 75, 95) and the straight flight areas (FL 45, 65, 85) are used for magnetic courses over the ground of 180 ° –359 °. See ICAO Annex 3 Appendix 2 and LuftVO §31 Paragraph 2.

Other countries have different transition heights and thus possibly less available flight areas. In the USA and Canada e.g. B. the transition altitude is determined by the beginning of airspace class Alpha at 18,000 feet, while the semicircle rule applies regardless of this from 3000 feet above ground. The semicircular flight altitudes, depending on the flight direction, are also not the same internationally. For example, Great Britain uses B. a quarter circle system (engl. Quadrantal rule ), while in Israel north and south directions are differentiated (instead of east / west directions). A new global standard is currently being developed.

Specialty

Depending on the meteorological conditions (temperature fluctuations), as can be seen from the barometric altitude formula , both the geometric flight altitude and the geometric distance between the flight levels can vary. However, this is not displayed in the altimeter. However, these are small differences that are insignificant for flight practice. Regardless of the geometric flight altitude, FL 60 always corresponds to an air pressure of 812.8 hPa, which is displayed on the altimeter as 6000 feet flight altitude when a reference air pressure of 1013.25 hPa is set.

Further regulations

The flight level on which a pilot wants to fly is indicated in the flight plan .

Air pressure and altitude

Standard atmosphere
 Height in m 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 6000 7000 8000 9000 10,000 11000 Pressure in hPa 1013.25 954.61 898.76 845.58 794.98 746.86 701.12 657.68 616.45 577.33 540.25 471.87 410.66 356.06 307.48 264.42 226.37 approx FL 0 16 33 49 66 82 98 115 131 148 164 197 230 262 295 328 361