FLIGHT LEVELS:-Flight level is a level of constant atmospheric pressure related to a reference datum of 29.92 inches of mercury. Each is stated in three digits that represent hundreds of feet—flight level 250 represents a barometric altimeter indication of 25,000′, flight level 255 as 25,500′.

A flight level (FL) is specific barometric pressure, expressed as a nominal altitude in hundreds of feet. The pressure is computed assuming an International standard sea-level pressure datum of 1013.25 hPa (29.92 inHg), and therefore is not necessarily the same as the aircraft’s true altitude either above mean sea level or above ground level.

Historically, altitude has been measured using a pressure altimeter, which is essentially a calibrated barometer. An altimeter measures air pressure, which decreases with increasing altitude following the barometric formula, and from the surrounding’s pressure calculates and displays the corresponding altitude.

To display altitude above sea level, a pilot must recalibrate the altimeter according to the local air pressure at sea level, to take into account natural variation of pressure over time and in different regions. If this is not done, two aircraft could be flying at the same altitude even though their altimeters appear to show that they are at considerably different altitudes.

Flight levels solve this problem by defining altitudes based on a standard air pressure at sea-level. All aircraft operating on flight levels calibrate to this setting regardless of the actual sea level pressure.

Flight levels are described by a number, which is this nominal altitude (“pressure altitude”) in feet, divided by 100, while being a multiple of 500 ft, therefore always ending on 0 or 5. Therefore an apparent altitude of, for example, 32,000 feet is referred to as “flight level 320”. To avoid collisions between two aircraft due to their being at the same altitude, their “real” altitudes (compared to ground level, for example) are not important; it is the difference in altitudes that determines whether they might collide. This difference can be determined from the air pressure at each craft and does not require knowledge of the local air pressure on the ground.

Flight levels are usually designated in writing as FLxxx, where xxx is a one- to three-digit number indicating the pressure altitude in units of 100 feet. In radio communications, FL290 would be pronounced as “flight level two niner zero”, in most jurisdictions. In some states, levels ending with 00 are read as “hundred”: FL200 is pronounced as “flight level two hundred”. The phrase “flight level” makes it clear that this refers to the standardized pressure altitude.

Flight levels are also used to separate Virtual Flight Rules (VFR) from Instrumental Flight Rules (IFR) traffic and the traffic flying in the opposite directions. It is based on the Magnetic track the aircraft(a/c) is following.

VFR traffic will always maintain “half levels” like FL 025, FL 105, FL 155, to a FL 195, the maximum FL for VFR flights. All flights above FL 200 must be under IFR rules.

IFR traffic will always maintain “full levels”, like FL 090, FL 140, FL 350.

Eastbound traffic (between Magnetic tracks 000° and 179°) will maintain ODD flight levels and westbound traffic (between Magnetic tracks 180° and 359°) will maintain EVEN flight levels. This ensures that the opposite traffic in IMC is separated 1000 feet with altimeters set to Standard.

Since altimeters become less reliable as the altitude increases, above FL 290 only ODD FL are used in the following manner:

Separation between opposite traffic above FL 290 is increased to 2000 feet. Obviously, FL 300 or FL 360 do not exist and cannot be used.

So, VFR flight Eastbound can use levels such as: FL 095, FL 135, FL 175.

IFR flight Westbound can use levels such as FL 120, FL 240, FL 300, FL 320

IFR flight Eastbound can use levels such as FL 150, FL 210, FL 290, FL 310

In most developed countries and some parts of the world like Tanzania the separation above FL 290 is reduced to 1000 feet, and the same ODD-EVEN rule apply. Such airspace is called RVSM (Reduced Vertical Separation Minima aerospace). A/c as well as the pilots must be certified by appropriate authorities to use this aerospace.


What is difference between height and altitude in flight levels?

Height: Vertical distance from the point of observation on the Earth’s surface to the point being measured. Altitude: Vertical distance from mean sea level to the point being measured.


What is the difference between flight level and altitude?

Strictly speaking a flight level is an indication of pressure, not of altitude. Only above the transition level (which depends on the local QNH(pressure at the airport level is converted to sea level,) but is typically 4000 feet above sea level) are flight levels used to indicate altitude; below the transition level feet are