Flight level

To see why flight levels are used, a discussion of the means of measuring altitude is necessary.

Historically, altitude has most easily been measured using an altimeter, which is essentially a calibrated barometer - it measures air pressure, which decreases with increasing altitude. To display altitude above sea level the pilot must recalibrate the altimeter according to the local air pressure from time to time, to take into account natural variation of pressure over time and in different regions. If this isn't done, different aircraft may be flying at different heights even though their altimeters show the same altitude. More critically, different aircraft may be flying at the same height even though their altimeters show different heights. Clearly this is a safety issue.

Flight levels solve this problem by defining altitudes based on a standard pressure of 1013.2 mb (29.92 inHg used in U.S. and Canada). All aircraft operating on flight levels calibrate to this same standard setting regardless of the actual sea level pressure. Flight levels are then assigned a number which is the apparent altitude ("pressure altitude") to the nearest thousand feet, divided by one hundred. Therefore an apparent altitude of 12,000 feet is referred to as Flight Level 120 (except in the United States and Canada -- see note below). Note that aircraft may be at some other actual height than 12,000 feet, but since they all agree on a standard pressure, no collision risk arises.

Flight levels are not used close to the ground, for perhaps obvious reasons - obstacles are fixed to the ground and so their absolute height needs to be known. A vertical region extending from a defined transition altitude to the lowest available flight level is known as the transition layer - pilots will use altitude based on the local pressure below this level, and flight levels above. The altitude of the lowest flight level varies from country. Most begin at 24,500 feet or FL 245. Other countries lowest flight levels begin at:

• FL 145 United Arab Emirates
• FL 150 Bahrain, Kuwait, Oman, Qatar, Saudi Arabia, Yemen
• FL 180 Canada, Puerto Rico, United States, US Virgin Islands
• FL 195 Belgium, Central America, Curaçao, Dominican Republic, Estonia, France, Italy, Lithuania, Malta, México, Netherlands, Panama, Switzerland
• FL 200 Iran, Israel, Jordan, Lebanon, Saudi Arabia (Jeddah FIR only)
• FL 220 Iraq
• FL 255 Egypt
• FL 265 Russia
• FL 285 Bosnia-Herzegovina, Croatia, Finland, Latvia, Poland, Sweden, Turkey, Yugoslavia

Quadrantal rule

(This does NOT apply in the United States or Canada)
Flight levels are 500 feet apart, but to further ensure the separation of aircraft, aircraft travelling in different directions in level flight (i.e. not climbing or descending) below FL 245 (24,500 feet) are required to adopt flight levels according to the quadrantal rule, as follows:

• Track 000 - 089° - odd thousands of feet (FL 70, 90, 110 etc)
• Track 090 - 179° - odd thousands + 500 (FL 75, 95, 115 etc)
• Track 180 - 269° - even thousands of feet (FL 80, 100, 120 etc)
• Track 270 - 359° - even thousands + 500 (FL 85, 105, 125 etc)
  

  Semicircular rule

• Track 000 - 179° - odd thousands (FL 250, 270, etc.)
• Track 180 - 359° - even thousands (FL 260, 280, etc.)
  At FL 290 and above, 4000 ft. intervals are used to separate same-direction aircraft (instead of 2000 ft. intervals below FL 290), and only odd Flight Levels are assigned, depending on the direction of flight:
• Track 000 - 179° - odd flight levels (FL 290, 330, 370, etc.)
• Track 180 - 359° - odd flight levels (FL 310, 350, 390, etc.)

In the U.S. and Canada, note that the foregoing information applies to flights under instrument flight rules (IFR). Different altitudes will apply for aircraft flying under visual flight rules (VFR) above 3000 AGL.

Reduced Vertical Separation Minimums

Reduced Vertical Separation Minimum or RVSM reduces the vertical separation above FL 290 from 2000 ft. to 1000 ft. This allows aircraft to safely fly more optimum routes, gain fuel savings and increase airspace capacity by adding six new flight levels. Only aircraft that have been certified to meet RVSM standards, with several exclusions, are allowed to fly in RVSM airspace. RVSM went into effect in Europe between FL 290 and FL 410 on January 24, 2002. The United States ,Canada and Mexico transitioned to RVSM between FL 290 and FL 410 on January 20, 2005. (Southwest Airlines retired their last 737-200 on January 17, 2004. America West operated their last 737-200 revenue flight on January 19, 2004. Both airlines were eliminating this type from their fleet and did not schedule them for RVSM updgrades.)

• Track 000 - 179° - odd thousands (FL 290, 310, 330, etc.)
• Track 180 - 359° - even thousands (FL 300, 320, 340, etc.)
  At FL 410 and above, 4000 ft. intervals are resumed to separate same-direction aircraft and only odd Flight Levels are assigned, depending on the direction of flight:
• Track 000 - 179° - odd flight levels (FL 410, 450, 490, etc.)
• Track 180 - 359° - odd flight levels (FL 430, 470, 510, etc.)
  

  Metric Flight Levels

  
  China, Mongolia, Russia and many CIS countries use flight levels specified in meters. Aircraft entering these areas normally make a slight climb or descent to adjust for this.

• Track 000-179°
• FL 0300 meter (1,000 ft.)
• FL 0900 meter (3,000 ft.)
• FL 1500 meter (4,900 ft.)
• FL 2100 meter (6,900 ft.)
• FL 2700 meter (8,900 ft.)
• FL 3300 meter (10,800 ft.)
• FL 3900 meter (12,800 ft.)
• FL 4500 meter (14,800 ft.)
• FL 5100 meter (16,700 ft.)
• FL 5700 meter (18,700 ft.)
• FL 6300 meter (20,700 ft.)
• FL 6900 meter (22,600 ft.)
• FL 7500 meter (24,600 ft.)
• FL 8100 meter (26,600 ft.)