Flight Level

A flight level (abbreviated FL) is a standardized pressure altitude expressed in hundreds of feet above the standard datum plane, used to describe the vertical position of an aircraft operating at or above 18,000 feet in the United States (or the equivalent transition altitude in other countries). Flight levels replace local altimeter settings with a universal reference pressure of 1013.25 hectopascals (29.92 inches of mercury), ensuring that all high-altitude aircraft share a common vertical reference regardless of local weather conditions.

What Is a Flight Level?

Below the transition altitude — 18,000 feet mean sea level (MSL) in the U.S., though it varies between 3,000 and 18,000 feet depending on the country — pilots set their altimeters to the local altimeter setting provided by ATC and reported in ATIS broadcasts. This ensures that everyone in the vicinity of an airport reads altitude relative to sea level, accounting for local atmospheric pressure changes. Above the transition altitude, pilots reset their altimeters to standard pressure (29.92 inHg / 1013.25 hPa) and begin flying by flight levels. At this standard setting, 35,000 feet becomes FL350, 39,000 feet becomes FL390, and 41,000 feet becomes FL410. The "FL" prefix signals that the altitude is a pressure altitude referenced to standard pressure, not actual sea level.

How It Works in Practice

ATC assigns cruising flight levels using a semi-circular rule to maintain separation between eastbound and westbound traffic. In U.S. domestic airspace above FL180, eastbound traffic (magnetic tracks 000° to 179°) is assigned odd flight levels — FL190, FL210, FL230, FL250 — while westbound traffic (180° to 359°) flies even levels — FL200, FL220, FL240, FL260. Above FL290 in Reduced Vertical Separation Minimum (RVSM) airspace, the separation standard narrows from 2,000 feet to 1,000 feet, allowing airlines to fly levels like FL290, FL300, FL310, FL320 in the same airspace band. An Airbus A380 crossing the North Atlantic might be assigned FL390, while a Boeing 737 at the same longitude flies FL370 or FL350 depending on weight and performance. When an aircraft descends through the transition level back toward an airport, it returns to altimeter-based altitude and the flight level label is no longer used.

Why It Matters

Flight levels provide the universal vertical separation language that keeps thousands of aircraft from colliding in shared oceanic and high-altitude domestic airspace. By using standard pressure, a controller at New York Center and a controller at London's Shanwick Oceanic Control Area can both describe the same aircraft as being at FL370 without needing to know whether local surface pressure is high or low at any given moment. The ICAO Annex 2 Rules of the Air mandate flight level usage above the transition altitude, and RVSM certification — which requires altimetry systems accurate to within 200 feet — allowed aviation authorities to double the number of available cruising levels above FL290, dramatically increasing capacity on busy transatlantic and transpacific corridors.

For pilots, operating at the optimal flight level is a fuel efficiency decision. Jet engines are most efficient at high altitudes where air density is low, but every aircraft has a "step climb" profile during which it ascends to progressively higher levels as it burns fuel and becomes lighter. A 777-300ER departing Dubai for Sydney might start at FL330 and step to FL370 and then FL390 over the 14-hour flight as fuel burns off and the aircraft's optimal altitude rises.

Key Facts and Figures

• The U.S. transition altitude is 18,000 feet MSL; above this, all IFR aircraft fly by flight levels.
• European countries use varying transition altitudes, typically between 3,000 and 9,000 feet above field elevation depending on local ATC procedures.
• RVSM airspace (FL290 to FL410 inclusive) doubles vertical capacity by reducing separation minima from 2,000 to 1,000 feet.
• The highest published jet route altitude in U.S. domestic airspace is FL450; above FL600 is typically reserved for military or special operations.
• Boeing 787 and Airbus A350 are certified to FL430, compared to FL390 for many older widebody types.
• China uses meters rather than feet for some domestic operations, requiring conversion when aircraft transition between Chinese and international ATC procedures.

Related Concepts

Altitude, RVSM, Transition Level, ATC, Cruise Altitude