Flight levels in aviation

🕗 4 minutes | August 6, 2024 | Text Mateusz Łapuć 

As you may already know, aviation relies on a vast number of terms, definitions, and regulations designed to standardize procedures, streamline traffic, and enhance safety. One of the most fundamental concepts used daily in aviation is flight levels (FL). But what exactly are they? Stay with us, and we’ll explain everything in this article!

A flight level (FL) is a standardized altitude reference used in aviation, measured in hundreds of pressure feet based on atmospheric pressure at a given height.

Simply put, flight levels are barometric pressure surfaces that help maintain vertical separation between aircraft during flight.

For example:

• FL300 = 30,000 feet (9.14 km)
• FL150 = 15,000 feet (4.57 km)
• FL100 = 10,000 feet (3.05 km)

Altitude measurement in aviation is based on atmospheric pressure. Aircraft are equipped with pressure altimeters, which measure air pressure at a given altitude and convert it into a height reading.

Flight levels are referenced to QNE, which is the standard atmospheric pressure at sea level (1013.25 hPa or 760 mmHg).

Flight levels are assigned according to specific rules and procedures that ensure safe vertical separation between aircraft. A widely accepted principle is that:

• Aircraft flying westbound are assigned even-numbered flight levels (e.g., FL380)
• Aircraft flying eastbound are assigned odd-numbered flight levels (e.g., FL370)

This reduces the risk of collision between aircraft flying in opposite directions.

Did you know that aviation has a system that allows aircraft to fly closer together vertically? This system is called RVSM (Reduced Vertical Separation Minima), and it reduces the required vertical separation between aircraft from 2,000 feet (0.61 km) to just 1,000 feet (0.3 km) at cruising altitudes.

Why is RVSM important?

• It increases airspace capacity
• It allows more efficient flight operations
• It ensures safety while maximizing traffic flow

Two key terms associated with flight levels are transition altitude (TA) and transition level (TL).

Transition altitude (TA)

The transition altitude is the altitude above sea level where aircraft switch from using a local pressure setting (QNH) to a standard pressure setting (QNE) for measuring altitude.

• Below the transition altitude, altimeters are set to the local QNH
• Above the transition altitude, altimeters use the standard pressure setting of 1013.25 hPa

Transition level (TL)

The transition level is the lowest available flight level above the transition altitude, where aircraft begin operating in flight levels instead of altitude measured above sea level.

Flight levels are an integral part of aviation operations, especially as the number of aircraft in the skies continues to rise.

However, rapidly changing weather conditions, such as storms, turbulence, and strong winds, can necessitate quick changes in assigned flight levels. This poses additional challenges for both pilots and air traffic controllers.

The flight level system plays a crucial role in maintaining safe and efficient air traffic management. Air Traffic Control (ATC) uses flight levels to:

• Maintain safe separation between aircraft
• Prevent mid-air collisions
• Ensure smooth traffic flow in increasingly busy airspace