Pilots and air traffic controllers rely on standardized systems to ensure the safe and efficient management of air traffic. One crucial aspect of this system is the concept of flight levels, which play a pivotal role in maintaining the separation of air traffic at different altitudes. Let’s take a look at what flight levels are, the significance of transition altitude and transition level, and why these components are essential for maintaining safe skies.
How do aircraft measure altitude?
Before we understand flight levels, we need to look at how aircraft actually measure our vertical distance in the first place.
The altimeter (seen here on the far right panel of an RAF Air Cadets Viking T1 Glider) is used to measure height or altitude. Image: C. Lomas.
Aircraft measure altitude using their altimeter. Traditionally, the altimeter is a pressure-based instrument (i.e. an instrument that measures the air pressure outside) and is either calibrated in Hectopascals ‘Hpa’ or Inches of Mercury ‘InHg’. That means to measure our height we need a datum – something to measure that height from. In aviation, we have two commonly used datums, feet above mean sea level, and feet above ground level. It is important to highlight that modern commercial aircraft use a variety of pressure based and computer-based measurements to feed the altimeter. We set our altimeter to measure in one of three ways, we call them ‘QNH’, ‘QFE’ and ‘QNE’.
Altimeter settingWhat it measures altitude againstDefinition
QNHPressure Altitude (ASL)Atmospheric pressure at sea level. Set on an altimeter to determine your altitude above sea level. This is referred to as ‘Altitude’.
On the ground at an airport that is 200 feet above sea level, the altimeter would read 200 feet.
QFEHeight above ground level (AGL)Atmospheric pressure at a specific location or airfield. Set on altimeter to indicate actual height above the airfield. This is referred to as ‘Height’.
On the ground at an airport that is 200 feet above sea level, the altimeter would read 0 feet.
QNEAltitude above Mean Sea Level (MSL)The level the aircraft is at compared to a pressure setting of 1013.2 Hpa or 29.92 InHg. This is referred to as Flight Level.
For simplicity, we may refer to all vertical distances as ‘altitude’ for the rest of this explainer.
General aviation aircraft often fly with reference to their height above the ground, or QFE. This is especially true if operating the circuit or traffic pattern of an airfield. In commercial aviation, vertical distance is mostly measured in feet above mean sea level, or ‘QNH. (General aviation aircraft can also use QNH).
However, as aircraft ascend to higher altitudes, using absolute altitude measures becomes impractical due to variations in atmospheric pressure. Let’s say that the altimeters of two aircraft are both showing 10,000 feet. One is flying in an area where the pressure is 1000 Hpa, the other in an area where the pressure is 1004 Hpa. Both pilots will see 10,000 feet on the gauges, but in reality the second aircraft will be 112 feet above the first. Essentially, if we flew around the world using a local QNH setting, the real height of 10,000 feet would be different for everyone, making air traffic control’s job a nightmare! To address this, the concept of flight levels (or QNE) was introduced.
What are flight levels?
A flight level (FL) is a standard pressure altitude, expressed in hundreds. Aircraft flying on flight levels all measure their altitude from the same pressure setting, 1013.2 Hpa (or 29.92 InHg). This is based on the principle of something called the International Standard Atmosphere. Pilots and air traffic controllers worldwide adhere to this standard to ensure consistency and uniformity in altitude reporting at higher altitudes.
For example, if an aircraft is flying at 10,000 feet on the standard pressure setting of 1013 Hpa, the aircraft is flying at “Flight level One Hundred” or ‘FL100’. This standardization simplifies communication and enhances situational awareness, allowing for effective air traffic management. Without flight levels, aircraft would not be able to fly under Reduced Vertical Separation Minima.
When do pilots use flight levels?
In short, pilots switch to measuring altitude using flight levels on the way up, and back to the local atmospheric pressure setting on the way down. The altitude at which this occurs is chosen by the authorities.
Transition altitude
The transition altitude is the point during ascent when an aircraft switches from using the local altimeter setting to the standard pressure setting. This transition altitude varies from region to region. In the UK and parts of Europe it is often between 6000-7000 feet. In the United States it can be anywhere up to 18,000 feet; it all depends on the complexity of the airspace. Above this altitude, pilots set their altimeters to the standard setting, ensuring accurate and consistent altitude reporting.
Transition level
As an aircraft descends, it reaches a specific altitude known as the transition level, where pilots switch from the standard pressure setting back to the local altimeter setting. This transition level is the counterpart to the transition altitude and is typically specified on the standard arrival ‘STAR’ or instrument approach chart pilots use when landing at airports, or given by ATC.
This approach chart for Ataturk Airport TEQ) specifies that the transition level is given “By ATC” in the top right-hand box. Image: Jeppessen.
The flight level system forms an integral part of the aviation framework, contributing to the safety and efficiency of all air travel. These standardized measures ensure that pilots and air traffic controllers can maintain safe separation and get you to your destination.
Cover photo: Goncalo Guimaraes, JetPhotos.
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