Atmospheric Pressure Codes (QNH - QFE - QNE)

QNH, QFE, and QNE are three fundamental altimeter settings based on reference pressure, used to measure aircraft altitude via barometric altimeters. Modern aviation is built upon integrated systems capable of precisely managing not only horizontal movement but also vertical positioning. The most common method for determining an aircraft’s height above ground or sea level is through barometric altimeters. For these altimeters to function accurately, the reference pressure values used must be determined without error. In international aviation practices, these reference pressures are classified into three distinct categories: QNH, QFE, and QNE. The Q-coding system is a standardized three-letter code system beginning with the letter Q. Each has a distinct purpose and operational context.

Principle of Operation of Barometric Altimeters

Barometric altimeters are mechanical or electronic devices that convert changes in atmospheric pressure into changes in altitude. Pressure decreases with increasing altitude; therefore, a height value is calculated based on the measured pressure. However, this height can be interpreted relative to different reference points depending on the pressure setting on the altimeter. To ensure flight safety, international regulations define the conditions under which each reference should be used.

QNH (Qualified Natural Horizon)

QNH (Qualified Natural Horizon) is an altimeter setting representing the local atmospheric pressure reduced to mean sea level. Translated into Turkish as “Measured Natural Horizon,” this concept denotes the sea-level-adjusted pressure at an airport. With the QNH value, pilots can accurately track altitudes (elevation) defined relative to their geographic location during takeoff and approach phases. QNH is used below the transition altitude, as defined by the International Civil Aviation Organization (ICAO) standard procedure.

When the aircraft touches down, the altimeter reading the exact elevation of the runway indicates that the QNH value has been correctly set. This pressure value can be obtained through ATIS (Automatic Terminal Information Service) broadcasts, air traffic control units, or meteorological reports such as METAR (Meteorological Aerodrome Report). However, an incorrect QNH setting can create serious safety risks, particularly during low-altitude operations. For instance, a deviation of just 10 hPa results in approximately a 300-foot vertical error. Such discrepancies can lead to collision risks during critical approach and landing phases, making the accurate and up-to-date use of QNH information critically important for operational safety.

QFE (Qualified Field Elevation)

QFE (Qualified Field Elevation) is the actual surface pressure at the elevation of the runway. Translated into Turkish as “Measured Height Above Ground,” this setting causes the altimeter to read zero when the aircraft is on the ground. The primary advantage of QFE is that it directly indicates the aircraft’s height above the runway surface. This feature makes QFE particularly preferred in missions requiring precise vertical positioning, such as military maneuvers, aerobatic flights, and helicopter operations. However, its use in civil aviation is uncommon because focusing on height above ground rather than altitude above sea level can create coordination challenges in navigation.

Since QFE references height above ground rather than altitude above sea level, it complicates the use of standard altitude values in communications with air traffic control (ATC) and may lead to navigational inconsistencies. In this context, the International Civil Aviation Organization (ICAO) recommends the use of QFE only under specific operational conditions.

QNE (Qualified Natural Earth)

QNE (Qualified Natural Earth) represents an altimeter setting based on the standardized pressure value of 1013.2 hPa (29.92 inHg) defined by the International Standard Atmosphere (ISA). Translated into Turkish as “Measured Natural Earth,” this fixed pressure value enables aircraft to determine their standardized vertical positions expressed as Flight Levels (FL).

With the QNE setting, all aircraft generate altitude information relative to a common reference pressure. This ensures the safe and consistent execution of vertical separation procedures, especially during high-altitude flight operations.

The QNE setting is typically used by aircraft flying above the transition altitude and serves as the standard pressure reference during the en-route phase. When aircraft enter regions with different transition altitudes, accurately knowing these altitudes and precisely timing the switch to QNE is critical to preventing mid-air collisions.