Altitudes

Updated at: 2026-01-31 10:58
Altitudes in aviation are not just “how high you are” — they are a standardized way to describe vertical position using specific reference pressures (QNH, QFE), reference surfaces (mean sea level (MSL), above ground level (AGL)), and formats (altitude vs flight level (FL)), spoken with consistent units (feet) and pressure settings (hPa, inHg).

Term definition

In aviation, altitude is the vertical distance of an aircraft above a defined reference. The reference may be a pressure surface (set on the altimeter) or a physical surface such as mean sea level (MSL) or the ground (AGL).
Altitudes are commonly written as numbers with units (e.g., “3,000 ft MSL”) and spoken using standardized air traffic control (ATC) phraseology (e.g., “three thousand”). In most international ATC environments, altitude is spoken in feet unless local procedures require meters.
A flight level (FL) is a pressure altitude referenced to a standard pressure setting rather than local pressure. Flight levels are written as “FL” followed by three digits (e.g., “FL180”) and spoken as “flight level one eight zero.”

Purpose

Standardized altitude references and phraseology exist to ensure that pilots and controllers have a common understanding of vertical separation and terrain clearance.
The purpose of specifying QNH, QFE, MSL, AGL, and FL is to remove ambiguity about what “altitude” means in a given context. The purpose of specifying hPa (hectopascals) or inHg (inches of mercury) is to ensure the altimeter is set correctly.

Use in aviation

QNH

QNH is the altimeter setting that makes the altimeter read elevation above mean sea level (MSL) when the aircraft is on the ground at an airfield (approximately; exactness depends on atmospheric conditions). When using QNH, an indicated altitude is interpreted as feet MSL.
QNH is the most common reference for altitudes used in terminal areas, approach, departure, and en route operations below the transition altitude. Controllers will often provide QNH in either hPa or inHg, depending on the region.
Typical ATC-style phrasing includes “QNH one zero one three” (hPa) or “altimeter two niner niner two” (inHg).

QFE

QFE is the altimeter setting that makes the altimeter read zero when the aircraft is on the ground at a specific reference point, typically the runway threshold or airfield reference point. When using QFE, an indicated altitude is interpreted as height above that airfield reference, i.e., a form of AGL-like indication.
QFE is used in some countries and operational contexts to simplify circuit (traffic pattern) height references. Where QFE is used, it must be clearly stated to prevent confusion with QNH-based altitudes.

MSL (Mean Sea Level)

Mean sea level (MSL) is a vertical reference surface approximating average ocean level. In aviation, published altitudes for terrain clearance, minimum safe altitudes, and many instrument procedure altitudes are referenced to MSL.
When pilots say “three thousand feet MSL,” they mean an altitude that can be compared directly to terrain and obstacle elevations shown on charts (which are typically given in feet MSL).

AGL (Above Ground Level)

Above ground level (AGL) describes height above the local terrain directly below the aircraft. AGL is commonly used for visual operations and performance/obstacle considerations such as traffic pattern altitude, minimum altitudes for certain operations, and decision heights on some approaches (depending on procedure design and terminology).
AGL is not typically what a standard barometric altimeter directly indicates unless QFE is used or the pilot applies a correction (for example, subtracting airfield elevation from an MSL altitude when near that airfield). Radio altimeters, when installed and used, can provide a direct AGL-type height above terrain, but barometric altimetry remains the primary reference for ATC separation.

FL (Flight Level)

A flight level (FL)<\/b> is a vertical position based on standard pressure<\/b> rather than local pressure. Flight levels are used above a published transition altitude (or above a transition layer) to standardize vertical separation between aircraft over wide areas where local pressure varies.
Flight levels are written with “FL” plus three digits (e.g., FL180) and spoken digit-by-digit in groups that match local phraseology, commonly “flight level one eight zero.”
Because flight levels are pressure-based, the aircraft’s true altitude above MSL will vary with atmospheric pressure and temperature, but separation between aircraft using the same standard reference remains consistent.

How altitudes are written and spoken

Feet

In most ICAO-based phraseology, altitude is spoken in feet unless otherwise specified. Written altitudes may appear as “ft” (e.g., “5,500 ft”) and spoken as “five thousand five hundred.”
When communicating with ATC, the unit “feet” is often implied, but many operators include it in training to reduce ambiguity, especially when transitioning between regions that may use different units or when discussing AGL vs MSL references.

Flight levels (FL) format

Flight levels are always communicated as flight level plus the level number (e.g., “flight level two four zero”). They are not spoken as “twenty-four thousand” when the clearance is a flight level.
In writing, “FL240” is preferred over “24,000 ft” when the reference is standard pressure. This distinguishes a pressure-based level from an altitude based on QNH.

Pressure units: hPa and inHg

Altimeter settings are provided in either hPa (hectopascals) or inHg (inches of mercury). Many ICAO states use hPa for QNH/QFE. The United States commonly uses inHg and the term 2altimeter2 rather than 2QNH2 in routine phraseology.
Pilots should read back pressure settings exactly as given, using standard number pronunciation. For inHg, ATC commonly uses “two niner” style phrasing (e.g., “two niner niner two”). For hPa, four digits are typically spoken (e.g., “one zero one six”).
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Operational considerations

1) Always confirm the reference: QNH vs QFE vs FL

The same number can mean different vertical positions depending on whether it is an altitude on QNH, a height on QFE, or a flight level on standard pressure. Operationally, pilots should confirm which reference is in use whenever there is any doubt.
  1. If ATC provides an altimeter setting/QNH, set it and treat cleared altitudes as feet MSL.
  2. If ATC explicitly uses QFE, understand that indicated altitude becomes height above the specified airfield reference.
  3. If ATC clears a flight level (FL), ensure standard pressure is set per local procedures and treat the clearance as a flight level, not an MSL altitude.

2) Transition altitude and transition level awareness

The switch between QNH-based altitudes and flight levels occurs around the transition altitude/transition level system used in a region. The exact values are published and can vary by country and sometimes by terminal area. The key operational point is that pilots must change the altimeter reference at the correct time to remain vertically separated from other traffic and compliant with clearances.

3) Temperature and pressure effects

Barometric altimeters infer altitude from pressure. Non-standard temperature and pressure distributions can cause indicated altitude to differ from true altitude. This matters most for terrain clearance and instrument procedures in cold conditions, where true altitude can be lower than indicated.
Operationally, pilots follow published cold temperature correction procedures where applicable and comply with ATC clearances using indicated altitude unless otherwise instructed.

4) Readback discipline and avoiding “say again” traps

Altitude and pressure setting errors are common contributors to loss of separation. Good practice is to read back: (1) the cleared altitude or flight level, and (2) the pressure setting when issued, using the same units and reference terms provided by ATC.
When a controller says “climb flight level one eight zero,” a correct readback includes the words “flight level.” When a controller says “descend and maintain three thousand,” a correct readback avoids “flight level” and does not add extra qualifiers that could confuse the clearance.

Examples

QNH / altimeter setting example (MSL-based altitude)

ATC: “Altimeter two niner niner two, descend and maintain three thousand.” Pilot: “Altimeter two niner niner two, leaving four thousand for three thousand.”

QFE example (height above airfield reference)

ATC: “QFE one zero zero seven, report downwind at one thousand.” Pilot: “QFE one zero zero seven, will report downwind at one thousand.”

Flight level example (standard pressure reference)

ATC: “Climb to flight level two four zero.” Pilot: “Climb to flight level two four zero.”

AGL vs MSL clarification example (pilot technique)

A pilot may say: “We are at three thousand feet MSL, approximately two thousand feet AGL.” This clarifies terrain clearance while keeping the primary altitude reference compatible with ATC and charted altitudes.
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Summary checklist (quick reference)

  • QNH: Altimeter setting for MSL-based indicated altitude; commonly used below transition altitude.
  • QFE: Altimeter setting for zero on the ground at the reference point; indicated altitude behaves like height above that point.
  • MSL: Reference for charted elevations and many published procedure altitudes.
  • AGL: Height above terrain; useful for visual operations and terrain clearance awareness.
  • FL: Standard pressure reference; used above transition altitude/level system; written as FLxxx.
  • Feet: Common unit for altitude in radiotelephony.
  • hPa and inHg: Common pressure units for QNH/QFE/altimeter settings; read back exactly as issued.






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