Units of Measurements in Aviation

The aviation industry uses a unique set of units of measurement that are designed to be safe, efficient, and consistent around the world. These units are derived from a combination of metric and imperial systems, strictly regulated by the International Civil Aviation Organization (ICAO).

The use of standardized units of measurement is essential for safety in aviation. It helps to ensure that pilots and air traffic controllers around the world are communicating with each other using the same language. This can help to prevent accidents and misunderstandings.

The ICAO plays a key role in standardizing units of measurement in aviation. The organization publishes Annex 5, a document specifying the units that should be used for different purposes. This document is used by airlines, aircraft manufacturers, and air traffic control organizations around the world. Understanding and using these units is vital for safe and efficient aviation operations.

Let’s delve into the intricacies of measurement units in aviation, covering various aspects such as distance, altitude, speed, weight, temperature, and pressure.

Measurement units in aviation

Here is a brief overview of key units used in aviation:

Length and Distance

Distance measurements in aviation primarily rely on two units: the nautical mile (NM) and the foot (ft). The primary unit of length in aviation is the nautical mile (NM).

• Nautical mile (NM): The nautical mile is the standard unit of distance in aviation. It is defined as 1,852 meters, which is approximately 1.151 miles. Nautical miles are used for all horizontal distances, such as the distance between airports or the distance between an aircraft and a waypoint.
• Foot (ft): Feet are used for short distances, such as the height of an aircraft above the ground or the length of a runway. One foot is equal to 0.3048 meters.

Altitude

Altitude measurements in aviation employ two distinct units: feet (ft) and flight levels (FL). Altitude is measured in feet above mean sea level (AMSL). This means that the altitude of an aircraft is its height above the average level of the Earth’s oceans. In some countries, meters are also used for measuring altitude.

• Foot (ft): Feet are used for altitude measurements below 18,000 feet.
• Flight level (FL): Flight levels are used for altitude measurements above 18,000 feet. A flight level is defined as 100 feet above the standard atmospheric pressure level at that altitude. For example, FL250 is equivalent to 25,000 feet.

Speed

Airspeed is measured in knots or Mach. A knot is defined as one nautical mile per hour. Mach is a dimensionless number that represents the ratio of an aircraft’s speed to the speed of sound. Mach is typically used for measuring airspeeds at high altitudes, where the speed of sound is lower.

Speed measurements in aviation primarily rely on two units: knots (kt) and Mach (M).

• Knot (kt): Knots are the standard unit of speed in aviation. One knot is equal to one nautical mile per hour. Knots are used for all airspeed measurements, as well as for groundspeed measurements.
• Mach (M): Mach is a unit of speed that is equal to the speed of sound. It is used for high-speed aircraft, such as fighter jets and supersonic airliners.

Pressure

Pressure measurements in aviation are essential for accurate altitude readings, weather analysis, and cabin pressurization systems. The primary unit of pressure in aviation is inches of mercury (inHg). Inches of mercury are used for altimeter settings, which are crucial for determining the aircraft’s altitude.

• Inches of mercury (inHg): Inches of mercury are the standard unit of pressure in aviation. Inches of mercury are used for altimeter settings, which are used to determine the aircraft’s altitude.

Weight

Weight measurements in aviation primarily utilize the pound (lb) unit. The weight of an aircraft is measured in pounds (lb) or kilograms (kg). The weight of an aircraft is an important factor in determining its performance, such as its takeoff distance and fuel consumption.

• Pound (lb): Pounds are the standard unit of weight in aviation. One pound is equal to 0.453592 kilograms. Pounds are used for aircraft weight, as well as for the weight of cargo and baggage.

Temperature

Air temperature is measured in degrees Celsius (°C) or degrees Fahrenheit (°F). Air temperature is an important factor in aviation weather forecasting, as it can affect aircraft performance and visibility.

• Degrees Celsius (°C): Degrees Celsius are the standard unit of temperature in aviation. Degrees Celsius are used for all temperature measurements, such as the temperature of the air outside the aircraft or the temperature of the engines.

Visibility

Visibility is measured in statute miles (sm) or meters (m). Visibility is an important factor in determining whether or not it is safe to take off or land an aircraft.

Fuel

The quantity of fuel on board an aircraft is measured in gallons (gal) or liters (L). Fuel flow is measured in gallons per hour (gph) or liters per hour (L/h). Fuel consumption is an important factor in determining the range of an aircraft.

Time

Aviation worldwide relies on Coordinated Universal Time (UTC) for standardized timekeeping. It eliminates the confusion caused by different time zones.

ICAO Annex 05

ICAO Annex 05 contains specifications for the use of a standardized system of units of measurement in international civil aviation air and ground operations. This standardized system of units of measurement is based on the International System of Units (SI) and certain non-SI units are considered necessary to meet the specialized requirements of international civil aviation.

SI Units

The International System of Units (SI), commonly known as the metric system, is the international standard for measurement.

The International System of Units developed and maintained by the General Conference of Weights and Measures (CGPM) is used as the standard system of units of measurement for all aspects of international civil aviation air and ground operations.

The SI Units are built upon seven base units, each representing a fundamental quantity. These base units, along with their symbols, are as follows:

1. Meter (m): Measures length or distance.
2. Kilogram (kg): Measures mass.
3. Second (s): Measures time.
4. Ampere (A): Measures electric current.
5. Kelvin (K): Measures temperature.
6. Mole (mol): Measures the amount of substance.
7. Candela (cd): Measures luminous intensity.

SI supplementary units

1. Radian (rad): Measures plane angle.
2. Steradian (sr): Measures solid angle.

SI Unit Prefixes

The following prefixes are used to represent large or small factors of numbers:

SI Derived Units

The International System of Units is a complete, coherent system that includes three classes of units: a) base units; b) supplementary units; and c) derived units.

Derived units of the SI are formed by combining base units, supplementary units, and other derived units according to the algebraic relations linking the corresponding quantities. The symbols for derived units are obtained through the mathematical signs for multiplication, division, and the use of exponents. Those derived SI units which have special names and symbols are listed below:

Non-SI units

The following non-SI units are used either in place of or in addition to, SI units as primary units of measurement. No end date has yet been set for the use of these non-SI units in civil aviation operations.

Definition and Meaning of Units

• Ampere (A): The ampere is that constant electric current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in a vacuum, would produce between these conductors a force equal to 2×10–7 newton per meter of length.
• Becquerel (Bq): The activity of a radionuclide having one spontaneous nuclear transition per second.
• Candela (cd): The luminous intensity, in the perpendicular direction, of a surface of 1/600000 square meters of a black body at the temperature of freezing platinum under a pressure of 101325 newtons per square meter.
• Celsius temperature (t°_C ): The Celsius temperature is equal to the difference t°_C = T – T₀ between two thermodynamic temperatures T and T₀ where T₀ equals 273.15 kelvin.
• Coulomb (C): The quantity of electricity transported in 1 second by a current of 1 ampere.
• Degree Celsius (°C): The special name for the unit kelvin for use in stating values of Celsius temperature.
• Farad (F): The capacitance of a capacitor between the plates of which there appears a difference of potential of 1 volt when it is charged by a quantity of electricity equal to 1 coulomb.
• Foot (ft): The length is equal to 0.3048 meters exactly.
• Gray (Gy): The energy imparted by ionizing radiation to a mass of matter corresponding to 1 joule per kilogram.
• Henry (H): The inductance of a closed circuit in which an electromotive force of 1 volt is produced when the electric current in the circuit varies uniformly at a rate of 1 ampere per second.
• Hertz (Hz): The frequency of a periodic phenomenon of which the period is 1 second.
• Joule (J): The work done when the point of application of a force of 1 newton has displaced a distance of 1 meter in the direction of the force.
• Kelvin (K): A unit of thermodynamic temperature which is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.
• Kilogram (kg): The unit of mass equal to the mass of the international prototype of the kilogram.
• Knot (kt): The speed is equal to 1 nautical mile per hour.
• Litre (L): A unit of volume restricted to the measurement of liquids and gases which is equal to 1 cubic decimetre. 
• Lumen (lm): The luminous flux emitted in a solid angle of 1 steradian by a point source having a uniform intensity of 1 candela.
• Lux (lx): The illuminance produced by a luminous flux of 1 lumen uniformly distributed over a surface of 1 square meter.
• Metre (m): The distance traveled by light in a vacuum during 1/299 792 458 of a second.
• Mole (mol): The amount of substance of a system that contains as many elementary entities as there are atoms in 0.012 kilograms of carbon-12. Note – When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.
• Nautical mile (NM): The length equal to 1 852 metres exactly.
• Newton (N): The force which when applied to a body having a mass of 1 kilogram gives it an acceleration of 1 meter per second squared.
• Ohm (Ω): The electric resistance between two points of a conductor when a constant difference of potential of 1 volt, applied between these two points, produces in this conductor a current of 1 ampere, this conductor not being the source of any electromotive force.
• Pascal (Pa): The pressure or stress of 1 newton per square meter.
• Radian (rad): The plane angle between two radii of a circle which cut off on the circumference of an arc equal in length to the radius.
• Second (s): The duration of 9192631770 periods of the radiation corresponds to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.
• Siemens (S): The electric conductance of a conductor in which a current of 1 ampere is produced by an electric potential difference of 1 volt.
• Sievert (Sv): The unit of radiation dose equivalent corresponding to 1 joule per kilogram.
• Steradian (sr): The solid angle which, having its vertex in the center of a sphere, cuts off an area of the surface of the sphere equal to that of a square with sides of length equal to the radius of the sphere.
• Tesla (T): The magnetic flux density given by a magnetic flux of 1 Weber per square meter.
• Tonne (t): The mass equal to 1 000 kilograms.
• Volt (V): The unit of electric potential difference and electromotive force which is the difference of electric potential between two points of a conductor carrying a constant current of 1 ampere, when the power dissipated between these points is equal to 1 watt.
• Watt (W): The power that gives rise to the production of energy at the rate of 1 joule per second.
• Weber (Wb): The magnetic flux which, linking a circuit of one turn, produces in it an electromotive force of 1 volt as it is reduced to zero at a uniform rate in 1 second.

Infographic of SI Units

Standard use of units of measurement

The following list gives the standard units of measurement used in aviation.

Quantity: Primary unit (symbol) and Non-SI alternative unit (symbol)

1. Direction / Space / Time

• altitude: m (ft for non-SI alternative)
• area: m²
• distance (long): km (NM for non-SI alternative) (As used in navigation, generally over 4,000 m.)
• distance (short): m
• elevation: m (ft for non-SI alternative)
• endurance: H and min
• height: m (ft for non-SI alternative)
• latitude: ⁰ ′ ″
• length: m
• longitude: ⁰ ′ ″
• plane angle (when required, decimal subdivisions of the degree are used): ⁰
• runway length: m
• runway visual range: m
• tank capacities (aircraft): L (As used for aircraft fuel, hydraulic fluids, water, oil, and high-pressure oxygen vessels.)
• time: s or min or h or d or week or month or year
• visibility: km (Visibility of less than 5 km may be given in m.)
• volume: m³
• wind direction: ⁰ (wind directions other than for a landing and take-off are expressed in degrees true; for landing and takeoff wind directions are expressed in degrees magnetic)

2. Mass Related

• air density: kg/m³
• area density: kg/m²
• cargo capacity: kg
• cargo density: kg/m³
• density (mass density): kg/m³
• fuel capacity (gravimetric): kg
• gas density: kg/m³
• gross mass or payload: kg or t
• hoisting provisions: kg
• linear density: kg/m
• liquid density: kg/m³
• mass: kg
• moment of inertia: kg m²
• moment of momentum: kg m²/s
• momentum: kg m/s

3. Force-related

• air pressure (general): kPa
• altimeter setting: hPa
• atmospheric pressure: hPa
• bending moment: kN m
• force: N
• fuel supply pressure: kPa
• hydraulic pressure: MPa
• modulus of elasticity: mN/m
• pressure: kN
• stress: MPa
• surface tension: mN/m
• thrust: kN
• torque: Nm
• vacuum: Pa

4. Mechanics

• airspeed: km/h (kt for non-SI alternative) (Airspeed is sometimes reported in flight operations in terms of the ratio MACH number.)
• angular acceleration: rad/s²
• angular velocity: rad/s
• energy or work: J
• equivalent shaft power: kW
• frequency: Hz
• ground speed: km/h (kt for non-SI alternative)
• impact: J/m²
• kinetic energy absorbed by brakes: MJ
• linear acceleration: m/s²
• power: kW
• rate of trim: °/s
• shaft power: kW
• velocity: m/s
• vertical speed: m/s (ft/min for non-SI alternative)
• wind speed: m/s (kt for non-SI alternative) (A conversion of 1 kt = 0.5 m/s is used in ICAO Annexes for the representation of wind speed.)

5. Flow

• engine airflow: kg/s
• engine water flow: kg/h
• fuel consumption (specific)
  • piston engines: kg/(kW h)
  • turbo-shaft engines: kg/(kW h)
  • jet engines: kg/(kW h)
• fuel flow: kg/h
• fuel tank filling rate (gravimetric): kg/min
• gas flow: kg/s
• liquid flow (gravimetric): g/s
• liquid flow (volumetric): L/s
• mass flow: kg/s
• oil consumption
  • gas turbine: kg/h
  • piston engines (specific): g/(kW h)
• oil flow: g/s
• pump capacity: L/s
• ventilation airflow: m³/min
• viscosity (dynamic): Pa s
• viscosity (kinematic): m²/s

6. Thermodynamics

• coefficient of heat transfer: W/(m² K)
• heat flow per unit area: J/m²
• heat flow rate: W
• humidity (absolute): g/kg
• coefficient of linear expansion: °C^-1
• quantity of heat: J
• temperature: °C

7. Electricity and magnetism

• capacitance: F
• conductance: S
• conductivity: S/m
• current density: A/m²
• electric current: A
• electric field strength: C/m²
• electric potential: V
• electromotive force: V
• magnetic field strength: A/m
• magnetic flux: Wb
• magnetic flux density: T
• power: W
• quantity of electricity: C
• resistance: Ω

8. Light and related electromagnetic radiations

• illuminance: lx
• luminance: cd/m²
• luminous exitance: lm/m²
• luminous flux: lm
• luminous intensity: cd
• quantity of light: lm s
• radiant energy: j
• wavelength: m

9. Acoustics

• frequency: Hz
• mass density: kg/m3
• noise level: dB
• period, periodic time: s
• sound intensity: W/m²
• sound power: W
• sound pressure: Pa
• sound level: dB (The decibel (dB) is a ratio that may be used as a unit for expressing sound pressure level and sound power level. When used, the reference level must be specified.)
• static pressure (instantaneous): Pa
• velocity of sound: m/s
• volume velocity (instantaneous): m³/s
• wavelength: m

10. Nuclear physics and ionizing radiation

• absorbed dose: Gy
• absorbed dose rate: Gy/s
• activity of radionuclides: Bq
• dose equivalent: Sv
• radiation exposure: C/kg
• exposure rate: C/kg s

Coordinated Universal Time

Coordinated Universal Time (UTC) has taken over the role of Greenwich Mean Time (GMT) as the globally accepted standard for clock time. It serves as the foundation for civil time in numerous countries and is the designated time for worldwide time signal broadcasts in aviation.

Horizontal reference system

WGS-84 serves as the horizontal (geodetic) reference system for air navigation. Aeronautical geographical coordinates, indicating latitude and longitude, are reported using the WGS-84 geodetic reference datum.

Vertical reference system

The mean sea level (MSL) datum, which gives the relationship of gravity-related height (elevation) to a surface known as the geoid, serves as the vertical reference system for air navigation. Note: The geoid globally most closely approximates MSL. It is defined as the equipotential surface in the gravity field of the Earth which coincides with the undisturbed MSL extended continuously through the continents.

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Aviation is a global industry that relies on precise and standardized measurements to ensure the safety and efficiency of flight operations. These measurements encompass various aspects of aircraft performance, navigation, and environmental conditions. To achieve global consistency, the International Civil Aviation Organization (ICAO) has established standardized units of measurement for aviation.