Units of Measurement and their Conversion

References

Objectives

Gain an understanding of:

  • Units of measurement used in aviation
  • Difference between countries in terms of use of specific units
  • Rules of thumb for conversion of units

Standard Units of Measurement

Units of measurement relevant for aircraft and principles of flight fall into six categories:

-vertical distance, -horizontal distance, -speed, -mass, -temperature -air pressure;

Standard units of measurement in aviation (Europe and North America) are shown in the following table:

Category of MeasurementUnit of MeasurementAbbreviation
Horizontal Distancenautical milesnm
Kilometerkm
Horizontal Distance: RVR, Rwy length, and visibility <5kmmetersm
feetft
statute milessm
Vertical Distance: Flight Levels, heights and elevationsfeetft
metersm
Speedknotskt
Vertical speedFeet per minuteft/min
Air PressureHecto Pascalshpa
Millibarsmb
Inches of mercuryIn.Hg
MassKilogramkg
Poundlb
Temperature and Dew Pointdegrees Celsius°C
Fahrenheit°F

Flight crews on international routes encounter different units of measurement for setting barometric altimeters.Throughout Europe, hectoPascal (hPa) [or Millibar (mb)], is the primary altimeter setting (1 hPa = 1 mb). Within North America, the primary altimeter setting is In.Hg. In some parts of the world, such as Russia, levels are measured in metres (300 m = 1000 ft), speed only in km/hour, and wind speeds can be given in meters per second (m/s).

According to ICAO Annex 5 Contracting States are required to notify the Organization of any differences between their national regulations and practices and the International Standards contained in Annex 5.

Altimeter settings are occasionally misheard when listening to ATIS or ATC and the error may sometimes go undetected. When hPa is used as altimeter setting, an error of 10 hPa will correspond to approximately 300 feet error in indicated altitude.

Time is presented in a standard way by Co-ordinated Universal Time (UTC) The use of UTC is recommended by such bodies as the General Conference on Weights and Measures (CGPM), the International Radio Consultative Committee (CCIR) and the World Administration Radio Conference (WARC).

International Standard Atmosphere

Also known as the ICAO Standard Atmosphere, ISA is a standard against which to compare the actual atmosphere at any point and time.

The ISA is based the following values of pressure, density, and temperature at mean sea level each of which decreases with increase in height:

  • Pressure of 1013.2 millibar - Pressure is taken to fall at about 1 millibar per 30 feet in the lower atmosphere (up to about 5,000 feet).
  • Temperature of +15 °C - Temperature falls at a rate of 2 °C per 1,000 feet until the tropopause is reached at 36,000 feet above which the temperature is assumed to be constant at -57 °C. (The precise numbers are 1.98 °C, -56.5 °C and 36,090 feet)
  • Density of 1,225 gm/m3.

The real atmosphere differs from ISA in many ways. Sea level pressure varies from day to day, and there are wide extremes of temperature at all levels.

Variation in pressure, vertically and horizontally, affects the operation of the pressure altimeter.

Conversion

Multiplybyto get
Knots1.85Kilometre per hour
Kilometers per hour0.54Knots
Meters3.28Feet
Nautical Miles1.85Kilometers
Kilograms2.20Pounds
Pounds0.45Kilograms
Celsius1.8 and add 32Fahrenheit
FahrenheitSubtract 32 and multiply by 0.56Celsius

Quiz Questions:

Q1: The most common unit of measurement of atmospheric pressure, HectoPascal (hPa) is often referred to as:?

  1. Inches of mercury (In. Hg)
  2. millimeters of mercury (mm Hg)
  3. millibars (mb)

Answer

Q2: One nautical mile is:

  1. 1852 meters or 5280 feet
  2. 1609 meters or 6076 feet
  3. 1852 meters or 6076 feet

Answer

Q3: In altimeter setting, an error of 10 hPa will correspond to approximately how many feet in indicated altitude?:

Answer

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