Rule 39C

Units of Distance, Speeds (Air Speeds) for navigation


Nautical Mile:  It is length of an arc of the meridian making an angle of one minute at the centre of curvature at that place.

–    ICAO has defined it as a distance of 1852 meters (6080 ft).

–    One minute of equator, meridian or latitude is one nm.

–    One minute of change of longitude is not 1 nm except at equator.

–    Actually the length of 1 nm is variable on the surface of the earth due to ellipticity  of the earth.  It is  1843 meters at equator, 1853 meters at about 45º N/S and 1862 meters at pole.


Kilometer: It is 1/10000th part of average distance from equator to pole on the surface of the earth. It is taken as a length  of 3280 ft.

Statute Mile: It is a standard length of 5280 ft established by the Royal Decree of British Monarch.

Conversion of Units of Distances:

                66 nm  =    76 sm

41 nm  =    76 km

41 sm  =    66 km

1 nm  =  1.15 sm = 1.85 km


Indicated Air Speed (IAS):  Speed as indicated by the needle on the face of Air Speed Indicator

Rectified Air Speed (RAS) or Calibrated Air Speed (CAS): IAS corrected for Position and Instrument error is called RAS or CAS

 Equivalent Air Speed (EAS): It is RAS corrected for compressibility error

–    This term is used for speeds 250 nm/h and above where compressibility factor becomes effective.

True Air Speed (TAS):

  1. i) For lower speeds (below 250 nm/h) RAS corrected for density error

is called TAS.

  1. ii) For higher speed (at or above 250 nm/h) EAS corrected for density error is called TAS.

Knot:  nm/h is called knot and represented by KT or kt.

Note: 1   To obtain TAS from RAS, Nav computer is used with Pressure Atitude and Corrected Outside Air Temperature (COAT) as inputs.

Note: 2  In the absence of Nav computer TAS may be found  by the formula:

TAS = RAS + 1.75% of RAS per thousands of feet.

For example if RAS = 150KT,Pressure Altitude = 10,000ft, then

TAS = 150 + (75/100)x150 x10= 150 + 26.25 = 176.25KT (Approx)

Mach Number(M): It is defined as the ratio of TAS to local speed of sound.

M = V/C  where V = TAS  C = Local speed of sound (C= 39√ TºA)

Example: Given M =0.78, PA = 29000ft. Calculate V.

Solution: ISA temp at F290 is (–29×2+15)-43ºC. TºA= 273-43=230ºA

C=39√ 230=591.46

V=MxC= 0.78×591.46=461.34 say 461kt.

Kilograms(kgs) and Pounds(lbs):   1kg = 2.202 lbs



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