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):
- i) For lower speeds (below 250 nm/h) RAS corrected for density error
is called TAS.
- 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
V=MxC= 0.78×591.46=461.34 say 461kt.
Kilograms(kgs) and Pounds(lbs): 1kg = 2.202 lbs