Departure  is rhumb line distance in NM (Nautical mile) in the East West direction of a given latitude for a given difference in longitudes.

 

Dep= d.long (in mins) x Cos Lat. x 6o

Note: It is max at Equator & Zero At Poles. Function Varies as Cos of Latitude.

 

Example :-

 

Question: An a/c  t/o from point X( 63* 25′ N 16* 25′ E) and flies in true west direction for 600 NM. Find latitude and longitude of destination.

Answer:

Dep= 600NM

Lat= 63* 25′

Hence applying the formula,

600 =d.long x Cos 63* 25′ x60

d.long=600/(Cos 63* 25′ x 60)

d.long= 22* 22′

Y= 63*25 N 22* 22′ E