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INSTRUMENT NAVIGATION
Ground speed checks may be taken for any length of time. The longer the time, the more
accurate the check. Whenever possible, 3 minutes is the recommended timing you should try
for.
Another method of computing ground speed is give below:
GS check commenced at 40 DME: time 0 + 00
GS check terminated at 57 DME:  time 0 + 03
57 - 40 = 17 DME
Since 3 minutes goes into 60 minutes (1 hour) 20 times, simply multiply 17 x 20. Your GS
in this case is 340. This method nets the same answer as the previous method of calculation.
Now look at J-2 (Figure 36). Note that you should switch your TACAN to SJI at 51 nm. If
doing so conflicts with your ability to take an accurate ground speed check, use your discretion
and with the permission of your instructor, you may be permitted to change the TACAN later
after your ground speed check has been completed. Remember a ground speed must be
attempted at least once per leg.
We have just talked about ground speeds and crab angles. These variables are needed to
determine the actual direction and velocity of the wind. You already know how to calculate
wind on the CR-2. Here you will learn a method of getting a rough wind, which can approach the
accuracy of the wind you determined from your CR-2.
Given TAS:
300
G/S:
320
MC:
270
MH:
(to hold MC) 274
First, it should be apparent that the aircraft has a 20 kts tailwind component. The crosswind
component guide number is 5 (300/60). The crab angle is 4 (274-270=4). It follows that 5
(Guide Number) times 4 (crab angle) equals 20 kts, your crosswind component.
Now simple geometry comes into play. Using Figure 37 and the theory of a right triangle,
draw a line which equates to your course and your head or tail wind component: (A).
Next, at the beginning of that segment; i.e. where it commences, draw a line representing
your crosswind component. We determined that since we were crabbing four degrees to the
right, that our crosswind component consists of 20 kts off the right wing: (B).
Now simply connect the end points of the two segments. That line represents your wind
velocity and direction.
Reviewing: If Segment A represents a course of 270, then the origination of segment C
(your wind) is from 045. And if the lengths of both lines (A and B) represent 20 kts, then the
length of line C, which is somewhat longer, represents about 28 knots.
RADIAL TRACKING AND COURSE CONTROL 3-27


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