Figure 36

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For purposes of wind calculation and for many other reasons, it is important that you be able
to calculate your aircraft's ground speed. It is a simple task but there are a few guidelines
you must follow. You will determine your ground speed by computing the change of your
TACAN DME. Recall now that DME gives you slant range to a TACAN station; as we get
further away from TACAN stations, the difference between DME (slant range) and ground range
decreases. To ensure that the mileage figures we use to compute our ground speeds are as close
as possible to actual ground miles do not utilize the DME within a radius of a TACAN which is
equal to your altitude in thousands of feet. For example, if you were on J-2 (Figure 36), flying
from MSY to SJI at FL 370, you would not commence a ground speed check until at least 37
DME from MSY and stop it by 37 DME of SJI.
Remembering that restriction, here is the procedure for conducting a ground speed check.
1. Watch your DME. As it passes through a whole mile, start your clock.
2. Run your clock for three minutes.
3. At the end of three minutes note the new DME. 3-28
Figure 36
4. Subtract the two numbers, divide by 3 (for three minutes) and you will have your ground
speed in nm per minute.
5. To change this to nautical miles/hour (knots), simply multiply this number by 60.
NOTE: A more convenient approach might be to double the DME difference and add a ZERO
to the end of your product (same as multiplying by 20).
e.g. Ground speed check commenced at 38 DME: time 0 + 00
Ground speed check terminated at 54 DME: time 0 + 03
Difference = 16. 16 = 5 1/3 NM per MIN (5 1/3 x 60 = 320 kts).
3-26 RADIAL TRACKING AND COURSE CONTROL

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