Head or Tail Wind Computation

STUDENT GUIDE

VISUAL NAVIGATION

What is the crosswind?

In this case, by doubling both the error and time interval we have 1

NM of error over 6 minutes of time. Using the equations previously described, this equates to 10

kts of crosswind.

EXAMPLE 7-2: After eight minutes of flight, a fix reveals the aircraft is 1 NM left of course.

What is the crosswind?

In this case, by halving both the error and time interval we have

NM of error over 4 minutes of time. Using the equations previously described, this equates to

7 kts of crosswind, which is typically rounded to either to 5 or 10 kts.

7.7.2. Head or Tail Wind Computation:

7.7.2.1. Head or tail wind components are nothing more than the difference between the ground

speed and the true airspeed. Rather than use a ground speed derived from a TACAN DME

reading, take the difference between the planned and actual time of arrival, and convert it into a

distance. This distance divided by the time flown equals the head or tailwind component. Again,

from Unit 5 of the T-34 VNAV text, the equation is:

Planned Aircraft Convert to

Speed (NM/sec) NM/hr

Time gained or lost (sec)

60 min

= Head/Tailwind (kts)

approx. time flown (min) since

sec

last compensation update

7.7.2.2. In the above equation, planned airspeed is a factor, and changes the equation for T-1

usage. Reducing terms and applying an airspeed of 300 kts, the equation becomes:

Time gained or lost (sec)

1NM

60 min

= Head/Tailwind (kts)

approx. time flown (min) since

12 sec

last compensation update

Or:

5NM =

Head/Tailwind (kts)

Time gained or lost (sec)

approx. time flown (min) since

min

last compensation update

7.7.2.3. For a time interval of 5 minutes, this equation becomes:

Time gained or lost (sec) =

Head/Tailwind (kts)

7.7.2.4. As with cross wind computations, this "Five minute rule" may be used with time

intervals between 4 and 6 minutes. (Additionally, this equation could also be used to compute

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