
 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 72: 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 ½
NM of error over 4 minutes of time. Using the equations previously described, this equates to
7½ 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 T34 VNAV text, the equation is:
Planned Aircraft Convert to
Speed (NM/sec) NM/hr
Time gained or lost (sec)
X
NM
X
60 min
= Head/Tailwind (kts)
approx. time flown (min) since
sec
hr
last compensation update
7.7.2.2. In the above equation, planned airspeed is a factor, and changes the equation for T1
usage. Reducing terms and applying an airspeed of 300 kts, the equation becomes:
Time gained or lost (sec)
X
1NM
X
60 min
= Head/Tailwind (kts)
approx. time flown (min) since
12 sec
hr
last compensation update
Or:
X
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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