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CHAPTER FIVE
507.
If wind is pushing the aircraft ahead or behind preflight time, the rate of error is proportional to
the head/tailwind velocity. The following formula applies:
Time gained or lost (sec)
X
NM
X
60 min
time flown (min) since last
sec
hr
compensation update (approx)
The head/tail wind component affects our timing and is a result of the difference between our
true airspeed and actual ground speed. The equation basically converts the time off to a distance,
which is then divided by time flown.
For a T-6A route planned at 180 knots, one NM equals 20 seconds. This is the source of the
second factor in the formula. Canceling terms, the simplified formula becomes:
X
1NM
X
60 min
Time gained or lost (sec)
time flown (min) since last
20sec
hr
compensation update (approx)
Reducing terms:
Time gained or lost (sec)
X
3.0 (or 6/2) =
time flown (min) since last
compensation update (approx)
Notice the equation is equivalent to time off divided by time flown multiplied by the miles per
minute. For a planned ground speed of 180 knots 3.0 is used. If planned at 300 knots ground
speed, what would replace 3.0? The answer is 5, and you will fly this airspeed later in the jets.
Below is an example of head/tail wind computation:
Example
At time 11+58 you pass abeam of a road bridge, anticipated crossing time of 11+48. You passed
the previous turnpoint at 7+46, and it had an updated time of 7+48. How much head/tailwind are
you encountering?
At the last turnpoint, you were 2 seconds early, at this turnpoint you are 10 seconds late, for a
difference of 12 seconds. Time flown is 11+48 - 7+46 = 4+12, approximately 4 minutes.
Because we arrived later we note we have a head wind, then:
12 sec x  6 =
18 knots