A footnote on calculating this in the cockpit:
Backpacker wrote:
So true heading is 012 degrees, while true track is 000 degrees. Ground speed is 100 * cos(12) = 97.81 knots.
The quick-and-dirty mental approximation for this, ( 1-cos(angle) ), is:
Percentage Speed reduction = (angle / 8) squared.
So (12/8) x (12/8) = 2.25%
For small numbers the percentage speed reduction is roughly the same as the percentage time increase, so the extra time is:
120 minutes x 2.25% = 2.7 minutes,
versus the 2.6 minutes in Backpacker's exact calculation.
In practice the effect is small, so the calculations can be rounded up to make the maths easier, to get a "worst case" arrival delay which is still typically small.