Flying in moving air
In a crosswind the nose points one way and the airplane goes another. Set the wind, then either hold a heading and drift, or hold a course and crab into it.
Through the air+Wind=Over the ground
Presets
- Heading
- 000°T
- Ground track
- 345°T
- Groundspeed
- 114kt
- Drift
- 15°left
Ground track
Through the air+Wind=Over the ground
9.5 NM over the ground
The nose points 000°, but the wind from 090° carries the airplane along 345°, 15° left of where it’s pointed.
Switch between holding the heading and holding the course, in the same wind, to compare the drift with the correction that cancels it.
The model
Velocity through the air plus the velocity of the air itself gives velocity over the ground. To hold a course, the airplane turns its nose into the wind until its own sideways velocity cancels the crosswind:
Vground = Vair + Vwind
sin(WCA) = crosswind / TAS
distance = groundspeed × time
WCA is the wind correction angle and TAS the true airspeed. Wind is named for where it blows from, so a wind from 090° pushes the airplane west and its arrow points away from the reported bearing.
Flight is steady and level at constant true airspeed, in a uniform wind. Bearings are true; there is no magnetic variation.
The FAA example preset is the worked problem in chapter 16 of the Pilot’s Handbook of Aeronautical Knowledge: course 090°, 120 kt true airspeed, wind from 045° at 40 kt. The handbook rounds the answer to heading 076° and groundspeed 88 kt. NASA Glenn’s page on relative velocity covers the vector sum.
Also in the lab: Wind tunnel, Why airplanes bank and Density altitude at KLAF.