Wind Drift Circle

How Wind Affects Aircraft in Straight and Level Flight

Wind is an ever-present force that influences every aspect of aviation. When an aircraft flies straight and level, maintaining a constant altitude and heading, it may seem unaffected by the wind. However, the reality is far more complex. Understanding how wind impacts flight is essential for any pilot.

Airspeed vs. Ground Speed

To understand wind's effects, it’s important to differentiate between airspeed and ground speed. Airspeed measures how fast an aircraft moves through the air, while ground speed refers to its actual speed over the earth's surface. When wind is introduced, these speeds often differ significantly.

For example, if a plane has an airspeed of 120 knots but faces a 20-knot headwind, its ground speed is reduced to 100 knots. On the other hand, with a 20-knot tailwind, the same aircraft would experience a ground speed of 140 knots. This concept is crucial for flight planning, fuel management, and time estimation.

Correcting Drift During Straight-and-Level Flight

When flying straight and level, maintaining your intended ground track is essential. Wind can cause the aircraft to drift off course, particularly when dealing with crosswinds. To counteract this drift, pilots apply a wind correction angle (WCA). This means angling the aircraft slightly into the wind, just enough to cancel out the sideways push caused by the wind.

The amount of correction required depends on three main factors:

• Wind Speed: Stronger winds result in greater drift and require a larger correction angle.

• Wind Direction: The closer the wind is to a direct crosswind, the more pronounced the drift.

• Airspeed: A slower airspeed means the wind will have a more noticeable effect on the aircraft’s ground track.

For example, if an airplane has an airspeed of 100 knots and encounters a 20-knot wind blowing directly from the side, the pilot may need to turn about 12 degrees into the wind to maintain the desired path. If the wind speed were only 10 knots, a correction angle of around 6 degrees would suffice. The goal is to maintain a straight track over the ground despite the wind’s influence. 

A helpful analogy is to think of a boat crossing a river with a strong current. If the boat aims directly across the river without accounting for the current, it will drift downstream. However, by pointing the boat slightly upstream, the captain can compensate for the current and reach the intended point on the opposite shore. Pilots apply the same concept in the sky when correcting for wind drift. [Figure 7-1]

Figure 7-1 Wind Drift (Airplane Flying Handbook FAA-H-8083-3C)

Understanding the Wind Drift Circle

The concept of a wind drift circle is a useful visualization for pilots. Imagine flying a circular path at a constant altitude in a steady wind. Because the wind is constant, the effect on your ground track will change depending on your position in the circle.

• When flying directly into the wind, your ground speed will be at its lowest.

• When flying with a tailwind, your ground speed will be at its highest.

• When flying perpendicular to the wind, you’ll experience the most significant sideways drift.

A wind drift circle helps pilots understand how the wind impacts their ground track and ground speed in different directions. Practicing a circular path in a known wind condition is an excellent way to visualize and adapt to wind drift. [Figure 7-2] and [Figure 7-3]

Figure 7-2 Effect of Wind During a Turn (Airplane Flying Handbook FAA-H-8083-3C)

Figure 7-3 Effect of Wind During a Turn (Airplane Flying Handbook FAA-H-8083-3C)

In Figure 7-2 and Figure 7-3 you can see the effects on your turn and turn radius with no winds and with winds. In Figure 7-3 you can see in the third photo they are maintaining a constant bank and therefore their radius is affected by the wind. But in the fourth photo they are using wind correction to maintain a constant radius of their turn.

How to Perform a Wind Drift Circle

1. Establish Straight and Level Flight: Maintain a constant altitude and airspeed. You can do this maneuver at any airspeed, however you'll want to choose an airspeed similar to the airspeed you'll be using for your other ground reference maneuvers. Also note that a slower airspeed may make the winds appear stronger than a faster airspeed. You'll also want to choose an altitude typically between 600-1000 feet AGL. 

2. Choose a Visual Reference Point: You'll want to choose a prominent landmark that you can clearly identify even after you get pushed away from it by the winds. You can also use a straight line reference point as long as there is a prominent intersection or some other type of visible landmark. I would recommend using a group of trees in a field, an intersection of two roads or highways, two fields, or any other prominent reference point. Really anything will work as long as it can be clearly identified.

3. Enter a constant rate turn: Enter a turn of a set bank angle (typically around 30 degrees) and hold that bank angle throughout the entire maneuver. Changing your bank angle during the turn will affect your ground track and may make the wind appear to be coming from a different direction.

4. Observe Aircraft Position After Completing a 180 Degree Turn: You'll want to note your entry heading or use a visual reference off your nose when you roll into the turn and then roll out on the same heading (when that reference point is off your nose again). Once you've completed this 180 degree turn, observe where your reference point is and where your aircraft is. By observing where this reference point is in relation to your aircraft, you will know what direction the wind is coming from. The further you've been pushed from this point, the stronger the winds are. If you're still relatively close to your point then you know the winds are light. If you've been pushed far from your point then you'll know the winds are strong.