I Survived a Downwind Turn

One of the most persistent misconceptions in aviation is that so-called downwind turns are dangerous. Proponents of this fallacy typically present the following basic argument:

Consider an airplane flying due north at, say, 100 KIAS into a 30 knot direct headwind. The airplane’s groundspeed is therefore 70 knots. If the airplane, still flying at an indicated airspeed of 100 knots, turns 180 degrees to a heading of due south, it needs to “gain” 60 knots to match its new groundspeed of 130 knots—that is, the sum of its airspeed and the wind velocity. A pilot who doesn’t take care to add power or push the nose down to help the airplane “gain energy” in the turn risks stalling and falling out the sky.

That argument may seem logical and consistent, but it doesn’t hold up when we actually do the experiment, as you can see in the video below.

I took advantage of an IFR training flight with a student in a C172 to capture the “downwind turn” phenomenon in action. We had flown an instrument approach and then climbed to 4000 ft to enter and fly the published hold at CARRO intersection, a fix located 24 nm southwest of the SEA VOR.

The track data shows that as we flew the outbound leg of the hold, we were cruising at about 100 KIAS almost directly into a wind of about 30 knots.

When we turned 180 degrees back inbound to CARRO, our airspeed remained essentially constant (the student was hand-flying the aircraft, which doesn’t have an autopilot).

Our groundspeed varied from about 80 knots outbound to some 120 knots on the inbound leg. But the pilot didn’t have to add power—or do anything else out of the ordinary—to keep the airplane flying. He just turned as if we were flying on a dead calm day. Because from the airplane’s perspective in the air, the 30-knot wind aloft didn’t exist.

And without looking outside at the ground or checking the groundspeed and wind displays on the PFD, we wouldn’t have sensed the wind, either.

We gained groundspeed thanks to the push from the wind.

As many experts have tried patiently to point out, the key to this situation is knowing that velocity and energy are measured with respect to specific frames of reference.

For example, the kinetic energy of an airplane that concerns us during takeoff, landing, and cruising to our destination—or when crashing into terrain—is a function of groundspeed, which is measured with respect to the earth.

If you are flying at, say, 60 KIAS in zero wind, when your wheels touch the runway you have the kinetic energy associated with 60 knots groundspeed and the aircraft’s mass.

Touch down at, say 60 KIAS into a 20 knot headwind, however, and you have the kinetic energy associated with a velocity of 40 knots, with respect to the earth.

But in both of those situations, your speed through the air remains 60 knots.

As the following excerpt from a column in AOPA Pilot by Catherine Cavagnaro succinctly explains, changes in the aircraft’s kinetic energy with respect to the ground are powered by the wind, and the change in groundspeed applies regardless of the aircraft’s mass or power, because the aircraft is always moving within and along with the air mass in which it is flying, just like a leaf, or a branch, or a boat floating downstream in a river.

A baseball that hits a wall gains no energy in the process. But when it hits a fast-swinging bat, it gains a significant amount before it soars toward the outfield. In the flight scenario, the airplane plays the role of the ball and the wind is the swinging bat. Without wind there is no change in energy from the departure leg to the downwind leg. But the 20-knot wind supplies the extra energy that increases the groundspeed of the airplane. It’s the same energy increase we enjoy as we travel cross-country with a strong tailwind. To avoid a stall in any turn, take the same precautions one does in an ordinary no-wind situation. (Proficiency: Relax and Go With The Flow, AOPA Pilot, August 1, 2019, by Catherine Cavagnaro)

In the air, your energy with respect to the airmass remains the same, regardless of which direction you fly and the presence of wind, if any. You can maneuver at will, and at a given airspeed, the airplane’s kinetic energy with respect to the airmass, regardless of the wind direction, doesn’t change. And when considering stall speeds, your airspeed (actually, angle of attack) measured with respect to the airmass is all that matters—because that velocity is what the wing experiences.

Of course, wind shear—an abrupt change in wind direction and/or speed—and vertical gusts–does affect an airplane. Those phenomena cause variations in angle of attack and G loads. But the myth of the downwind turn isn’t about wind shear. It’s an error caused by conflating frames of reference.

For more information about this topic, see the following articles and other videos:

And for a wonderful presentation about frames of reference and the fundamental principle of relatively (not limited to Einstein), see: Relativity Crash Course by Ramamurti Shankar, a professor of physics at Yale University.