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What Is Dutch Roll, And How Do You Prevent It?

ThierryB.

Imagine you're cruising at FL350 sitting left seat in a 737. The yaw damper failed sometime after takeoff, so you've let the first officer do the flying. You've stashed some water bottles behind the right seat, and reach around to grab one when your knee hits the yoke. You bank to the right. What happens next is quite a ride - Dutch roll.

What Is Dutch Roll?

Dutch roll happens naturally in many aircraft. Sometimes it's performed intentionally as an aerobatic maneuver - and other times it happens accidentally and makes everyone feel like an out of control aerobatic passenger.

Dutch roll is a series of out-of-phase turns, when the aircraft rolls in one direction and yaws in the other. Its name comes from the motion of a classic Dutch skating technique. Check it out here in a plane: (Sorry for the shaky video, it was the best example we could find.)

And now, compare it to these people skating...

Even if you never understand why Dutch roll happens, you'll understand where the name came from... Now for the important part: why Dutch roll happens.

Dutch Roll Naturally Happens Because Of Your Airplane's Stability

Let's look at two types of stability - roll stability (also known as lateral stability) and yaw stability. Stability describes what happens to your aircraft when it's disturbed from a steady state - does it stay in a new position, return to the original position, or continue to move farther away?

Strong Roll Stability

Let's go back to the 737 example - you've started an uncoordinated roll to the right. That tilts the lift vector to the right, causing a sideslip motion. Instead of the airflow blowing straight down your nose, it's now coming from the right. Take a look at the example below - see how the air is hitting the right wing's leading edge straight on, and the left wing's leading edge at an angle?

right-sideslip

From our article on wing sweep, you learned that only the air flowing parallel to the chord line creates lift. So, in the example above, the right wing has more air flowing parallel to the chord line than the left wing, meaning the right wing generates more lift. This effect is known as dihedral effect because it simulates the stabilizing effect of dihedral. It results in positive roll stability; the extra lift rolls the aircraft back towards level. However, this extra lift also generates drag that pulls your nose to the right.

dihedral-effect

Weaker Yaw Stability

Your vertical stabilizer helps stop the yaw motion caused by the right wing's drag. With your nose yawed to the right, the relative airflow approaches the vertical stabilizer from the left. Your vertical stabilizer generates lift towards the right, yawing the nose back to the left. (Yes, the vertical stabilizer actually generates lift - it's not just air "pushing" the tail back into position).

tail-lift

But, with a typical swept wing aircraft, this yaw stability isn't as strong as the roll stability caused by the sweepback. While the tail's still trying to line up the nose, the aircraft has over-banked to the left, causing a left sideslip. Now the sweepback starts to raise the left wing, rolling your 737 right. The drag from the left wing starts to pull the nose to the left.

left-sideslip

Most modern swept wing aircraft have yaw dampers that automatically correct for Dutch roll by quickly adjusting the rudder. If your yaw damper's inoperative, stopping the roll can be more tricky. Many modern swept-wing jets will fly themselves out of Dutch roll if you stop adding control inputs. However, some of the older jets, like the 727, can be difficult to recover. But we'll leave that technique to your sim instructor.

Aleks Udris

Aleks is a Boldmethod co-founder and technical director. He's worked in safety and operations in the airline industry, and was a flight instructor and course manager for the University of North Dakota. You can reach him at aleks@boldmethod.com.

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