On first guess it is probably the residual excess of the holiday festivities around the belt line, or more likely, an entertaining aerobatic manoeuvre. The truth is not far off, in the sense that it can be an unintentional, or intentional.
A Dutch roll, which is more likely to occur in airliners with swept wings, is essentially an oscillatory motion where the aircraft yaws (moves level sideways) and rolls (wing drops left or right) at the same time. This occurs when the dihedral effects of an aircraft are more powerful than the directional stability.(1). The diherdral refers to the upward slant of the wings (if they slant downwards this is known as anhedral), and the primary purpose of this is to stabilise the aircraft, by returning it to a ‘wings level’ position following a turn (roll) to the left or right, or an unintentional upset, like a gust of wind.
Most notably, the aircraft yaws in one direction, and rolls in the opposite direction, making for a rather uncomfortable sensation!
Click on the animation
Why is it called a ‘Dutch’ roll?
The origins of the name are somewhat a myth, but the most plausible explanation would be the comparison to an ice-skating motion of left to right, while leaning to the opposite side. ‘Dutch’ simply because the dutch are well known for success in competitive ice-skating.
What happens in a Dutch roll?
Most modern aircraft are designed to be naturally stable, so in order to initiate a turn, the pilot needs purposefully upset this condition, by changing the position of the aileron on one wing. This causes more lift to be generated by one wing than the other wing. A consequence of this, is that an uneven distribution of drag occurs on the nose of the aircraft, causing it to point in the opposite direction of the roll, and the aircraft will also yaw in this opposite direction.
Similarly, this could also occur is a strong gust of wind lifts one wing. However, the design of the aircraft is intended so that if there is an ‘upset’, the dihedral of the wings will naturally restore a wings level position. When it comes to the yaw, however, the aircraft restorative design force is not as strong as that of the dihedral. The vertical stabiliser counter-acts the effect of drag created by the down going wing in the roll, yet it lags behind the corrective action of the dihedral, and so another roll and yaw in the other direction will occur.
Is it dangerous?
A Dutch roll is not essentially dangerous if recognised. As designs in both the roll and yaw planes are inherently positively stable, the associated fish-tailing and oscillations will eventually dissipate. The good news is that modern airliners have nifty things called yaw dampers, which sense the roll induced yaw and make the appropriate rudder corrections so that the effects are never even felt. In a less adequately equipped lighter aircraft, you want to make sure that your turns are co-ordinated using rudder, keeping the ball of your turn co-ordinator firmly in the middle.
- Ref: Skybrary