Why Does the Tail of an Aircraft Wag? A Small Computer Stops It

An aircraft disturbed in yaw does not simply return to straight flight but enters a combined motion in which the nose swings one way while the aircraft rolls the other, then reverses, producing a slow corkscrewing wallow with a period of several seconds. The motion is inherent to the aerodynamics rather than a fault, and it arises from an imbalance between two natural stabilities, one that resists sideslip by rolling and one that resists it by weathercocking the nose into the airflow. Where the first is strong relative to the second, the oscillation damps out slowly or not at all.

The characteristics of a fast jet transport aggravate every part of it:

• •Sweep increases the rolling response to sideslip considerably
• •That strengthens exactly the effect that drives the oscillation
• •A short tail arm reduces the weathercocking that would resist it
• •High altitude reduces air density and weakens damping further
• •So the motion is worst at cruise, where passengers notice it
• •It is uncomfortable rather than dangerous in most conditions

The correction is straightforward once the motion is understood. A gyroscopic or inertial sensor measures the rate at which the aircraft is yawing, and the system moves the rudder to oppose that rate continuously and automatically, applying a correction proportional to how fast the nose is swinging. Crucially it responds to the rate of yaw rather than to the direction the aircraft is pointing, so it damps oscillation without fighting a deliberate turn, and a filter removes the steady component so that a sustained turn passes through unopposed. The authority given to it is deliberately limited, so a failure cannot move the rudder far.

Losing the system is not usually an emergency and it changes how the aircraft must be flown, which is why the failure has its own procedures. The oscillation returns, so the ride becomes uncomfortable at altitude and the crew generally descends to denser air where the natural damping is stronger. Speed may be restricted. Handling in turbulence deteriorates noticeably. Large transport aircraft carry more than one independent system for that reason, and dispatching with one inoperative is permitted under defined conditions and restrictions rather than being prohibited outright, which is a standard pattern for equipment that improves comfort and margin rather than being essential to flight.

Correcting the motion by hand is possible in principle and is actively discouraged, and an accident made the reason unmistakable. The oscillation period is close to the timescale on which a pilot naturally responds, so corrections tend to arrive out of phase and reinforce the motion rather than damping it, which is pilot-induced oscillation. Vigorous alternating rudder inputs also load the fin far more heavily than a single full deflection, because each reversal adds to the load already present. An accident in New York in 2001 in which a fin separated following rapid alternating rudder inputs led directly to changes in training on exactly this point.

A disturbed aircraft can enter a slow combined roll and yaw oscillation driven by an imbalance between two natural stabilities, and wing sweep, a short tail and thin air at cruise all make it worse. A sensor measuring yaw rate drives the rudder to oppose it continuously, filtered so deliberate turns pass through. Manual correction tends to arrive out of phase and reinforce the motion instead.