Yaw dampers

The tail of the airplane begins repeatedly wagging left and right about its vertical axis as you try to dampen those oscillations with your feet dancing on the rudder pedals. Those efforts will eventually become exhausting for the pilot in command and probably send passengers diving for the sick sacks, especially if they’re sitting in the last two seats. There is a better way: an independent stabilization system known as a yaw damper, today found on aircraft of all categories including large jets.

But let’s begin small. The yaw damper on a single-engine Cirrus SR22, for example, senses that wigwagging through a series of accelerometers or rate sensors located in the rudder. The sensors translate those motions into just the proper amount of calming mechanical inputs to the rudder. Those Cirrus yaw damper servos are also in constant communication with the avionics on board, including the air data attitude and heading reference system (ADAHRS) that monitors the aircraft’s every pitch, roll, and yaw movement.

Because a yaw damper senses skids and slips on the aircraft, it also can provide just enough rudder in a turn to create a near perfectly coordinated movement. This is one reason many pilots who are used to flying sophisticated, yaw-damper-equipped turbine aircraft can be lousy at flying a small aircraft. Feeling a taildragger skid or slip through turns for a few hours is normally all that’s needed to reacquaint that pilot with how much rudder to add to remain coordinated.

On a swept-wing aircraft, a Cessna Citation Latitude for example, the yaw damper has the additional purpose of inhibiting the Dutch rolling tendency, a kind of wallowing combination of yawing and rolling motions of the wing and tail. Dutch rolls occur when the roll stability of the aircraft is greater than its yaw stability. In turbulence, then, the wings attempt to roll back to their neutral position before the tail settles down, inducing a series of oscillating overcorrections. The yaw damper’s job is to smooth out all the aircraft’s gyrations, because without it, flight in a swept-wing aircraft would also quickly become extremely uncomfortable to people in the cabin.

The yaw damper’s job is to smooth out all the aircraft’s gyrations, because without it, flight in a swept-wing aircraft would quickly become extremely uncomfortable to people in the cabin.By contrast, the Cirrus single-engine Vision Jet’s two ventral carbon fiber fins are attached to the empennage in a V-tail configuration, like the old V-tail Beech Bonanza. That configuration demands a slightly different kind of system to completely dampen the yawing tendency. The Vision Jet’s ventral fins provide attach points for the yaw stability augmentation system controlled by a servo motor on the autopilot. The control surface, which is hinged to the ventral fin, rotates asymmetrically to actively augment lateral and directional stability. The stability augmentation system shuts off when the autopilot yaw damper automatically engages above 200 feet.

Typically, yaw dampers are engaged a few hundred feet in the air after takeoff and switched off on short final. In fact, pilots are warned against using the yaw damper on many aircraft during takeoff and landing because the system will fight the pilot’s rudder inputs as they attempt to keep the aircraft correctly aligned on the runway centerline. Attempting a takeoff in a large aircraft with the yaw damper engaged could lead to the airplane correcting on its own for adverse yaw in the event of a powerplant failure. That would make identification of the failed powerplant more difficult.

There are exceptions to everything, of course. On a jumbo aircraft like the Airbus A380, the yaw damper is actually switched on before takeoff and switched off during the after-landing checklist when clearing the runway. The yaw damper helps align the aircraft with the runway centerline during an autoland procedure.

In other aircraft such as the Boeing 787, the yaw damper turns on as soon as the aircraft is powered up. However, because the 787 is also a fly-by-wire aircraft, the amount of effort the yaw damper is adding to the flying of the aircraft changes depending upon whether all flight control systems are operating normally. When any flight control system is degraded for any reason, yaw damper input may be reduced.

A yaw damper can also assist the pilot of a multiengine aircraft during the loss of one engine by sensing the yaw toward the failed engine and correcting for it. Depending upon the type of aircraft too, an inoperative yaw damper could be listed in the minimum equipment list as a no-go item, grounding the aircraft. On others, an inoperative yaw damper might only restrict the aircraft in some way, such as maximum usable altitude.
Rob Mark is an aviation journalist and the publisher of JetWhine.com.