Yaw Damper Failure
 

If Yaw damper failure occurs , Pilots are supposed to fly slow and Low.

Flying slow : Will reduce the lift on outer wing and corresponding drag ,
hence stopping the yaw to the opposite side ,
Thus cancelling the dutch roll.

Flying Low : What is the logic behind this ?
Can anybody help in explaining this ?

Denser air, aircraft is more stable. Do you really want to risk any kind of roll or yaw oscillation at FL370?

Dutch roll occurs when lateral stability is more powerful than directional stability.


As altitude (and therefore TAS) increases, the directional stability reduces at a greater rate than lateral.


Therefore the aeroplane is more prone to the phenomenon at higher altitude.


Seemples.

Thanks
Flyboyike and LM

Just checked my QRH, it just says to avoid moderate or severe turbulence, and use flaps 30 with a cross wind exceeding 30kts. No further operational restriction, so i can happily fly along at FL410 and M.819 if i wish to.

Not the same for all types. For many the YD must be working in order for the autopilot to work. Not autopilot means no RVSM which means no FL410....heck, not above FL270 in some areas. Also, not sure I'd be a happy chappy without a YD up high for passenger comfort never mind anything else.

Generally the larger the aircraft the more intense the effect.

Yaw is controlled by the rudder. The yaw damper operates the rudder to dampen the un commanded yaw. The book says with most aircraft, control the roll with aileron, should the YD be u/s. Why? The yaw can be dampened manually with gentle rudder input. There is quite a lag. Hint, "control column left, apply right rudder gently and momentarily" to reduce, then almost eliminate the unwanted oscillations. Get it wrong and you could have a problem of course. I am talking heavy, 4 engine first generation jets, with under slung engines. Worked for me and I haven't "wiped off any pods" due to side loads using this method.

I never wrote that! :}

Sure, I know it is completely possible to manually dampen yaw osculations, no doubt though that the pax in the back of the craft used many more barf bags. Modern YD action is a rapid movement that (if you could watch the rudder) would look like a flutter providing much straighter flight.

We honed our yaw dampening skills using rudder on cargo flights. Better the correct rudder input than no input when the YD became U/S. It could become quite hard work using aileron/spoilers to try and dampen the dutch roll, not to mention the increase in fuel burn and passenger discomfort. The book method is legal a**e covering, along with x-wind, non elimination of drift before touch down.

You would be surprised what abuse cargo would take and not a sick bag in sight.

FREIGHT IS GREAT BROTHER, BOXES DON'T BITCH! Sorry for yelling...

Density and damping
 

The "fly low" advice comes because damping is affected by altitude, particularly in yaw. Put in very simple terms, the yaw damping comes from the vertical stabiliser moving sideways through the air as the tail yaws from side to side. The denser the air is, the more damping there is, which means the yaw oscillations will die out quicker.

The rolling motion in a dutch roll comes from sideslip oscillations, which are an inevitable consequence of yaw oscillations. Again, roll is damped more by increasing density because the wings moving up and down during a roll have more air molecules to react against. This is particularly important for swept wing aircraft, which tend to have high lateral stability and thus roll a great deal with relatively little sideslip present.

Involved with test flights of the L1011 tristar, switching off the YD's was carried out. The oscillation was so violent I could cope only for 30 seconds before YD re-engagement...

I took my check ride in the actual aircraft when I got my type ride in the 727. Part of the maneuvers I was required to do, was to have the yaw dampers turned off, have the aircraft placed in a Dutch roll condition by the safety pilot in the right seat, then I was to recover from the Dutch Roll.

In the 727 you use opposite aileron of the direction of the roll and when the aircraft was stable, the yaw dampers were turned back on and we continued on with the rest of the check ride.

Failing the Dutch roll recovery was an instant fail for the check ride.