View Full Version : Yaw Dampers

14th Nov 2004, 21:57
Hello all,

Can someone please describe the basic functions of the Yaw Damper. Is it a mandatory item on aircraft that have it fitted.

Cheers all.

14th Nov 2004, 22:46
A search for Yaw Damper would have revealed a day's reading.
A Yaw Damper 'damps' i.e. reduces to a minimum Dutch Roll. Where fitted as discrete YDs there are usually two, only one being required for dispatch.
Suggest go read previous posts.

15th Nov 2004, 10:13
I can give you some idea of a very basic one as fitted to some BAe models.

In fact some J31's don't even have them fitted which must be a very horrible ride down the back.

Where fitted they look at the directional gyro outputs and apply an oppersite rudder input to stop dutch roll. Normally turned on after takeoff and off just before landing unless linked with a flap setting ie there off when you select full flap. If they remain in after your on the deck it is very hard if not impossible to remain on the center line on the roll out.

The system it self seems very robust and any problems getting it to engage is more likely to show problems with the flux valves and DG than anything else.

As you get into more complex aircraft which have more issues with stablity in dutch roll. The failure of the yaw damper is a concern for aircraft control, the yaw damper is duplicated and there is also the ablity to take off with it engaged and also land with it engaged some also are smart enough to balance turns.

Hope this help


15th Nov 2004, 21:30
Thanks a lot guys,

Apologies, I didn't think of running a search.:ok:

15th Nov 2004, 23:00
Cutting and abrupt I may do, it's my nature and at my age unlikely to change, however while I might be brief I try not to be wrong and therefore give the wrong idea to someone with a genuine query.

A Yaw Damper takes as it's input the output from a Yaw RATE Gyro, and via it's circuitry developes an opposing Rudder deflection thus damping the Dutch Roll tendancy.

It has NOTHING to do with Directional Gyros or Flux Valves.

If you have been taught or given this impression somewhere then it is totally wrong.

15th Nov 2004, 23:39

Which modern jet has a rate gyro? Non that I know, but they all have yaw dampers.

16th Nov 2004, 03:05

Most modern jets use IRU's which incorporate 3 RATE gyro's. Yaw rate being fed from these.

Several Yaw dampers on older a/c had a built in rate gyro, others used derived rate fed from a basic DG.

However damping of dutch roll (can we still say that in these overly PC times??) is, and will always be, a rate function. It is what is known as a rate/rate system i.e. system response is a product of the RATE of displacement rather than size of the displacement.

16th Nov 2004, 05:33
On most modern jets, the Yaw Damper is not switched off during takeoff or landing. In fact, if both YDs are failed, it is considered an abnormal operation often requiring a different landing flap e.g. Flaps 20 on a CRJ when the norm would be Flaps 45. Dual YD failure also sets a yellow YAW DAMPER Caution message on the EICAS.

16th Nov 2004, 10:42

I thought the IRUs used 3 "laser gyros" and then clever maths to sense the yaw.

16th Nov 2004, 10:59
Just to add about the comment about the flux valves. its the sperry SZ-400 yaw damper i was on about.

And it takes its input from 2 C14 directional gyros and 1 GH-14 Gyro horizon according to my flat mates Jetstream tech manual


16th Nov 2004, 14:39
First generation jet transports (early models of same...speaking B707 here) had a parallel yaw damper, where the rudder pedals moved continuously as the rate of input from the vertical gyro was received to the yaw damper actuator.

On these early models, it was required to switch the yaw damper OFF for takeoff and landing, because if it was not done, rudder forces for the pilot would be insurmountable in the event of an outboard engine failure...and marginally just possible with an inboard engine failed.
Even tho these aircraft had a hydraulic powered rudder, control forces were heavy with an outboard engine failed at takeoff thrust.

Later models of the 707 had series yaw dampers, and these, due to the much improved design, were full time...ie: not required to be switched off, except if they malfunctioned...and the usual malfunctions were, excessive rate response or rudder hardover...the latter being very bad news, especially just after takeoff.

Second generation jet transports (speaking Lockheed TriStar here) had both types.
The SAS (stability augmentation system) supplied series yaw damping/turn co-ordination...and parallel operation for runway alignment and rollout...the parallel mode used for automatic approach/landings only, with one or two autopilots engaged.

18th Nov 2004, 15:38

I thought the IRUs used 3 "laser gyros" and then clever maths to sense the yaw.

They do. A LASER gyro is a RATE sensing device. The interference pattern created by the two laser beams (ok one laser beam split into 2) changes as the RATE of rotation about the axis changes. The clever maths converts this rate of change of the interference fringe into rotational rate and by intergration rotational distance (i.e angle)

18th Nov 2004, 17:46
Thanks mono. As I said, they don't have rate "gyros".

19th Nov 2004, 19:10
Yaw dampers are not just for preventing dutch roll, but also prevent adverse yaw while applying ailerons for a turn. This is why pilots of aircraft with yaw dampers do not need to apply co-ordinated rudder with aileron movement as is required on light aircraft.

The 757 has 2 yaw dampers, each capable of displacing the rudder 3 degrees at low speed, so with both operating they can apply 6 degrees of rudder movement. When compared with the 30 degrees of movement available from the pedals at low speed then there is no need for them to be switched off for T/Os and landings.

Pogo P

Mad (Flt) Scientist
20th Nov 2004, 17:20
When compared with the 30 degrees of movement available from the pedals at low speed then there is no need for them to be switched off for T/Os and landings.

The issue with turning YDs off for takeoff and/or landing is the possible impact on minimum control speeds through a possible reduction in rudder travel (due to the YDs 'pushing' the other way and removing some control power; or due to 'lost motion' in the YD system preventing a full pedal demand getting to the rudder PCUs). You can't just look at the relative displacements and say it doesn't matter because it's only 20% of travel (and anyway, 20% of travel is a LOT when you look at min control speed impact).

Generally, credit is taken for full travel authority when determining min control speeds. therefore it has to be demonstrated/shown that the YD system will not reduce the available travel - usually by some 'smarts' in the system which takes itself briefly offline while the min control speed rudder application is happening.

20th Nov 2004, 17:25
would a flying buddy be willing to explain to a non flying buddy - but who dreamed of becoming one - how the yaw damper works? I have read about it but still have difficulties w/ this. Tx.

pls disregard this request; I read through the whole posting! ;)

21st Nov 2004, 12:54
As stated by others, the newer aircraft have full time yaw dampers which can be turned off if a problem exists. On the B-747 the yaw rate gyros are actually on the front of the yaw computers (upper and lower). Turn coordination with flaps out of the up position is a function of Number 1 INS. If No.1 INS fails and number 3 is selected, turn coordination will not shift. It is lost.

22nd Nov 2004, 02:29
In a previous post on this thread, someone mentioned that it is not necessary to use rudder for coordination in a turn in an aircraft with a yaw damper installed. Is this true for all aircraft or jet aircraft? In the MD-80 for example, it sure seems like you need to coordinate rudder (judging by the ball, and seat of the pants). Am I wrong?

Mad (Flt) Scientist
22nd Nov 2004, 09:00
Depends on whether the yaw damper system has a turn coordination function and how well designed it is for all configurations.

In theory, with a well designed system you shouldn't need to manually coordinate the turn, the system will do it. However, there may be circumstances for which the system was not designed where it needs some help (or where it is, in fact, disabled). It all depends on the design....