Yaw Damper 747-400

[NSEU]

A question raised on another forum ...

If a 747-400 single Yaw Damper (Upper or Lower) system fails, will the serviceable YD system produce twice the rudder deflection it normally would for a given set of circumstances (assuming the effectiveness of upper and lower surfaces was equal)? i.e. up to the normal authority limit

Or would the serviceable Yaw Damper system simply output a given (tabulated) deflection for the current sensor (IRU/ADC) inputs? (but perhaps prolong the rudder deflection until it had achieved what it was trying to do).

Hope this makes sense.

Thanks.
Rgds.
NSEU

[GlueBall]

Never had a failed YD, but if I remember correctly: Ea YD has equal aerodynamic authority, approx 4 degrees, primarily for augmenting turn coordination when flaps are extended. So, "double duty" not required, as one operating YD is sufficient for all operations. [There is no A/P input to rudder system].

[Intruder]

There is A/P input to the rudder when in Approach mode below 1200', where it applies crosswind correction.

[parabellum]

Yes, as the auto pilot kicks in to the rudder system it is possible to feel it through the feet. Beware in the GA as when the modes change during clean-up the rudder reverts to manual control and if an engine has been lost considerable leg pressure may be required so be prepared.

[NSEU]

Thanks gents.

Just to clarify the autoland stuff... some sideslip (5 degrees) for crosswind correction is introduced at 500' (200' if the crosswinds are not so bad) by the a/p. Asymmetry (engine out) correction starts at 1,300' AGL.

I've been told that crosswind correction is removed during go-around, but not asymmetry correction (i.e. until GA mode is replaced, as, I think, Parabellum is saying)

Rgds.
NSEU

[HarryMann]

Only one answer even referrring to yaw damping... no answers yet

I suspect the answer is NO, and you'll end up with a bit of dutch roll during cruise, very much doubt the amplitude would be doubled.
You might even need to consult Boeing directly on this one

[NSEU]

Lots of information to take in here... may take me a while.

Interestingly, in the DDG for the Yaw Damper (Upper or Lower), there are no notes for pilots, suggesting normal ops (?)

Cheers.
NSEU

[747MC]

-400 Yaw Damp

Quote - AMM - "The yaw damper provides damping via the upper and lower rudders to eliminate dutch roll. When the airplane is rolled into a turn, a yawing moment is generated by changing lift vectors due to the changes in relative wind. This moment produces a forward component (reduced drag) on the downgoing wing and a rearward component (increased drag) on the upgoing wing. The result of the yaw moments produce yaw opposite to the intended turn direction. Rudder deflections to counter roll induced yaw are required to achieve a coordinated turn.

The yaw damper modules provide self-tests, full flight regime yaw damping control, and turn coordination. Yaw correction and turn coordination are computed from yaw rate, lateral acceleration, indicated angle of attack, roll angle, roll rate, and true airspeed

Each yaw damper is limited to ± 1 degree of rudder, whereas the total output command of the control law from each yaw damper module may never exceed ± 4 degrees rudder. The authority is further limited as the number of valid ADC and IRU inputs decreases."

The simple answer to your question is each YD system is independent of the other, and has the ability to entirely control the yaw, as a single system and simplify a co-ordinated turn. together they work together to reduce the input that would be singularly required by the other - either upper or lower. Due to the surface area and moment arm the deflection of either is not linear ... only applied to null the forces on read on the the combined sensors.

As far as the ANC03IA001 event is concerned the yaw dampner itself had nothing to do with the failure it was a failure of the PCM " rudder power control module manifold fractured, allowing the yaw damper piston to travel beyond its normal position. This resulted in a full left command input to the main control valve. The main control valve drove the rudder to the full left position. The only association to the YD system is that is contained in the same housing as the PCM and fine tunes the output of the main piston actuators to add or subtract to the input according to the inputs above. As a direct result of this incident an AD was issued on the PCM assembly.

[tie domi]

a/p yaw dampners

bELOW 1300 FEET WHEN IN LAND MODE,IE ALL 3 A/P ENGAGED RUDDERS CONTROLLED BY A/P WHEN IN G/A SAME APPLIES UNTIL ANOTHER NON LAND MODE, LNAV/VNAV,HDG,V/S IS ENGAGED.SEE A/P IN FCOM UNDER LAND 3/2 INFORMATION

[Rainboe]

I suspect the answer is NO, and you'll end up with a bit of dutch roll during cruise, very much doubt the amplitude would be doubled.

I'm sure I've flown the 747 with an unserviceable yaw damper system, and more than once. Absolutely no evidence of Dutch Roll in the cruise. The yaw damper system makes co-ordinated turns more elegant and comfortable, but it is not a Dutch roll-prone aeroplane. The VC10 could do some amazing DRs, with the whole tailplane violently waggling- it was quite surreal observing the tail through the aft overhead periscope.

[Pitch Up Authority]

Do they communicate with eachother

Hello

I was wondering if the operational YD gets some info that the other has failed?

If not it looks that, being a computer, it will try to execute its program within its/the normal limitations including the validity of its inputs.

That may not be enough, hence different crosswind limitations. But these are no different in case the upper or lower has failed.

[Intruder]

I just flew a flight in a 747 Classic with a failed upper yaw damper. There was no dutch roll at altitude. There is no restriction on Cat II approaches, and indeed I had to fly one into ANC.

I don't know how much rudder deflection it used, but it was enough to fully control the airplane without any perceptible difference in performance.

[Foo Dog]

This doesn't answer the original question but it does provide some insight to the overall topic.

I found this in D. P. Davies, "HANDLING THE BIG JETS" page 261 in the paragraph under stability...

"There is no significant dutch roll in any flight regime and while yaw dampers are fitted they are not required for despatch".

For an multi-autopilot (engine out go-around), yaw is initially controlled by the autopilots. Manual rudder input must be applied when selecting another roll mode, pitch mode, or when altitude capature occurs above 400 feet AGL because the autopilot reverts back to single-autopilot operation.

Hope this helps.

[Jumbo Driver]

I agree with Rainboe and others (above) in that I don't believe Dutch Roll is a factor in either the Classic or 744 airframes. I have flown the Classic with one Yaw Damper u/s on more than one occasion and experienced no adverse handling characteristics whatever. I have to hand an elderly copy of the the DDM for the Classic and it says that one Yaw Damper may be inoperative without restriction and adds ...

NOTE: Although flight control without Yaw Dampers is satisfactory, the structural substantiation of the aircraft requires one to be serviceable.

Furthermore, it lists Yaw Dampers under Automatic Flight (22) rather than under Flight Controls (27), which reinforces the supposition that Yaw Dampers are there on the 747 more to balance normal turning flight than to counter any Dutch Roll tendency, as in the VC10.

With this in mind, and with regard to the original question, I would suggest that the feedback loop in the yaw damper demand circuit would simply apply sufficient yaw to keep the aircraft in balance, irrespective of the the number of rudder surfaces actually being deflected. Therefore, in single Yaw Damper operation, the aircraft would still remain in balance by virtue of this feedback signal to the remaining Yaw Damper and not because of any wiffle-tree or other similar mechanical arrangement (as in the VC10 Feel System).

HTH

JD