Mystery of Yaw Damper

[Dreamlinerwannabe]

I have heard of this term for awhile and I never know how does it look like.

I then go to google, made me even more confused.

yaw damper - Google Search (http://www.google.com.hk/search?hl=en&site=imghp&tbm=isch&source=hp&biw=1920&bih=937&q=yaw+damper&oq=yaw+damper&gs_l=img.3..0j0i24l9.1546.2886.0.3028.10.8.0.2.2.0.37.218.8. 8.0....0...1ac.1.26.img..0.10.224.3tIhBPZ_vqg)

When I input Yaw Damper on google search for images,

I see whole light aircraft, whole big jet, instrument panel....

So my question is, how does yaw damper look like? Which picture is true?

Thanks!

[de facto]

Hi there,

First picture is a B737 yaw damper.

[Rick777]

They are probably all true. The yaw damper is a piece of equipment that lives in the tail section of a lot of aircraft and may be mechanical or hydraulic operated. All the pilot will ever see of it is the switch to turn it on, and some planes don't have a switch.

[717tech]

Some aircraft claim to have one (Dash-8 200/300) but I don't think it actually does a thing!

[LimaFoxTango]

Some aircraft claim to have one (Dash-8 200/300) but I don't think it actually does a thing!

Au contraire, try flying with it off and you'll see.

[aterpster]

727 had two of them. Turn them both off at high altitude and you could very well roll over on your back.

[Rick777]

The KC135 didn't have one and neither did some 707s. They had a rudder axis on the auto pilot which usually worked fine, but had to be turned off for take off and landing. Always funny to watch new guys Dutch rolling down final.

[fantom]

Goodness me! Dutch roll/ yaw damper. Do pilots know anything these days?

[aterpster]

Rick777:

My company's early 707s had a part-time yaw damper that had to be off for takeoff and landing. Perhaps mid-way, or earlier, the deliveries changed to a full-time "series" yaw damper. At that point in production the anti-skid brakes changed from those awful thumping (can't recall the name) anti-skid to full modulating anti-skid.

[tdracer]

Large, low wing commercial aircraft tend to have nasty Dutch Roll characteristics (high wing aircraft are generally much better). While there are some design characteristics that can reduce Dutch Roll, for the most part they tend to make other aspects of aircraft handling/control worse.....

The exception is the yaw damper - a properly functioning yaw damper makes Dutch Roll a non-issue. A yaw damper is pretty much what it sounds like - an automated system that moves the rudder automatically to address oscillatory yaw behavior.

I've always heard the 707 Dutch Roll behavior was really bad (with regard to what aterpster wrote - I hadn't heard the 727 was as bad or worse than the 707 - not disagreeing, just relating Boeing tribal lore).

The last fatal Boeing Commercial flight test crash was a 707 back in 1959. Boeing had introduced a new 707 yaw damper, and the customer pilot was doing his best to test it out by making large control inputs to induce Dutch Roll. One of the inputs was so severe that the subsequent recovery resulted in engines 1, 2, and 4 departing the aircraft :sad:. The loss of three engines and associated damage was such that they couldn't make it back to an airport and had to set it down in a field north of Seattle. Four of the eight occupants didn't survive. The story as I heard it was, the pilots and flight engineer stayed in the flight deck, while the other occupants braced for the crash in the back - those in the flight deck perished while those in the back survived.

ASN Aircraft accident Boeing 707-227 N7071 Arlington, WA (http://www.aviation-safety.net/database/record.php?id=19591019-0)

[TLB]

Had one of those on the old CF-101 Voodoo. Part of the Automatic Flight Control System, it had a separate ON/OFF switch and performed two functions:

1. if you manually applied a bunch of rudder, say to the left, and then let go & centralized the rudder, the aircraft would yaw back the other way (to the right) and after one cycle the yaw damper would in fact dampen out all the yaw instead of yawing back & forth until the aircraft stability would eventually zero out the yaw;

2. more importantly, it would automatically counter adverse yaw effects. In other words, if you want to turn left, you input left aileron and the aircraft starts to roll to the left, but it will also yaw in the opposite direction (yaw right in this case). The yaw damper would recognize this and automatically input left rudder to counter the right yaw. You could keep your feet off the rudders and roll left or right and the yaw damper would always keep the ‘ball in the centre’.

[aterpster]

tdracer:

I've always heard the 707 Dutch Roll behavior was really bad (with regard to what aterpster wrote - I hadn't heard the 727 was as bad or worse than the 707 - not disagreeing, just relating Boeing tribal lore).

The fact Boing had to split the rudder on the 727 so it could have two yaw dampers should tell you Boeing fact from Boeing lore. :)

[grounded27]

Yaw Damp in modern aircraft is a calculation in the FCC that constantly corrects rudder position, if you saw the rudder in flight it would look like it was fluttering constantly. Even in the 727 yaw damp is more or less a function of the auto pilot. If I remember older systems were less rate/gyro driven but simply tightened up rudder authority with airspeed hence the above statement about switching off on the 707 for t/o and landing.

[aterpster]

grounded:

Yaw Damp in modern aircraft is a calculation in the FCC that constantly corrects rudder position, if you saw the rudder in flight it would look like it was fluttering constantly. Even in the 727 yaw damp is more or less a function of the auto pilot.

The 727 didn't have a FCC, rather a lousy two-axis Sperry auto-pilot. The two yaw dampers were independent of that lousy auto-pilot in all respects.

[Rick777]

I never flew a 727 with yaw damper off, but the sim was a lot harder to recover than the 707. Or maybe it was too long flying new airplanes. I went from A320 to left seat 727. Just wanted to say I flew it.

[Piper19]

To the original message; none of the google results show a yaw damper. The first picture tells you about the 737 yaw damper, but shows one of the PCUs I think. The yaw damper is in that same area. It depends from aircraft to aircraft how big it is, but it's basically not 1 item. Older aircraft tend to have more mechanical linkages whereas modern aircraft calculate the amount of rudder deflection in computers. The yaw damper is closely connected mechanically to pilots pedals, rudder ratio changer, autopilot actuators and the trim/neutral offset cam. Rods and springs make sure that e.g. the rudder pedals do not move when yaw damper makes inputs etc...
I've uploaded a technical drawing for you
Image - TinyPic - Free Image Hosting, Photo Sharing & Video Hosting (http://tinypic.com/r/34y8ynb/5)

[doubleu-anker]

Yes, of course, the yaw damper controlled rudder input. If you lost the Y/D the book said attempt to control the Dutch roll with aileron/spoiler input. However if you knew when to manually apply rudder input, to control the D/R, it was far more effective than fighting it with ailerons/spoilers. Had to "know what you were doing" though, or didn't even think about it. With A/P engaged the clue for rudder input, was in the control wheel displacement. Taking lag into consideration, the rudder input was applied when the control wheel was in the exact opposite position (on the A/C I used to operate) as the handling pilot would initially expect.

[stilton]

I don't agree that Boeing had a split Rudder on the B727 so they could have two yaw dampers (or dampeners as some of our southern friends called them)


The 757 /767 has two yaw dampers but does not have a split rudder.


I think it was more for redundancy of having two separate powered rudders on the 727.


I do agree, after seven years of flying that lovely Aircraft that you don't want to go above 25000 feet without at least one Yaw Damper and without any it had very unforgiving dutch roll.

[J.L.Seagull]

To the original poster... and to those looking for a chunk of metal.. i.e. an actual component called the "Yaw Damper"... IT DOESN'T EXIST! (At least not on the A330, A340, B777 and all modern aircraft.)

As grounded27 mentioned, Yaw Damping on FBW aircraft is a software function of the of the PFCS. Using inertial references, the primary flight control computers calculate an opposing input and feed it to the rudder servos.

A330s and A340s have a BYDU (back-up yaw damper). If the PRIMs fail, hydraulic fluid (B&Y) enters the BYDU, drives a generator to supply electricity for a small computational unit and a set of gyros. Signal output then commands a hydraulic actuator inside the BYDU to give inputs to the rudder servos input system, thus driving the rudder to provide a yaw damping function.

[Leftofcentre2009]

The pictures of the actual mechanical Yaw Dampers that you see in your search results are off railway vehicles.

The mechanics of it are similar to a motor car shock absorber in that it is a cylinder and piston filled with oil. It is connected to the rail vehicle chassis/body at one end and the wheel bogie at the other end and prevents the bogie from "shimmying" at high speed.

In the UK, rail regulations stipulate that trains must have them fitted if they travel faster than 75mph.

Now back to aviation . . .

[Desert185]

Td racer:I've always heard the 707 Dutch Roll behavior was really bad (with regard to what aterpster wrote - I hadn't heard the 727 was as bad or worse than the 707 - not disagreeing, just relating Boeing tribal lore).

Never flew the 707. The DC-8, by comparison, has one rudder and one YD. The YD can be deferred for dispatch with no speed or altitude limits.

The 727, on the other hand, was a dutch rolling animal. A YD on each of the two rudders cured the problem, but there were limitations if one was inop, and as I recall, no flight if both inop.

The gentle-giant 747 has one on each rudder, but only requires one for dispatch.

A company I worked for who had a survey Twin Otter added a YD to stabilize the platform for survey work. The aircraft would move around a bit in yaw while in turbulence and hurt the data. It didn't seem to have enough muscle to do the job and would eventually result in a skid. We usually turned it off, as they couldn't resolve the problem.

For the OP, I couldn't find a picture, but the one on the Twin Otter looked like a DG compass gyro. If that helps...

[aterpster]

Desert 185:

The 727, on the other hand, was a dutch rolling animal. A YD on each of the two rudders cured the problem, but there were limitations if one was inop, and as I recall, no flight if both inop.

Yes, immediately descend to something on the order of not higher than FL 260 and observe a speed limit schedule. This was certification stuff that left no discretion.

[aterpster]

This is a lousy scan of a 727 in flight. I have underlined the two black guarded yaw damper switches.

That's all we knew about them. We were pilots not avionics techs. :)

http://i201.photobucket.com/albums/aa214/aterpster/727dampers_zps2aecdda3.jpg

[Leftofcentre2009]

Why is engine #3 somewhat throttled back?

ps. i think you need to clean the scanner glass :8

[aterpster]

leftofcentre:

Why is engine #3 somewhat throttled back?

Rigging and all that. The F/E went for EPR first, and other readings second, not throttle position.

ps. i think you need to clean the scanner glass http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/nerd.gif

Why didn't I ever think of that? :oh:

Actually, the photo was quite old when I got my first scanner.

[stilton]

Looks like an old -100 Cockpit.


There was a list of speed / altitude restrictions to comply with if you had both yaw dampers inop.


They were different depending on whether you were in a -100 or -200 (the -100 was less restrictive)


If you descended to 25000 feet you could operate up to 350 kias in a -200.

[aterpster]

stilton:

Looks like an old -100 Cockpit.

Indeed is was.


There was a list of speed / altitude restrictions to comply with if you had both yaw dampers inop.


They were different depending on whether you were in a -100 or -200 (the -100 was less restrictive)


If you descended to 25000 feet you could operate up to 350 kias in a -200.

Which means the -200 was less restrictive.

[lynn789]

can the yaw damper operate on any je t airliner when its gliding or must the dutch roll be accepted?

[Chris Scott]

The VC10 has a pronounced Dutch roll tendency at medium and high altitudes, whereas - IIRC - the B707 seems more likely to do it on the approach, no doubt often with an element of PIO involved (for the newbees, that's pilot-induced-oscillation).

Although yaw dampers always make small, quick inputs to the rudder to correct incipient Dutch roll (which presumably starts with yaw), the recovery technique pilots use for recovering a developed one involves aileron, NOT rudder. On the VC10 at about FL 200, the trainer used to switch off the yaw dampers, induce the Dutch roll (can't remember how) allow the roll to develop to about 40 deg each way, and then tell the trainee to recover using full aileron/spoiler against the direction of roll just before the attitude passed zero bank. That worked well.

The VC10 has 3 rudders, each with its own yaw damper. Turning any of them back on stopped the Dutch roll in a couple of seconds.

Yes, the early B707s had a parallel yaw damper on the single rudder, which had to be turned off for T/O and landing. It wasn't very good, and the rudder pedals moved to reflect its inputs. I remember passengering in G-AYSI, an early dash-320, from Gatwick to Nairobi and she Dutch-rolled gently all the way there. Later B707s have a series yaw damper (like the VC10 and BAC 1-11), which works well and doesn't move the rudder pedals.

[stilton]

I stand corrected Aterpster you are quite right the -200 was less restrictive !

[tom775257]

A question for people. Regarding the A320, with a dual FAC fautl (or YAW DAMPER SYS), you are told yaw damping inop. There are no indepth notes about this that I can find.

How does the A320 behave with no yaw damping? Do the other computers manage to prevent any serious dutch roll?

[rudderrudderrat]

Hi tom775257,
Do the other computers manage to prevent any serious dutch roll? Affirm.
The FBW roll computers will damp any unwanted roll rate before the roll could develop from the yaw oscillations. (In the same way Chris Scott describes recovery)

[tom775257]

rudderrudderrat: Many thanks for the information.

[TURIN]

Why is engine #3 somewhat throttled back?



Getting close to a trim run required methinks. 737-200 was allowed about one full knob difference, dunno about the 727 never worked that one. Similar engines though.

[flyboyike]

That's why FADEC is nice. No knob differences.

[grounded27]

The 727 didn't have a FCC, rather a lousy two-axis Sperry auto-pilot. The two yaw dampers were independent of that lousy auto-pilot in all respects.

I never said the 727 had FCC's.



Strait out of the B-722 SP-150 Maintenance Manual.

B.The autopilot and yaw damper system, hereafter referred to as the autopilot, provides automatic control in yaw, roll and pitch axes. Each axis is connected electromechanically to the hydraulic power units of the flight control system.

(1)Control of the yaw axis is by a dual yaw damper system, which is shown in (figure) and consists of the following: upper and lower yaw damper couplers, related rudder power units, rudders, rudder position sensors, engage switches, a yaw damper ground test switch, an overall autopilot ground test switch, an indicator that displays dual rudder position and system engagement, and relays located in an autopilot accessory unit. An air data computer provides airspeed modulated attenuation of yaw damper rate gyro error signals.


(2)Engagement of the system yaw damper stabilizes the airplane by using yaw rate error signals characteristic of the natural yaw frequency of the airplane. These rate signals, when processed through the system, individually deflect the two rudders a maximum of ± 5 degrees in opposition to yaw rate error. Yaw damper interlocks within the autopilot require engaging either yaw damper prior to engaging the autopilot. Normally, both couplers are engaged when airplane power is applied since the engage switches are lock guarded to ON.


(3)The pilot may yaw the airplane by rudder pedal action when the yaw damper system is engaged without feeling excessive pedal opposition from the system. This is possible since the two inputs to the rudder power units (manual rudder pedals and couplers) are independent and neither input is fed back into the other. Thus, the airplane may be manually yaw maneuvered in a normal manner without input opposition from the yaw damper system. Simultaneous dual inputs of manual and coupler yaw cammands are algebraically summed within the rudder power unit to produce the required rudder displacement.






Granted is no IRU/FCC set up but it is designed on the same but crude principals. Not the Hydro-mechanical Damper mentioned before.

[JammedStab]

We intentionally turned off both yaw dampers in the 727 once. As part of a post maintenance inspection. I can't remember the exact altitude but it was above FL300.

Initial reaction in the smooth air was no effect. Then a small amount of rudder input was made and as expected, the yawing started. not really bad but it was there. No doubt, the greater the input, the greater the motion would be.

The procedure for maintaining control was to avoid any further rudder input and use sharp, properly timed aileron inputs to minimize the oscillation. Choose a particular wing as your reference. When that wing starts rising from its maximum amount of downward displacement, initiate the quick, aileron input and return to neutral. But while the oscillations were controllable, they did continue. Just like in the sim. Make sure to use the chosen wing as your reference in your mind. Upgoing right wing means the right side of your artificial horizon line is going down. It is easy to mix up and make the wrong input.

What would also happen is while initially, you might have say 20° of bank either side of neutral with neutral being wings level, that neutral position would move to one side or the other, so now there might still be 20° banks happening but on either side of a new neutral point of, for example, 10° left bank. Weird.

The true initial effective measure to counteract this phenomenon in the 727-200 was to use speedbrake. Problem solved it seemed to me.

[aterpster]

grounded27:

Yaw Damp in modern aircraft is a calculation in the FCC that constantly corrects rudder position, if you saw the rudder in flight it would look like it was fluttering constantly. Even in the 727 yaw damp is more or less a function of the auto pilot. If I remember older systems were less rate/gyro driven but simply tightened up rudder authority with airspeed hence the above statement about switching off on the 707 for t/o and landing.

Your latest post referencing the SP-150 seems to indicate it and the two yaw dampers were completely separate. That is how we were trained in any case. We could dispatch with an inop SP-150 but both yaw dampers had to be operating.

BTW, our last 10 727-200s were the "advanced" model with, I believe, the Sperry 250. It would do a poor man's CAT three quasi-auto-land. It would flare but not de-crab nor provide roll-out guidance. We were trained on it in the sim but no one in his right mind would actually use it. :)

[Yankee Whisky]

Had an opportunity once (when we were still "honest" citizens) when I was allowed to visit the flight deck because the Captain was a club friend of mine. I asked the question about the infamous "Dutch roll" and he let me fly the aircraft with the damper off. It did not take long for me to become "out of sinc" with the yawing and the aircraft would have increased the yawing until a nasty roll would result. It takes some hours of experience for a pilot hand flying an aircraft until he can anticipate the motions and make corrections in sync. I compare it somewhat to being towed behind a power plane in a glider.:) I saw that yawing may also have been reduced because of only using the inboard "flapperons" instead of outboard ailerons like in the old days. I could be wrong, but it seems logical to not cause drag on the outboard wing section, which only adds to increase the Dutch roll.

[flyboyike]

Good post, Yankee Whisky. Nice amount of nostalgia, just the right bit of jargon, good stuff.

[grounded27]

BTW, our last 10 727-200s were the "advanced" model with, I believe, the Sperry 250. It would do a poor man's CAT three quasi-auto-land. It would flare but not de-crab nor provide roll-out guidance. We were trained on it in the sim but no one in his right mind would actually use it. http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/smile.gif

I worked a very large fleet of dual mode SP-150 722's that were just about all maintained to CATIII A minima. They were reliable, the most common problem with the 727 A/P was a result of elevator position sensors, caused alot of disconnects during autoland logic, porpusing, disconnects et cetera. I believe the a/b mode SP-150 would likely be the advanced mode you are thinking of.

[Chris Scott]

Quote from JammedStab:
"Upgoing right wing means the right side of your artificial horizon line is going down. It is easy to mix up and make the wrong input."

True. Further to my earlier post about recovery on the VC10, my previous (older) types had had A/Hs on which the bank indicator pointer was at the bottom of the instrument, whereas the VC10's followed the later practice of incorporating a skid/slip bubble-indicator at the bottom, displacing the bank indicator to the top. That is less intuitive, IMO, and can create confusion. It could easily have led me to apply aileron/roll-spoiler in the wrong direction. Not a good idea... Has anyone else ever had that problem?

[FE Hoppy]

It could easily have led me to apply aileron/roll-spoiler in the wrong direction. Not a good idea... Has anyone else ever had that problem?

See it all the time in the sim on initial type ratings. It's a major learning point for many. We see it with both EDS/EFIS and HUD. It's easily corrected by learning to look at the centre of the ADI first and only use the sky pointer for fine tuning.

[Pugilistic Animus]

Simply read Handling the Big Jets....:)

[Chris Scott]

Quite. No doubt Dai Davies would be turning in his grave (assuming he is dead?) on reading most of our stuff.

I now find that he is yet to get a Wiki entry, despite that book - and being responsible for the redesign of the early B707's empennage (ventral fin added) to improve directional stability.

[Chris Scott]

This may be old-hat to the afficionados, but I've found a well-researched article on the 'net, originally published in a magazine called "Airways" (?) in 2010, here courtesy of the Renton (Washington, US) website:

http://rentonwa.gov/uploadedFiles/Living/PBPW/AIRPORT/04_Public_Relations/BOAC%20and%20the%20Rolls%20Royce%20-%20Davis%20combined.pdf

Compare the fins (VSs) on the photos on pages 32 & 33. The account of the British certification process that led to the modifications starts on the right column of page 35.

I hadn't realised that the ventral fin retrofitted to those early models was dual-purpose - it also stopped the a/c over-rotating on take-off, in the absence of a suitable stick-shaker for that purpose. ** The other part of the B707 "fix", the top extension, is clearly visible when you compare the two photos.

The B707-320B/C "Advanced" types, such as I flew in Caledonian/BCAL, had dispensed with the ventral fin, apparently for the reasons stated on page 36 (yaw-damper and a stick-shaker).

OFF-TOPIC
Although the stick-shaker was supposed to prevent over-rotation on T/O, I once over-rotated (and probably rotated at too-high a rate) on a MTOW departure at LAX, and had to check forward slightly before the a/c would unstick.

** The British certification authority, the ARB, had pioneered the test of taking-off with the tail scraping along the runway, and the concept of Vmu (minimum unstick speed), as a result of accidents on the Comet 1.

[yanrair]

Maxarets? Think that is the name.

[FE Hoppy]

Maxarets are mechanical anti skid devices.

[Chu Chu]

I ran across an incident report about a 737 yaw damper behaving badly:

Air Accidents Investigation: 1/1998 G-BGJI (http://www.aaib.gov.uk/publications/formal_reports/1_1998_g_bgji.cfm)

[DozyWannabe]

Quite. No doubt Dai Davies would be turning in his grave (assuming he is dead?) on reading most of our stuff.

I'm not so sure. His MO always seemed to be to make sure that the engineers got things as right as possible so that pilots would only have to use their superior skills in seriously abnormal situations. That said, the fact that he wrote HTBJ implies that he felt pilots should have a deeper understanding of what advanced aircraft were doing!

I now find that he is yet to get a Wiki entry, despite that book - and being responsible for the redesign of the early B707's empennage (ventral fin added) to improve directional stability.

Same for his colleague (and later successor) Gordon Corps - who was instrumental in the Concorde minimanche testing and development programme, and later, the A320 flight control system design and development. I've long said that if more people knew about him then there would be far less misunderstanding of the impetus behind that system. I've also long held that one of the biggest tragedies in aviation was his premature death from altitude sickness before he had the chance to write *his* book (which could have had the potential to do for line pilot understanding of modern digital FBW systems what HTBJ did for swept-wing jets).

Early yaw dampers, stick-shakers and stick-pushers were essentially the electro/hydro-mechanical forebears to the modern flight control systems with envelope protections.

[Grunff]

Here is rather self-explaining video of what happens when you disconnect yaw damper(s). It's made by Dutch students...doing, errr... Dutch roll... PH-LAB (4/7): Dutch Roll - YouTube