Mystery of Yaw Damper
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Td racer:
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...
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 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...
Last edited by Desert185; 2nd Sep 2013 at 11:21.
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Desert 185:
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.
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.
Last edited by aterpster; 2nd Sep 2013 at 12:33.
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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.
That's all we knew about them. We were pilots not avionics techs.
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leftofcentre:
Rigging and all that. The F/E went for EPR first, and other readings second, not throttle position.
Why didn't I ever think of that? 
Actually, the photo was quite old when I got my first scanner.
Why is engine #3 somewhat throttled back?
ps. i think you need to clean the scanner glass 
Actually, the photo was quite old when I got my first scanner.
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.
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.
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stilton:
Indeed is was.
Which means the -200 was less restrictive.
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.
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.
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.
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.
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A320 Dual FAC fault
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?
How does the A320 behave with no yaw damping? Do the other computers manage to prevent any serious dutch roll?
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Hi tom775257,
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)
Do the other computers manage to prevent any serious dutch roll?
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)
Why is engine #3 somewhat throttled back?
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ATERPSTER
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.
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.
(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.
Last edited by grounded27; 5th Sep 2013 at 22:28.
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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.
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.
Last edited by JammedStab; 5th Sep 2013 at 23:22.
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grounded27:
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.
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.
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.
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Yaw Damper
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.
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.
Last edited by Yankee Whisky; 6th Sep 2013 at 00:08.