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Some excerpts from the Pan Am 707 AOM indicate that the 707 can have one of two different yaw-damper systems, "series" and "parallel", with differing capabilities:

YAW DAMPER

The series yaw damper is always ON. On parallel yaw damper airplanes the yaw damper should be turned on when comfortably airborne (1000 ft. suggested). Remember to trim and release rudder pedal forces before engaging so that the damper has the proper neutral point.

FIRE

[...]

PHASE II

  1. Autopilot/Yaw Damper - OFF

    (Leave Series Yaw Damper - ON) Since rudder trim will be necessary, disengage parallel yaw damper/auto-pilot until any trimming of the aircraft is done. The aircraft can be trimmed with the series yaw damper ON. After the aircraft has been trimmed, the parallel yaw damper and the autopilot may be re-engaged. [...]

An FAA notice sent out following a 1969 707 crash also mentions this difference:

[...] EITHER OF THE FOLLOWING PILOT ACTIONS WILL IMMEDIATELY RELEASE THE TAB LOCK LEVER WHICH MECHANICALLY LINKS THE CONTROL TAB TO THE RUDDER PEDALS FOR MANUAL RUDDER OPERATION:

  1. TURNING OFF THE RUDDER POWER SWITCH.

  2. RETURN RUDDER PEDALS TO A NEUTRAL POSITION.

    NOTE: CAUTION MUST BE USED IN TURNING OFF THE RUDDER POWER SWITCH WITH THE RUDDER AT OR NEAR FULL DEFLECTION TO PREVENT A RAPID CHANGE IN RUDDER POSITION WHICH MAY ADVERSELY AFFECT AIRCRAFT CONTROL.

    NOTE: THESE CHARACTERISTICS APPLY ONLY TO AIRCRAFT HAVING THE SERIES YAW DAMPER. [FAA Notice 8430.115, reproduced in NTSB/AAR-70/08, Appendix G, p. iii (page 56 of the PDF); my emphasis.]

What is the difference between the series and parallel yaw-damper systems, and why do they have differing operational limitations?

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Background

  • Early 707s had only the parallel type
  • Later 707s had both the parallel and series (matching the quotes in the question)
  • The E-3 Sentry (derived from the 707) also has both
  • Later later 707s had only the series type
  • When both exist, the parallel is a backup.

Differences

enter image description here
Simplified diagram of a parallel system; source: NASA

The parallel type moves the control cables and thus the pedals. This makes the parallel work against a rudder input for example when an engine fails, and that's why it's turned off before trimming, and turned on when airborne (it would counter the pilot's yaw input when an engine fails during the takeoff run).

The series is in series and doesn't have the above issues.

The 707 series yaw damper and its centering issue during that accident

enter image description here
The schematic is from Boeing's patent; exact implementation may vary, but the principle remains

The gist of it is this: yaw rate (from a bob weight or rate gyro) is sent to a motor (56) that changes the length of the control rod (39) that controls the hydraulic actuator (3) moving the rudder. This lengthening/shortening of the rod is how the rudder action is not fed back to the cables (43), and also explains why when the powered rudder is turned off (manual reversion), the rudder needs to be recentered by force (as happened on that accident).


Interesting recollection:

Although only slowly divergent, the 707's Dutch rolling always needed a counteracting yaw damper, and even with that, it still gently weaved its way around the world. Later versions had a series unit which applied imperceptible corrections, but our early marks had a parallel damper, so the rudder pedals moved with its inputs. Because it would oppose any rudder applied to counteract the swing from a failed engine, it was only engaged after the flaps had been retracted on climb-out, and was disengaged in the event of engine failure or during the landing check on final approach. Many over-loaded neophyte co-pilots would respond to the landing checklist prompt of 'Yaw Damper' not with "Disconnected" but "Roger, my damper". (707jet.com)

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A serial system takes inputs, generates outputs, and there is no feedback to the input. A parallel system is a subset of a larger system, where the parallel system outputs also change the inputs. Similar to the inner/outer-loop story: a serial flight control system does its job without the pilot noticing any changed behaviour on the flight controls, while a parallel system does move the flight control, or changes the force.

The serial yaw damper would do its job, wit the only noticeable result being that the aeroplane behaved in a stable manner in the yaw axis. The parallel one would move the pedal trim position while operating.

When turning OFF rudder power while the serial yaw damper is still commanding a rudder deflection (unbeknownst to the pilot, since they are not aware of its output), there will be a large jolt on the pedals when they are suddenly loaded with airforces, resulting in a surge in yaw deflection.

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  • $\begingroup$ That is not what "series" and "parallel" usually mean; going by the normal definitions of the two terms, one would expect a parallel system to simply have a pair of yaw dampers acting in parallel (presumably for redundancy). $\endgroup$
    – Vikki
    Dec 25 '21 at 3:23
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    $\begingroup$ Yes then the yaw dampers are parallel to each other, and will be fighting each other when one has failed. The serial damper will always be at he output end of the preceeding system, most likely a hydraulic actuator. $\endgroup$
    – Koyovis
    Dec 25 '21 at 5:18

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