Failure - Interactions
An Engineer's Lesson:
eg, the KC-135's "SYD"
introduces another failure mode
"Shell 77" / 3May2013, KC-135R, 63-9977,
Board's Report: 64-pages, pdf= 3.4MB
See the USAF investigation of KC-135R, 63-9977, inflight-upset, slow onset Dutch Roll, progressed to LoC-I and inflight-breakup:
" . . . use of rudder, while in a Dutch roll, increased the aircraft's oscillatory instability. . . . ensuing large side-to-side movements . . . inadvertent fluctuations in rudder position. These fluctuating rudder movements, coupled with slight right rudder use . . . compounded the Dutch roll severity and produced extreme airframe stress that caused the KC-135's tail section to separate . . ."
Better: see the AW&ST review of this "Shell-77" accident --
. . . a malfunction with the aircraft's series yaw damper . . . crew identified the ... source ... but instead of turning the system off, they attempted to correct the yawing motion by using rudder and aileron inputs as well as turning on the autopilot, causing the oscillations to worsen.
Osborne, Tony. "Factors … Lack of training ... inexperience with 'Dutch roll' … "
Aviation Week & Space Technology (March 24, 2014).
Local Spokane KXLY-TV offered a detailed web-story describing the lessons from the mishap-investigation:
. . . an aircraft malfunction … aggravated by the autopilot and … by some crew inputs, but only because they didn't recognize that it was a Dutch roll … only … see it in a simulator and … the simulator doesn't do a very good job of presenting the Dutch roll …
Control of an airplane is complicated because . . . flight takes place in three-dimensional space. Fortunately, analysis is simplified by the mirror (or lateral) symmetry of the usual airplane. By virtue of this symmetry, the vehicle’s motions can be divided into two classes, longitudinal and lateral, that can be treated independently. Longitudinal motions take place in the plane of symmetry of the airplane; they include vertical and pitching motions such as can be brought about by deflection of the elevator.
Lateral motions act to displace the plane of symmetry; they consist of sideways and rolling and yawing motions, such as those caused by movement of the ailerons and rudder. The various lateral motions are interrelated in a complex way and are more difficult to understand and discuss than longitudinal ones. . . .
The over-all oscillation, known as "Dutch Roll",
results from the interaction of these
moments and forces.
The Dutch Roll is a combination of
yawing, rolling, and lateral displacement . . .
Epistemology of the term Dutch Roll in flight dynamics:
Bill Cook (p181) cites Hunsacker's early usage:
"... type of oscillation ... possibly derived from the fact that
Dutch ice skaters, as they negotiated the long, narrow canals,
typically would roll from the outside edge of one skate
to the outside edge of the other skate
in a series of linked, S turns . . ."
Hunsacker had spent his teen years in Michigan (another "Ghost of Saginaw"), so he was exposed to ice skating (and perhaps enjoyed our common-sense of "flying" on skates) -- but his airplane "Dutch Roll" analogy-metaphor to ice skating will be lost on most of us [we grew up skating for speed, with a stick in our hands]. Hunsacker's "outside" blade-edge metaphor misleads his reader (as un-natural or "not idiomatic") , since we prairie-grown skaters never used that lazybones- skating technique.
See the video demonstration of that
slow-speed "dutch roll" skating method.
There you have it: Hunsacker's ice-skating metaphor might still be salvaged, applied to airliner flight dynamics. But we will leave that chore for you writer-skaters. [At one very large airline it seemed they had hired PILOTS that included an entire hockey team from the USAF Academy -- a sort of northern counter-culture to contrast against their company's southern mindset].
Any future airplane- Dutch Roll skating analogy-metaphor might better focus on the lateral-force component used during skating, and during airliner Dutch Roll testing.
The "explanation" shown below appeared as Attachment "A",
near the end of the CAB's twelve-page Aircraft Accident Report.
Careful: this "explanation" of Dutch Roll mentions yaw and roll moments,
but it omitted any mention of the lateral-force component,
as cited in the Boeing Airliner (excerpt shown further down this page).
NO-Rudder technique for Dutch Roll Recovery
Dutch Roll recovery techniques evolved:
Prior to the Oso-upset-breakup-fire of 19Oct' 1959, Boeing had published their suggestions for Dutch Roll recovery in the Boeing Airliner, June 1959.
Russel H. Baum, age 32, at Boeing since mid-1957,
total Flight Hrs= 5,015; time on B707 was 369-hrs.
Instructor-Pilot for training customer-pilots.
Boeing Airliner (February 1960) p6, Figure 5:
During rudder movement of 10 degrees to either side of neutral, the "dead spot" in the slide valve prevents ... boosting the rudder
In-progress: standby . . .
For both the N7071 / 19Oct59, and N7506A /1Mar62, the C.A.B.'s AAR's never define the most-recent configuration-modification of the mishap aircraft (omitted).
Never discussed in the AAR, nor elsewhere, was the Rudder-response DURING the divergent Dutch Roll of 19Oct59:
Note, deduced from their wording used in the Oso-AAR:
the Braniff B707-227, N7071, must then have had the "power-augmented rudder" described in the Boeing Airliner, Feb'1960, "707 Rudder Boost". That term "power-augmented rudder" was employed by Boeing in the July 1960 "Flight Improvement Program." Boeing Airliner. (July, 1960), 3-7.
Even with that first (hydraulic-) rudder-boost design, during oscillations of Dutch Roll, the Rudder deflections would VARY with the alternating sideslip-angle:
Rudder "floats" with the sideslip airloads-forces,
"That is, the rudder tends to trail to the downwind side during sideslip. . . ."
-- Even with the partial (hydraulic-) rudder boost installed and operational, the Rudder couldn't be "fixed" (deflection held at zero), because the hydraulic boost only activated after the Rudder DEFLECTION had exceeded 10-15 degrees (the central control region either side of neutral remained unboosted with normal control of Rudder handled by the TAB. The pilot could only "fix" his pedals, which only "fixed" the Rudder Control TAB.
-- If the Rudder "floats" during sideslip oscillations of Dutch Roll, or during sideslip of engine-out thrust asymmetry, then the mechanically-linked TAB would also "float" to a "balanced" tab-direction -- further exciting the Rudder to increased deflection -- a feedback interaction that would further excite the amplitude of the yaw-roll excursion.
Boeing Airliner. Feb'1960 "707 Rudder Boost",
Figure 5: "During rudder movement of 10 degrees to either side of neutral, the dead spot in the slide valve prevents the ... PCU from boosting the rudder...."
A "powered", "irreversible" control surface:
the Rudder is termed "fixed" (won't float during Beta-excursions of Dutch Roll nor sudden engine-out).
The various modifications, Tip-Extension of the Fin, and full-range boosted-Rudder, and Yaw Damper, provided B707-pilots with a more effective Fin-Rudder.
The C.A.B.'s Aircraft Accident Report for the upset of N7071 does NOT include any mention of any Yaw Damper:
This early B707-227, N7071, had the early "parallel" Yaw Damper installed [an inference from Boeing Airliner, June'59].
The term "yaw damper" never appears in the CAB's Oso-AAR, never mentioned: On one hand that omission suggested that thus the yaw damper was NOT deactivated, no yaw damper, NOT INSTALLED. However, in contradiction, there appeared an earlier Boeing Airliner, June'1959, p3-7, "707 Yaw Damper Operation" -- which cites the "yaw damper" as part of the basic configuration for B707-227's.
The C.A.B.'s Aircraft Accident Report for N7071 does mention a "rudder boost" -- but that brief mention is misleading: The initial hydraulic-boost design, for that early B707-227 rudder, ONLY ACTIVATED hydraulic assist with Rudder-travel beyond 10°. Essentially, during the sideslip excursions of Dutch Roll, even if the pilot held his Pedals "fixed" (pedals linked to only to the Control Tab), the Rudder itself was "free" to float, or trail, during the oscillating β -excursions.
Story from _Seattle_Times_, Friday, Nov'20th, 1959, p10.