The    initial  upset:

Discrepant-Rudder,     of a mere 2-degrees,    excited   a roll-rate   of 6 degrees/second

AirAsia  QZ8501 /  28Dec2014,  A320,   PK-AXC

Another  recent  military mishap report    documented  Discrepant Rudder   inputs  from a faulty   Yaw Damper:  


The dynamic-stability  of the KC-135R  was  upset,

   after  an unrecognized  failure-mode,  

        which excited  this    "hunting rudder"   response.


"Shell-77"  / 3May2013,   KC-135R,   63-8877,

       discrepant-rudder:   Dutch Roll,   LoC-I,     Breakup

Board's Rpt, 64-pages: 

TWA841's  Initial Upset:  

Flight Data Recorder  Heading-trace,  

sudden  changes,                             

recording   the   YAW  to the right.     

The apparent step-changes  were   recorded, documented,  by the Data Recorder.  

Experienced investigators had seen this before:  an artifact of the  friction,  of the heading stylus inscribing  on the metal foil.

An earlier lesson from the AA Flt One investigation,  regarding   FDR's  HDG- trace   during initial-upset.

The investigation of   sudden aberrations,   almost step-changes,  in the FDR's  recorded  Mag-HDG   were   found  on the foil for the AA Flight One  initial-upset, 1Mar62  (see CAB's AAR for AA Flt One):


​ "...absolutely precise  time  correlation of the four traces was impossible  and    errors of one to two seconds between them exist.    An additional error   is introduced by the   friction and play in the recorder,     resulting in short period  (up to two seconds)   aberrations of  ...  heading traces   from the smoothly varying changes   made by the airplane.     Characteristic of these are the numerous steps   . . .      Although  ...  headings   are usually expressed in terms of exact values  throughout the flight recorder discussion,   it must be understood that they are approximate but generally accurate to within plus or minus one second...."

[CAB's Report on AA Flt One,  pg 10.]



"... Starting  at time 1008:07  the heading trace   began to record changes   the significance  of which cannot be precisely defined.      From this point in time,  therefore,   the recorded heading changes will be described but the possible meanings to be   ascribed to them,   except where the   reason for a change is clear,  will be discussed under analysis...."  

 [CAB's Report on AA Flt One,  pg 14.]

     The INITIAL Upset:

             FDR  Mag'  HEADING

                    -- the Yaw Right,

                                                     and then the slope-reversal.

The human-analyst chose these points,  a human's choice.  The times of the chosen pairs  were not coordinated between the different traces   (the points selected on the G-trace differ-in-time from the points selected on the Hdg-trace).


The FDR- raw-data, shows a "buzz"  (approximately oscillating +/- .04 in the G-trace,  that starts about five seconds before the first activity in the Mag-HDG  (the INITIAL  upset-yaw was preceded by the "buzz" in the G-trace).


The FDR- raw-data, shows a "buzz"  (approximately oscillating +/- .04 differences in the G-trace),  that starts at recorder time 23:23,  about five seconds before the first activity in the Mag' HDG-trace.    The changes in the HDG-samples at 23:30  appear as almost "step" changes  (suggesting the  FDR HDG-stylus' scribe on the foil is unrealistic or aberrant),   appearing as a perhaps   stiction-delayed   STYLUS jerk-response  to HDG-inputs  that had occurred over a longer time-interval.   

FDR --  Data Recorder,  raw-data

from   billy Hopper's     choice   of manual data-points


After reviewing several presentations  (various graphic-plots of time-history)  of the data  from the mishap- FDR,

            the most useful data are  those  raw X-Y pairs  sampled, and presented in the tables of raw data.

Human Factors Report, 

The Initial Upset

FL390,  smooth,  Crz Mach 0.8m,  


Failure - Interactions

Hand-on   Rudder Trim Knob

    installed-position-CHANGED:

        switch-proximity,  

         inadvertent  

            pilot-input of LEFT RUDDER Trim

In recent years,   the best  data  from such   Yaw x Roll = DIVE 

             discrepant-rudder    INTIAL  upset   comes from  JTSB's  investigation  of


 Air Nippon,  September 6, 2011,   B737-700,   JA16AN;

             Yaw x Roll upset    (LoC-I),

                       CRZ   41,000 Ft,   approximately 69 NM east of Kushimoto,

                  Wakayama Prefecture, Japan; ≈  22:49JST.


Data and images are included in the      JTSB    "Serious Incident Report"

More than a decade later,   during  B737  investigations of the 1990's,

      and   during reconsideration  of  the    B727-upset  of  TWA841,

           NTSB investigators  again  studied  the  FDR-Heading  excursions

 just  prior  to the   INITIAL  upset:     B737  7Sep1994  at PIT.          

     The Boeing-NTSB  Investigation:

        All the conflict was about  

                this  fraction  of one Minute,

                       less than  20-seconds   in time,

                             just PRIOR to initial upset.

   

The initial     "buzz",      "slight vibration",      "gentle buffet",       "shudder"  :  


  Buzz due to   beta-vibration  as the  discrepant rudder  produced an increasing angle of sideslip?


    Perhaps this "buzz"  was   instead  the    "popping sound"  
generated during engine compressor-stalls  excited with onset of sideslip-Beta?


     Or, perhaps due to a slight flutter of  
                               that RHS O/B Aileron  (after a fractured Hinge-Bolt)?
     
      Or,  perhaps partly a simple  spoiler-buzz  
                               as the Aileron-Spoiler Mixer  added Spoiler deflection on the LHS-Wing?

        Or,  was  the   "buffet"  due to the aerodynamic disturbance
                                  as  the L.E. Slats  suddenly deployed?????


   For investigators'  analysis  of   this    "buzz"  or  "shudder",
            the simple   onboard  Flight Data Recorder
                            offered only  its G-trace, that one stylus'  etching  on a slow moving foil.

      The investigators  also could use the FDR's  Heading-trace  to  confirm the reports
          from the pilot and some passengers,    "it went off to the right".

   .  .  .

    Excerpts from the DEPOSITION of the mishap Captain

                        NTSB Depositional Proceeding, April 12, 1979, at Inglewood, CA .