Modification center,   at the old B-17 factory at Plant-2,   included  the  Fin-Tip Extension.

The various Fin-Rudder modifications continued through the next decade,  with the Rudder-TAB  rotation  redesigned to  an  Anti-Balance function, with various "boost"  iterations  [from the initial manual-Tab  to a "power augmented rudder"  then later a "full range boosted Rudder"] ,  and  various iterations of  Rudder-Tab ratios.  The artificial stability was added:  first effort was the  "parallel" Yaw Damper,  then after AA Flt-One,  a redesigned  "series"  Yaw Damper.

But what was the real cause of

all the   loss of control   training accidents?      

Investigators of the 1960's, '70's, early '80's,

  would   have you believe   that it was

             simply a case of pilot error.

As we now know it [training mishap fatalities]

were a result of poorly designed and

  improperly certificated airplanes ...

attempting to fly with thrust-asymmetry

[engine failure] at too low speeds.   

This hindsight conclusion is

not meant to cast any aspersions on anyone . . .

but only to put into  perspective

the evolution of the design of jet transports. . . .

To understand this, it is necessary to review

    just how the aircraft were certificated

​          for  minimum control speed.

Marthinsen, H.F.  "Engine-Out Flight Training Revisited". Presented at ISASI's  13th Int'l Seminar, Tel Aviv, Oct' 11-15, 1982; ISASI Proceedings 1982,  p46-63. 
B-707 : evolving Fin-Rudder iterations

By the first week of February 1960, 

   UK's  ARB had  withheld certification of B-707;

     more than a year before the C.A.B.'s accident report was adopted,

          the manufacturer   initiated   Fin-Rudder modifications:


Fin-Tip   Extension,

                               and    the   hydraulic-boost,

                                                              with an   anti-balance   Tab.

The initial B707  Rudder-design was  for

             a manual Tab,       a "balance" Tab:       no hydraulic-boost.  

     The Rudder Pedals were connected to   the Tab,  the Tab   then flew the Rudder.  

    Initial Fin-Rudder  design:   

                  a shorter-height  VertStab,    Rudder --  manual,   spring- Tab     [no hydraulic boost].

  The early   B-707 Rudder   was operated only by   aerodynamic forces   on  the TAB  and

       by      differential  pressures   acting  on

             the    BALANCE PANELS    ahead of   the   rudder Hinge Line  on  the fin. 

    Per _Flight_,   25July58,   p145:  

  "Flying Controls  Internal balance platesaerodynamic tabs . . . are the basis of

    the Boeing 707's  mainly manual control system,   which also provides proportional feel,

       but   rudder boost   is  used on   the 707-220   and   Intercontinental  models

            for   maximum   rudder  deflection   in  the   low-speed,  engine-out   case...."

 public  hearing   was ordered by the Board  and   held in two phases

 The   operational   phase     of   the investigation   was held

       at the Henry Perkins Hotel,   Riverhead,   Long Island,   New York,

              on  August 27, 1959

 The   technical   phase   of   the investigation   was  held at

         the   Forest Hills Inn,   Forest Hills,  Long Island,   New York,

​                     October 7, 1959.

Press story reported on  first day of Public Hearing, Thursday, August 27th 1959:

        Thrust asymmetry   yaw,     roll.

Failure - Interactions

B-707  Service Bulletin     dated   Feb' 5th,  1960 .

​     " . . .    the machine was   already FAA-certificated  . . .
            the FAA Test Pilot   had    not   been   supported  by  Washington,
in   his   attempt   to   reject  the  airplane."

"... It came as   something of a shock   . . .    that  following flight tests   by ...    Air Registration Board ...

   British approval   of the 707    was still  withheld... 

 it was    lateral behaviour  . . .  at  slow speeds    ...  called to question ...

​​​ ARB’s   Chief  Test Pilot, 

      D.P. Davies     Interview — 

                 RAeS audio,  web    or    iTunes   :   

Davies   recalls  his battle  with  Boeing,         

                                  FAA,    the ARB’s Board, 
              over  the certification  of the  Boeing 707 .
   D.P. Davies’   commentary   about   

             early  B-707  S&C testing  
                     begins   at    audio-time  of  1h:14m:32s .

"... The Boeing 707  really was one of the world's 

      leaders  in big transport airplanes  . . . 
        the first . . .  bought by BOAC was
             one of the  big  intercontinental  -436' s.
                 . . .  a  big  hairy-chested  aeroplane.  

  But it was   unreasonably  demanding   to fly.

  The  primary  flight controls   were ...  manual
         – in spite of its size – ...
          ...   But the other thing was that

                  it   was   very   heavy   to fly. . . .

. . .   there  were   large  problems
               in   directional    
stability  &  control .
     Fundamentally   the  Fin   was   too small. 

And this led to all  the problems associated –
    divergent  Dutch  Roll,
      violent  roll  following  engine failure inflight,

 high   minimum control speeds  [inflight],
                all  compounded  by  high foot-forces 

                       in  engine-out conditions,  and 
        extremely high foot forces

          in  [1:16:14]   two-engine-out   conditions.
             . . .  all made worse by   . . . 
        the   unachievable  LOW 

            [published]   minimum control speeds 
                   on   takeoff,   and    on   go-around,

           ...  I was appalled  . .  

   It didn't take any wit  . . . 
         to   turn  the  aircraft  down  . . . 
       Boeng   simply  couldn't  believe  that   we 

                                   were turning the airplane down.

B-707 Rudder : 3 Balance Bays, 2 Weight Bays

More  details from  the C.A.B.'s   "Peconic"  investigation  on next page

 . . .   [these   notes  are  still in-progress  so  scroll through that next page ... ] 

 . . .  the   beginning  of   rudder   "blowback,"  

           where  the  rudder hinge moment    is   overcome  by   slipstream  forces

                                and  causes    the  rudder    to streamline ​ . . . 

Probable Cause
The Board determines the probable cause of this accident was
the crew's failure
to recognize and correct the development of excessive yaw
which caused an
unintentional rolling maneuver
at an altitude too low to permit complete recovery.

[C.A.B.'s  AAR,   p11.]      

_Flight_, 5Feb'1960, pg-187, ARB's complaints about B707 directional control.
_Flight_, 5Feb'1960, pg 187 -- ARB's complaints over B-707 directional control.