Note  the  investigator's   hint   of  

a possible    double-failure,

 or   failure-interaction

"only the material failure

of two systems

. . .  a complete explanation"

. . .   the  FAA   . . .  

to the Spoiler malfunction hypothesis, we  cannot  agree  . . . 

Wreckage-Examination

Rudder   and    Rudder Pedals

20 degrees right Rudder


No abnormality was found

Wreckage- examination

Stabilizer-Trim   10.5  A.N.U.

USA's . . . Manufacturer . . . Comments . . .

Analysis of  . . . abnormal manoeuvres . . .  only left . . . Bank up to 60-degrees . . . 

. . . the various controls were found . . .  an attempt to ... bank to the right . . .  the elevator ... UPwards . . .  Stabilizer was already ... 10.5 units Nose Up. . . .    

Interpretation -- results

Three Hypotheses Considered

Wreckage-Exam' --  Flight Controls

Spoilers

-- other Failures

. . . rapid slipping due to aerodynamic loads  would have brought the  screw nut in contact with the mechanical stop . . .

    3rd Hypothesis

Slipping Horizontal Stabilizer

Outboard Ailerons

. . . possible causes of the accident  

was therefore limited to a

flight controls failure.

. . . It was, therefore, concluded that the stabilizer's position was abnormal for the approach. . . .

Wreckage   Examination


Track  deduced from Flight Data Recorder

Witnesses

Weather was good

Reconstruction of the trajectory

Trouble Reports

from prior flights


The Aircraft

Investigation and Evidence



The Crew --   this section,  from the report,   is omitted from this web-page, since the pilots'   background  and recent flight experience  was  typical, unrelated to the possible failure-interactions encountered:

              see the pdf-version for a complete  copy.


   Aileron-Roll control:  

                      blocked, jammed.

Replaced Left and Right Aileron-Trim assemblies on 11-Jan'1961.

   9-Feb'  Test Flight:  Abnormal response

                               of the Stabilizer Trim . . .  

   Replaced the

         Horizontal Stabilizer Trim-MOTOR.


. . .  on final to Rwy 20 . . . near threshold . . . power and retracted Gear . . .  climbed . . . made several circles left . . .  during turns bank angle increased . . . until finally it crashed . . . 



From   11-Jan'61  to  9-Feb'61

 the Aircraft was in Overhaul.


Sabena,   Boeing 707-329,   OO-SJB,   15Feb1961

    accident at Berg, 2 km northeast of the threshold of Runway 20 at Brussels National Airport,  Belgium

               on 15 February 1961.   Report released by Minister of Communications, Belgium.


                Excerpts from    ICAO Circular 69-AN/61     [Aircraft Accident Digest#13],     pgs  43-58.

Of the several hypotheses

. . . in the accident report

. . .  the most plausible

. . . a malfunction of

the stabilizer . . . permitting

. . . 10.5 degrees aircraft nose up position. . . .


  the only means to prevent

. . . pitching up into a stall

. . . full forward Column and enter a turn . . .

Comments of  the State of   Manufacture


Sabena,   Boeing 707-329,   OO-SJB,   15 Feb' 1961

Rudder Control Centering Spring Cable Guard . . . May 1962

Probable Cause

 . . . Failure of the  Flying Controls . . . 

while  . . .  possible to  advance certain  hypotheses . . . 

 not . . . entirely satisfactory . . . 

Only . . .  failure of  two  systems

could lead to  a  complete  explanation,

but left the way open to

an   arbitrary   choice

because there was  not  sufficient evidence

to  corroborate  it.


Assumption:

. . .   the commission assumed

. . .  a  spoiler  malfunction. . . .  

This assumption is supported by . . .   

NOT  

the  Horizontal-Stabilizer:

The commission concluded that the probable cause of the accident could hardly be attributed to a malfunctioning of the stabilizer . . . 

Considered  as improbable  were   a failed  Rudder,   or  Elevator.


   ==>   But found  Horizontal Stabilizer  ABNORMAL.

. . .  concluded . . . abnormality . . .  forced the aircraft to the left and prevented the pilot from re-establishing level flight . . . 

  Assumptions  --   1st hypothesis

            -- failed control of Horiz-Stab- Pitch Trim

Lateral Controls

Inboard Ailerons

Note:     These  Investigators 

explicitly  cited their  assumptions

in various sections of their report.

Assumptions -- 1st hypothesis . . .

[Horizontal Stabilizer]  position  . . .

  a failure in the control system . . . 

the Commission . . . difficult to believe

. . .  did not . . . operate the Cut-Out Switch

 . . . a technical failure . . .

 a period of 1 min 30 sec during which

. . . a rapid decrease in speed and altitude . . .  

the evolutions . . . showed clearly that it was not possible to control the attitude . . .

 Commission . . . tried to establish a correlation between . . . failures with ... manoeuvres made by the crew . . .

. . . in the landing configuration  and at a height . . . possible to . . .  land normally . . . Contrary to all expectations . . . started a manoeuvre . . . resembled an overshoot.  This changed . . .  into "an abnormal sequence of evolutions",  characterized by left ... banks and sharp left turns. . . .

. . .  flight recorder indicated . . . 220 kt . . .

         loss of altitude . . .  1000 ft/min  . . .

  reduction . . .  from 215 kt to 145 kt . . .

 the pilot . . . in quick succession . . .

power reduction . . . flaps to 30 . . .

 landing gear . . . flaps to 40 . . . flaps to 50 . . .  trim during reduction in speed . . .

    8 seconds of trim motor action . . . 

Discussion of the data

Engines,  Electric,  Hydraulic,

 Mach-Trim,  Autopilot


Failure - Interactions