from prior flights
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.
Replaced Left and Right Aileron-Trim assemblies on 11-Jan'1961.
9-Feb' Test Flight: Abnormal response
of the Stabilizer Trim . . .
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.
Sabena, Boeing 707-329, OO-SJB, 15 Feb' 1961
Rudder Control Centering Spring Cable Guard . . . May 1962
. . . 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.
. . . the commission assumed
. . . a spoiler malfunction. . . .
This assumption is supported by . . .
. . . rapid slipping due to aerodynamic loads would have brought the screw nut in contact with the mechanical stop . . .
Slipping Horizontal Stabilizer
Assumptions -- 1st hypothesis
-- failed control of Horiz-Stab- Pitch Trim
Engines, Electric, Hydraulic,
Reconstruction of the trajectory
Rudder and Rudder Pedals
20 degrees right Rudder
No abnormality was found
Stabilizer-Trim 10.5 A.N.U.
USA's . . . Manufacturer . . . Comments . . .
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
. . . concluded . . . abnormality . . . forced the aircraft to the left and prevented the pilot from re-establishing level flight . . .
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
. . . 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 . . .
. . . 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. . . .
Track deduced from Flight Data Recorder
Note the investigator's hint of
a possible double-failure,
"only the material failure
of two systems
. . . a complete explanation"
. . . the FAA . . .
to the Spoiler malfunction hypothesis, we cannot agree . . .
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.
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. . . .