B-908 CRASH MAINTENANCE REPORT

• Document Type: FOIA
• Collection: FOIA Collection
• Document Number (FOIA) /ESDN (CREST): 02145410
• Release Decision: RIFPUB
• Original Classification: U
• Document Page Count: 7
• Document Creation Date: March 9, 2023
• Document Release Date: August 21, 2020
• Case Number: F-2015-00256

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B-908 Crash
Maintenance Report
I. Investigation of Maintenance
A. Elimination of Possible Causes
CON tr: ENT I AL1
All investigators agreed almost from the beginning that such causes as
weather, navigational error, collision with flying objects and sudden ill-
ness of both pilots at the same time could be eliminated. It was quickly
determined that the aircraft was not overloaded and that the load was within
established balance limits.
B. Establishment of Possible Causes
1. This accident was most unusual in that it occurred in daylight during
clear weather and over flat terrain. The aircraft had taken off to the
south from Sui Nan Airfield and turned to the left 180�-to proceed north
to Taipei. Several competent witnesses observed the takeoff and initial
climb. The aircraft behaved. normally. The Control Tower operator
watched the aircraft until it passed to the northeast of the airfield
at which time one of the pilots called the tower and advised they were
climbing on course and would see them tomorrow.
2. A number of other ground. witnesses saw the aircraft as it approached 7eng
Yuan which is north of Taichung. The consensus of the witness reports
is that the aircraft was travelling to the north when it made a sharR. .
turn to the west and thereafter started to descend at a rapid rate. As
it neared the ground the wings were said to "wave" and it hit the ground,,
at about a 300 angle and with the left wing low.
No witnesses saw any part fall from the aircraft prior to impact. Several
witnesses said they saw puffs of smoke but this could be imagination or
vapor trails from propellers. Most witnesses in,the same area saw no
smoke and no fire.
4. No witnesses saw or heard anything which would indicate engine. trouble.
5. The sudden transition from normal climbing flight in the direction of
Taipei to a sharp turn to the left and diving into the ground almost
certainly establishes only two possible causes:
a. A sudden and major mechanical trouble which made it impossible for
the pilots to control the aircraft.
b. Sudden incapacitation or restraint of both pilots which rendered
them unable to control the aircraft through some act of other
aircraft occupants.
6. Thus investigation can be narrowed down to a search for any possible
major mechanical trouble and an investigation of the other aircraft
occupants for possible opportunity and means and motive to attack or
restrain the flight crew.
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C. Investigation of Possible Mechanical Causes for the Accident
1. Specially trained American government investigators spent three weeks,
partly at Taichung but largely in Tainan,. in examination of the wreckage
to determine possible causes, Air Asia personnel who know the C-46
aircraft intimately examined the wreckage at Taichung but were told by,
CAA upon its removal to Tainan that they could not participate in the
examination. In these examinations all aspects that would relate to
a sudden and major mechanical trouble making it impossible for the
pilots to control the aircraft were considered,
� 2. Examination of maintenance records disclosed that' all routine maintenance
was performed as scheduled. Furthermore, the aitcraft had flown on 37
consecutive flights prior to the accident with not one single reported
trouble. This is unusual for any aircraft and indicated excellent
mechanical condition.
3. Of first concern was an explosion or fire in the air. A fire would not
normally have caused the aircraft to descend from an altitude of appro-'
ximately 1200/1500 feet and hit the ground in within 20 to 30 seconds
thereafter unless it burned long enough for a major part such as an
engine or a wing to fall off. No witness saw any fire in the air. A
decisive percentage of the witnesses saw no smoke. No part of the
aircraft was found anywhere but at the crash site. Furthermore,
examination of the wreckage disclosed no evidence of a fire prior to
impact or an explosion of any kind.
4. An American expert made a detailed study of the two engines. He found ,
no evidence of failure of either engine. All damage was only that which
was caused by impact.
5. The expert next examined the two propellers. 'These propellers are Curtiss-.
Wright electrically operated propellers which are governed to maintain'a
constant speed (revolutions per minute) by a mechanism which automatically
change blade pitch as necessary in a range between 170 (fine pitch) and �
47� (coarse pitch). All parts of the right propeller were found and the
blade pitch was determined positively to be 300, the blade pitch which
would exist at normal engine power and normal propeller speed. If the
engine power had been less than normal or the propeller had been over-
speeding the propeller blades would have to be at a fine pitch blade
angle setting, but they were not.
6. The motor .and reduction gear assembly of the left propeller was not
found, despite very extensive and costly efforts to locate it. We must
assume that someone unknown removed it from- the scene of the accident,
possibly during the first night after the accident, because, many people
specifically searched for this assembly on the following day, 21 June.
However, the gear.which turni the propeller blades was still in the
propeller together with the propeller fixed stop plate,. The American
expert's report is qaoted as follows: �
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"Detailed Report
"Blade gear damage is described elsewhere, however) it is essentially the
same as that of the right propeller and represents blade angles at impact
of approximately 300.
"Of the power unit, only the power gear and adaptor plate remained. As
of this writing, the remainder has not been recovered. Initial attempts
to identify reference marks on the power gear from which to establish its
position relative to blade angle were unsuccessful because of corrosion.
Subsequently, by etching it was possible to identity a reference mark
from which the 170 reference spline was established. Comparison with a,
serviceable unit established its angular position when removed from the
.hub to be approximately 300,k To establish precise correlation of its
position with a serviceable unit required rotating the power gear two
splines from the normal 170 reference spline. Each spline represents
. a blade angle change of 0.30, consequently the variable introduced by
the two 'spline position uncertainty is 0.6�.
".Summary '
The left propeller power unit.was not recovered except for the power
gear and adaptor plate. The power gear and adaptor plate were jammed
in a fixed position in the hub. This position was noted and subsequently
determined to represent an approximate 300 blade angle."
Thus the report finds the blade angle of the left propeller to be the
same as that of the right propeller, the blade angle which would exist
%
at normal engine power and speed and not at engine power less than normal
or with an overspeeding propeller.
7. In addition to the above we have the followinl evidence that the left
propeller was operating normally:
a. All witnesses state that they heard no unusual engine sound except
loudness. If a propeller were overspeeding a high pitched whine
would be very noticeable.
b. The manifold pressures of both engines were the same. An overspeeding
propeller would noticeably change manifold pressure, or the pilot's
first reaction would be to reduce manifold pressure.
*c. There is no evidence of overspeed dathage to the nose section of the
left engine.
d. Magnaflux and careful magnifying glass examination of Dower gear teeth
shows cracks at roots of individual gear teeth and impact marks on
sides of gear teeth which are related to gear tooth engagement at 'the
time of impact. By analysis of the manner in which blades were pulled
from the hub these damage marks on the power gear would indicate a
power gear position at time of impact corresponding to roughly 300
blade angle position.
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The damage of the left propeller blade gears of the left propeller
in relation to the damage of the bladesthemselves has been compared
with similar damage of the right propeller. There is definite and
positive correlation which also is evidence that the left propeller
had the same blade angle as the right propeller (and thus had about
� the same speed of rotation).
f. Subsequent to the departure of the American experts Air Asia personnel
found deep gouges inside the propeller hub of the left, as we11 as ..
right, propellers. These gouges ,were caused by the blade gears as
they were pulled out from the hub. Careful measurements and drawings
� were-made to match the gouges to the blade gears of the propellers.
� In all cases where gouges were clearly defined it was found that they
showed blade angles of very close to 300 at time of impact. This
evidence alone is positive proof that both propellers were-at the same .
blade angle setting and there was no engine Or propeller trouble.
It can be stated with certainty that the left propeller was not over-
speeding.
g. Location of the left power unit would not materially add to the above
evidence and in fact could not be expected to give any more evidence
than already exists.
8. The next part of the investigation was an examination for some structural
or control system defect. All major structural parts were examined by
ah American expert. His examination disclosed no structural defects or
damage which did not occur-as the result of impact.
9. Examination of the control system was very thorough.. It started with
examination of the cockpit trim controls. All of these were found to
be in an almost neutral position on the night of the accident. Later
another American expert found a piece of trim tab control cable which
he thought should .be sent to a'laboratory in Washington for analysis.
10. One expert later examined all parts of the control system in Tainan. He
found no defects that did not appear to be the result of the crash.
However, he sent one piece of aileron control cable to Washington for
laboratory examination.
II. Review of CAA's "Salient Points of Investigation Report on CAT B-908 (C-46) Crash",
dated 15 July 1964.
A. Elevator Trim Tab Control Cable
1. In paragraph 3 of the CAA report it is stated: �
"USCAB experts who tested on the control cables affirmed in a report that
the cable used for control of left elevator trim tab showed substan-
tial wear and tear, although the breakage was more probably due to impact
with the ground. The majority Board opinion is that even if these cables
became broken during flight prior to crash, the crew should have en-
countered no particular difficulty in controlling the aircraft in con-
tinued flight."
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2. Yet in paragraph 5 of the dame report the following statement is made:
"Based on substantial wear and tear of left control cable the Board
deemed that normal time maintenance for the aircraft was not attentively
carried out
This conclusion obviously cannot .be made from the statement in paragraph
3 of the report. In fact, the report made by the laboratory in Washingtoi
upon which this conclusion would presumably be based, describes the
damage to the cable in great detail but does not say what caused the
damage.
This cable is connected to a drum or "motor" in the stabilizer (horizontal
tail) which turns a shaft to operate the trim tab. The cable is 3/32 in"
diameter cable which is composed of 49 separate small wires. The cable
is designed to be strong enough to withstand at least 920 pounds of
tension. In tests several cables of this size broke at 1200 pounds
tension, The normal force on the cable needed to operate the trim tab
is only 20 pounds of tension. Thus the cable is at least 50 times stronger
than necessary for the normal load.
4. All control cables in the entire aircraft were completely removed and
inspected as part of CATIs routine aircraft overhaul procedures at the
end of January 1964. Furthermore, during another inspection on 20 April
1964 it was found that the left elevator trim tab had a little too much
"play" (movement when pushed with the hand) which Was caused by wear
inside the trim tab motor. On that date the trim tab motor together.
with the cable was replaced with overhauled parts. These facts are both
proof of excellent maintenance and show that the cable was used for only`
two months prior to the accident, Normally this cable will remain in
use for many years.
5. Examination of the left horizontal stabilizer where this section of the
cable is located shows why the cable was found worn and torn. The crash
damage to the cable guide, pulley, pulley mounting bracket, and two
fuselage bulkheads of the tail section (all of which is documented by
photographs) positively show that prior to the impact the cable was
intact and was strong enough (actually) far stronger than required) to
perform its function in control of the aircraft.
At the time of impact when the aircraft was broken into pieces almost all
cables were broken. At that time the left elevator trim tab cable which �
was sent for examination pulled away from the pulley which changes its
'direction of travel 900 to allow it to go from the fuselage to the left
stabilizer, With the pulley no longer guiding it this cable and its
companion cable were forced, against the sides of two holes in a bulkhead
of the fuselage tail and cut into the bulkhead for a distance of several
inches. The photograph (attached) of this .bulkhead shows that both of
these trim tab cabled had great and approXimately equal strength prior.
to impacts.
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action understandably wore through the outer part .of this cable.
ugh very minor wear of the cable prior to impact is possible the
re of the cable was caused by being pulled through aircraft structure
inally by overload in tension at the time of the crash. The pull
s cable almost certainly exceeded 1,000 pounds to have caused it
ar through the bulkhead as shown, whereas nornal pull is only
unds.
evidence, which is of great significance to technically trained
s, is proof that this conclusion of paragraph 5 of the CAA report
Jletely erroneous,
eller
ragraph 4 of the CAA report appears the statement:.
though there was a difference in RPM of more than 1000, The
deemed that such would obviously give.rise to overppeeding of
propeller,"
agraph 5 of the CAA report it is said:
on the overspeeding of left propeller, the Board deemed that
time maintenance for the aircraft was not attentively carried
noted that none of the American experts, which we have quoted,
red in either of the above statements.
erican expert's report positively established that the left pro-
was set at a blade angle of 300 at,the time of impact. .At this
(which is the same as the right propeller) the left propeller would
e at about the same speed as the right propeller and could not
ly overspeed.
A report apparently was based on an examination made of the tacho-
indicator for the left engine. American expert's report on the
tor states as follows:
achometer was examined for evidence of engine RPM at impact. The
er shaft gears do show some impact damage in more than one place
e periphery of the. gear. Theseimarks are to be examined under �
er magnification than was available to determine if significant
ndications can be obtained, .It must be noted that the pointers
omparatively lightly restrained under normal operation and that
ple.damage marks may result in inability to obtain a single.reliablQ
eading."
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A careful reading of this statement clearly shows that examination of
the tachometer for engine (and consequently propeller) speed could give
only a doubtful indication at best. Yet it appears the doubtful indica-
tion of this tachometer, situated in the aircraft cockpit and separated
from the propeller and engine mechanisms by electric wire and an electric
generator which transformed the speed of the engine to an analagous
electric voltage) was accepted in place of the positive indications
directly from the propeller mechanism itself.
6. Thus, there is no evidence that the left propeller was overspeeding. It
is apparent that the CAA investigator neither understood how the pro-
peller operator its ,effects nor read the expert's report, which said
something quite different.
III. Conclusion
CAAts finding that "normal time maintenance of the aircraft was not attentively
carried out" is not only contrary to available significant technical evidence
of positive reliability and to the statements of eyewitnesses but contrary
also to the reports of American experts and laboratory analyses. There is
in fact no evidence of any kind uncovered to Cate that there was Lax mechanical
trouble of any nature at the time of the crash. Such findings by CAA, follow-
ing an investigation in which CAAts participation was cursory at best and which
was rushed to conclusion without due consideration, are an offense to reason
and due process and a grievous injustice.
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