As a result of this occurrence, the Bureau of Air Safety
Investigation is investigating a perceived safety deficiency
involving the airworthiness of wooden structural components in
passenger-carrying aircraft. This deficiency relates to the quality
assurance, repair and on-going maintenance of wooden structure
aircraft.
Any recommendation issued as a result of this deficiency
analysis will be published in the Bureau's Quarterly Safety
Deficiency Report.
The investigation could not determine the origin of the right
upper wing or when it was installed. Therefore, its history and
airworthiness could not be determined. Despite the extent of damage
incurred by the right upper wing during the 1993 accident, the
fabric was not removed from the wing to conduct a thorough
inspection of the wing structure. During the course of the repair,
the wing structure was noted as appearing old. However, had a
comparison with the logbook description of the wing been made at
the time, it may have been evident that a deeper examination of the
wing, to preclude the possibility of more extensive damage to the
aged and unknown wing structure, would have been prudent.
It is possible that the compression failures and shakes found in
the wooden wing structures subsequent to the 1998 accident, were
caused by break-up forces. However, due to the degree of reported
damage to the wings during the 1991 and 1993 accidents, it is
likely that the damage was present prior to the accident
flight.
The presence of either microscopic compression failures or
visible shakes would have seriously reduced the load-bearing
capacity of the wood. The 1993 inspection was conducted, as
recommended by the maintenance manual, visually through holes cut
in the fabric. The appropriateness of this type of inspection could
be questionable considering the difficulty associated with visually
detecting compression failures and shakes in wood components. If
the compression failures and shakes existed prior to the accident,
then once the wood was subjected to a load in excess of its reduced
load-bearing capacity, it would have catastrophically failed
without any warning.
The effect of any mis-rigging of the aircraft's upper right wing
could not be determined. However, if the centre section was
out-of-square, then the right upper wing may have carried extra
loading for an extensive period. Although the right wing rear spar
had evidence of significant weakening around the fuselage
attachment fitting, it was considered unlikely that this was the
area that initiated the wing break-up because the spar was
predominantly subjected to compression loads at that point.
Witness evidence and wreckage disposition indicated that the
right upper wing failed while the pilot was pulling out from a
loop. The wing failed in the area of the right upper wing spar
where the inter-plane strut was attached. Evidence indicated that
the upper right wing spar was significantly weakened around the
inter-plane strut attachment point by the effects of fungal decay
and a partially de-bonded doubler.
Because the loop profile was described as being egg-shaped and
the aircraft was possibly being flown at a lower height than
normal, the pilot might have used more nose-up elevator control
than usual during the pullout. The pullout from the loop may have
induced a higher than usual g-loading on the wing structure,
however, the loading could not be determined from the available
evidence. In any case, the g-limits for the DH-82A were not
published and the aircraft was not fitted with a g-meter.
Therefore, the pilot was probably unaware of the aircraft's g-limit
and of the g-loading he was putting on the aircraft structure just
prior to the accident.
Despite the aircraft's flight manual requiring the wing slats to
be locked before conducting aerobatics, it is possible that the
pilot might not have locked them before commencing the looping
manoeuvres. However, any opening of the wing slats should not have
caused a serviceable wing to fail, although the upper right wing
forward spar was already structurally weakened by fungal decay,
delaminated doubler at the inter-plane strut area and possibly by
pre-existing microscopic compression failures and shakes. A violent
opening of the right slat may have applied some additional loading
to the most critical area of the spar. If the slat deployed during
the pull out from the loop, the additional loading may have
contributed to the failure of the already weakened right upper wing
spar.
CONCLUSIONS
Findings
The pilot may have conducted a more positive pullout from the
loop than usual and in doing so probably applied a higher than
normal g-loading to the aircraft. This, associated with the
possible deployment of the wing slats, contributed to the in-flight
failure of the already weakened upper right wing.
The pilot hired a de Havilland DH82A Tiger Moth VH-TMK to
undertake a 30-minute local pleasure flight with a friend. Although
the pilot was qualified to conduct aerobatics, he was not
authorised by the aircraft's operator to do so during the accident
flight. The aircraft departed Jandakot at approximately 1715
Western Standard Time (WST) and proceeded south to the training
area. About 20 minutes later, witnesses saw the aircraft performing
an egg-shaped loop at a lower altitude than usual. One witness
reported that the aircraft had been conducting continuous loops. As
the aircraft was pulling out at the bottom of a loop, witnesses
heard a loud crack accompanied by a tearing sound. Two witnesses
reported hearing three "bangs". A large, yellow object was seen to
separate from the aircraft. The aircraft then appeared to stop and
pitch nose-down before entering a spiral dive. The right wing was
seen to fold back against the fuselage before the aircraft entered
the dive. As the aircraft descended, the left wings folded back,
shedding wing components. The aircraft impacted the ground in a
near vertical attitude and caught fire. Bystanders were unable to
assist the occupants.
Aircraft characteristics
TMK was a single-bay biplane with a wood and metal structure
covered by fabric. Metal, aerofoil shaped, flying and landing wires
braced the wings. Wing slats were mounted on the outboard leading
edge of the upper wings above the inter-plane strut attachment
points. British Aerospace, the type certificate holder for the
DH-82A, reported that this area underwent the greatest bending
stresses when the wing was placed under aerodynamic load and,
therefore, determined the ultimate load limit of the wing. The
ultimate load limit was determined to be 7.5g, although the
aircraft's structural g-limit was not published in the DH-82A
flight manual. TMK was certified in the normal and acrobatic
categories.
The wing slats were lift augmentation devices that reduced the
aircraft's stalling speed by about 2 kts. As the wing approached
the stall, the aerodynamic centre of pressure moved forward,
causing the slat to automatically deploy. Many pilots used the
initial stages of this deployment as an indication that the wing
was approaching the stall. The slat assembly was attached to nose
ribs which, in turn, were glued to the upper and lower surfaces of
the spar. The slats were locked in the closed position by a
slat-locking lever located in the cockpit. The slat-locking lever
had a positive locking mechanism to preclude inadvertent movement;
however, it was known that if the locking mechanism was worn, the
lever might disengage when the aircraft experienced g-forces during
aerobatics.
The aircraft's flight manual required the slat-locking lever to
be locked before the conduct of aerobatics. Engineering analysis by
British Aerospace determined that slats deploying during a looping
manoeuvre would not cause a serviceable wing to fail.
Discussions with experienced DH-82A pilots indicated that they
had, on occasion, inadvertently conducted aerobatics with the slats
unlocked or when the slats had opened due to the slat-locking lever
disengaging from the locking detent. One pilot reported that
although he had experienced a violent opening of the slats under
heavy g-loadings, there had been no damage to the wing. British
Aerospace reported that their archives indicated that there was no
previous record of slat separation being implicated in the
structural failure of a DH-82 wing.
Wreckage examination
Pieces of wood and fabric were strewn along the flight path from
north to south, over a distance of approximately 750 m. Numerous
small pieces of the wooden wing internal structure were found early
in the wreckage trail along with the two yellow painted aluminium
wing slats. A fierce fire consumed the aircraft after it struck the
ground, destroying most of the remaining wood and fabric
components. The engine was operating when the aircraft hit the
ground. The large, yellow object seen separating from the aircraft
could not be positively identified.
The fire had extensively damaged the slat-locking mechanism and
the investigation could not determine whether the mechanism was
previously worn or damaged. However, the slat-locking lever was
found to be in the unlocked position. The investigation could not
determine whether the slat-locking lever was unlocked during the
aerobatics or became unlocked during the subsequent in flight
break-up or ground impact. Both slats were bent upwards in a
V-shape around the centre attachment. The outboard part of the
right wing slat had additional deformation and contained a deep
cut. The cut was consistent with the slat impacting either the
right wing's flying or landing wires. Within the wreckage trail,
the slats were found beyond the separated pieces of wing spar and
internal structure.
Detailed analysis of the recovered pieces of the wing structure
found that the upper half of the right upper wing front spar was
affected by fungal decay. The centre of the affected area was just
above the hole where the slat-locking cable passed through the
spar.
A wooden doubler made from ash timber, attached to the front
side of the spar at the inter-plane strut attachment, was
delaminated from the spar. Engineering advice was that delamination
of the doubler would increase the bending stress in the spar at the
inter-plane strut attachment point by about 33 per cent. The grain
slope of the doubler was also found to be extremely steep, which
significantly reduced its load carrying ability.
The upper right wing rear spar root end had fractured along a
line that ran through the centre of the outer row of fuselage
attachment fitting bolts. The fracture surface was discoloured and
the wood weakened by reacting with corroded attachment fitting
bolts. Pieces of the front and rear spars of the upper right wing
had numerous slip planes and creases in the wood cell walls, which
indicated that the pieces had experienced compression overload.
When wood has been subjected to compression overload along the
wood grains, the grains exhibit microscopic slip planes and creases
(also known as failures). If a large section of wood is subjected
to compression overload a well-defined visible wrinkle across the
face of the wood, known as a compression shake, may be present. The
presence of either microscopic compression failures or visible
shakes seriously reduces the load bearing capacity of the wood.
However, experience indicates that even the visible compression
shakes may be difficult to detect. The evidence of a shake is
usually associated with a sudden change of the spar's
cross-section, which is often directly at the side of a doubler.
Shakes could be extremely subtle and hidden by paintwork or other
surface features that hinder their detection. Despite the
difficulty associated with detecting compression shakes in wood,
the aircraft's maintenance manual recommended that such inspections
be visual, conducted through inspection holes in the wing's
fabric.
A compression shake may result from abnormal bending overloads
often experienced during relatively innocuous situations such as a
heavy landing or a landing gear collapse. Unlike a crack in metal,
a compression shake in wood does not progress during the aircraft's
normal utilisation. However, once the wood is subjected to a load
in excess of its reduced load bearing capacity, it may
catastrophically fail without any warning.
Aircraft history
The history of the aircraft prior to a rebuild in 1980 could not
be established. Since the rebuild it had been used for training,
private flying, and commercial operations, including the carrying
of fare-paying passengers on joy flights.
Prior to this accident, TMK had been involved in two other
accidents. In 1991 the aircraft had a heavy landing and in addition
to other damage, it was recorded that only the lower right wing
required repair. The second accident occurred approximately 2 years
later, when an engine failure resulted in a forced landing. In
addition to other repairs, structural damage was such that the
lower left wing spars and the lower right wing required
replacement. The right upper wing front and rear spar inter-plane
strut attachment fittings were extensively damaged and required
replacement. The inspection of the right upper wing critical points
was conducted through inspection holes in the wing fabric. The
licensed aircraft maintenance engineer (LAME) who conducted this
inspection reported that the wing structure appeared old.
During the 1980 rebuild, the right upper wing front spar was
recorded in the aircraft's logbook as being replaced with one
manufactured by Perfectus Airscrew Pty Ltd. However, after the 1998
accident, the recovered right upper wing front spar components were
identified as not being of Perfectus origin. There was no entry in
the logbook to indicate that the right upper wing or spar was
replaced during the period between the 1980 rebuild and the 1998
accident.
It was established that following the 1993 accident, the
aircraft was difficult to rig and there were problems with the
aircraft's flying characteristics. Despite minor wing rigging
adjustments, no attempt was made at this time to review the
aircraft's rigging in accordance with the aircraft's maintenance
manual. The right rear centre section spar attachment point was,
reportedly, three-eighths of an inch lower than the left.
In November 1997, the wings were re-rigged as a result of pilots
reporting that the aircraft had a tendency to roll to the right
when it was flown hands-free. The logbook showed that the aircraft
wings were re-rigged in accordance with the aircraft's maintenance
manual. However, the new maintenance organisation reported that the
wings were not removed during the procedure. British Aerospace
confirmed that the DH-82A maintenance manual required the removal
of the wings to effect accurate rigging of the centre section. When
the maintenance organisation re-rigged the aircraft, it did not
find the reported misalignment of the centre section. Failure to
correctly rig the centre section and wings could induce additional
stresses into the wing structure.
Repair and maintenance aspects
During the course of the investigation, it was reported that
significant structural defects and deterioration in other Tiger
Moth aircraft had been discovered. Although these aircraft had been
previously inspected, the defects found included: incorrectly
manufactured wing spars; wing tie rods made from incorrect, and
much weaker, material; cracked and deteriorated spars; corroded
fuselage frames; incorrect materials used in the wings; and
incorrect repair and construction techniques. These defects and the
evidence found on the accident aircraft, appeared to indicate that
periodic inspections on some aircraft were being conducted
inadequately, and that some LAMEs were approving materials and work
that were deficient.
Personal information
The pilot held a private pilot's license and had a valid class 2
medical category.