As a result of this occurrence, the Australian Transport Safety
Bureau issues the following safety recommendation:
RECOMMENDATION 20030027
The Australian Transport Safety Bureau recommends that the Civil
Aviation Safety Authority review the inspection procedures with
regard to the continuing airworthiness of wooden wings and
propellers that were manufactured with the use of Casein
adhesive.
The investigation could not determine the reason for the pilot
losing control of the aircraft. There was no physical evidence
indicating that the right wheel destroying two taxiway lights
during takeoff damaged any major structural element or in any way
contributed to the accident. Also, no evidence was found to support
the reported observations of the left or the right wings folding up
or a part of the aircraft separating in flight. The evidence
suggested that the portion of the right wing rear spar interplane
strut that was found approximately 73 metres from the accident
site, was thrown to its location as a result of impact forces when
the right wings struck the tree.
The aircraft was observed cruising in level flight when it
apparently departed controlled flight. It was not observed changing
altitude or commencing a turn and would have been experiencing
approximately 1g loads only, well below the minimum design load of
5g.
The timber examination reports noted failures of the laminated
members, on wings of the original manufacture. This was probably
due to the Casein glue having been attacked by micro-organisms
after the wood and the glue moisture content rose above 18%. The
attack by micro-organisms was least evident in areas close to the
edges of the glued joints. Circulating air quickly dries these
areas causing any attack by micro-organisms to cease while it can
continue on the inner areas of the joint that remain moist. This,
and the fact that the glued joints were further secured by nails,
bolts and screws would most likely prevent detection of any glued
joint that had been attacked by micro-organisms. It is also
considered likely that the presence of `Irish linen' and the thick
coat of paint on the propeller would disguise any underlying
delamination.
The wreckage and the exposed wood were soaked wet while on site.
The possibility of attack by micro-organisms commencing at that
time was not considered likely, because once the wreckage was
removed from the site it was stored in dry environment. That
prevented elevated moisture content and attack by
micro-organisms.
CONCLUSIONS
The pilot was appropriately qualified and endorsed on the
aircraft.
No evidence was found indicating that the aircraft striking and
destroying two taxiway lights during take-off contributed to the
accident.
It could not be determined if any part of the aircraft structure
or the propeller failed prior to the aircraft departing from the
level flight.
The wood that the wings were constructed from was of a high
quality and there was no evidence to indicate that a failure of
wood was a factor in the accident.
A number of laminated members on three wings of the original
manufacture and the propeller failed at the glue line rather than
in the wood, probably as a result of Casein glue having been
attacked by micro-organisms.
Current inspection procedures would not allow detection of
delaminated Casein glued joints.
The pilot hired the de Havilland DH82A Tiger Moth aircraft,
registered VH-AJG, to undertake a local pleasure flight with a
friend. The aircraft took off from a grassed area parallel to the
Royal Australian Air Force (RAAF) base Williamtown's main runway
between the eastern end of the runway and taxiway `A'. During the
takeoff, the aircraft was observed to veer right and strike and
destroy two taxiway lights with the right wheel. The pilot
continued the takeoff and the aircraft departed the Williamtown
circuit area at 1428 Eastern Summer Time.
Based on witness reports and radio transmissions by the pilot,
the aircraft initially conducted a sightseeing flight over the
coastal suburbs of Newcastle City. About 20 minutes after takeoff,
the pilot broadcast that he was transiting north through the
Williamtown Mandatory Broadcast Zone west of the coast. The
aircraft subsequently joined the Williamtown circuit from the north
at 1519. The actual flight profile and manoeuvres conducted during
the flight are not known. Shortly after joining the circuit, the
aircraft was observed to depart level flight and impact the ground
approximately 2 km southwest of the Williamtown airport. Both
occupants were fatally injured and there was no evidence of fire in
flight or after the impact.
A helicopter with an instructor and student on board was in the
circuit area, about 1.5 km behind the Tiger Moth. The helicopter
was about 800 ft above ground level and maintaining approximately
60 kts. The helicopter crew estimated that the Tiger Moth was
flying at the same speed and altitude. The helicopter's pilots
reported observing the left wings fold up, the aircraft rotate and
fall almost vertically in a steep nose-down attitude rotating only
a couple of times before impacting the ground.
A witness on the ground reported observing the aircraft's right
wings fold back, followed by the aircraft spinning or spiralling to
the ground. Another witness reported seeing the right wings folding
up, making the wings into an `L' shape and about a metre long
silver pole flying up from the cockpit area. The aircraft then
started turning to the right. None of the witnesses reported
observing the aircraft changing altitude or commencing a turn prior
to the loss of control.
Personnel information
The pilot held a valid Australian Special Pilot Licence and had
accumulated 360.6 hours total of which 4.1 hours were on the Tiger
Moth. The pilot's tail-wheel experience totalled 7.9 hours. He was
endorsed on the type and flew earlier that day with an instructor.
The accident flight was his first unsupervised flight on the
type.
The Australian Special Pilot Licence authorises a pilot with a
current overseas private or higher class of licence to undertake
private flights in Australia. The pilot held a United Kingdom
private licence. He held an Australian Class 2 (private) medical
certificate valid until October 2005, issued by the Civil Aviation
Safety Authority (CASA) on the basis of his United Kingdom medical.
There was no evidence of any physiological condition affecting the
pilot that may have contributed to the occurrence.
Operation and weather
The aircraft was operated by the RAAF Williamtown Flying Club
located on the base and the accident flight was the aircraft's
third flight of the day. The weather was reported to be fine with a
light north-easterly breeze.
The aircraft and its history
The Tiger Moth was a fabric covered biplane aircraft with two
open cockpits in tandem arrangement. The pilot sat in the rear
cockpit. The truss type fuselage was made from steel tubes while
the wings and tail surfaces were constructed from wood. The
two-spar, single-bay wings were reinforced by a system of drag
struts, drag and anti-drag wires. Each spar was made from a single
piece of wood with a reinforcing doubler glued at the interplane
strut attachment. The spars were fitted with metal fittings used to
attach the wings to the fuselage and cabane centre section upper
struts. The flying, landing and cabane wires, tie rods and
interplane struts gave the wings the required rigidity.
The aircraft was stressed to withstand maximum loads of
approximately 7.5g (acceleration due to earth gravity). Information
from the manufacturer indicated that even with the reinforcing
doubler delaminated and ineffective, the aircraft was designed to
withstand manoeuvre loads of about 5g.
The aircraft was manufactured in 1942 and used by the RAAF until
August 1947, when it was decommissioned and received civilian
registration. The aircraft was substantially damaged during an
accident in 1967. It was then dismantled and stored until 1988 when
it was rebuilt and had since accumulated approximately 48.35 flying
hours. The fuselage truss structure was repaired and an overhauled
engine was installed. The aircraft was fitted with four wings and
propeller of original manufacture. The previous history of the
wings and the propeller could not be determined.
In November 2001, the left lower wing was damaged when the
landing gear collapsed on landing in Newcastle after a ferry flight
from Bankstown. A new wing was manufactured and fitted in December
2001. On 13 January 2002, the left lower wing contacted the ground
while the aircraft taxied after a flight.
The aircraft was used to perform only limited aerobatic
manoeuvrers such as barrel rolls, loops and stalls. A few steep
turns and dives were performed during the flights on the morning of
16 February 2002.
The aircraft was maintained in accordance with the applicable
and current maintenance requirements. The maintenance release was
valid until 9 March 2002. Examination of maintenance documents
indicated that all required maintenance had been carried out. The
aircraft had no known maintenance deficiencies and was considered
capable of normal operation prior to the accident.
Accident site and wreckage examination
The impact site was an area of dense undergrowth, tall grass and
10 to 12 m high trees. The ground was soft and waterlogged. The
wreckage was contained within a small area at the foot of a tall
tree, its distribution indicated that the aircraft impacted in a
steep nose-down attitude while rotating to the left. The aircraft
and its four wings were extensively damaged. The wing spars were
splintered and broken at numerous locations. The right wings struck
a tree during impact and their damage was significantly more severe
than the damage sustained by the left wings.
With the exception of both propeller blades and parts of the
right wing rear spar interplane struts, all aircraft components and
extremities were accounted for at the accident site. The propeller
boss that remained attached to the engine shaft was the only part
of the propeller recovered. Propeller contact marks on the tree
indicated that the propeller was rotating at the time of impact. A
part of the missing interplane strut was found some six months
after the accident approximately 73 m from the accident site.
Examination of the engine, systems and flight controls did not
reveal any pre-impact defect that would have prevented them from
normal operation. The left front and rear flying wires and the left
cabane wire were severed in overload. There was no fuel remaining
in the ruptured fuel tanks, but fuel was evident at the accident
site.
Sections of the wing spars were examined by a timber specialist
who concluded that the wood was of high quality and in good
condition. It was free of decay and there was no evidence to
indicate that a failure of the wood was a factor in the accident.
Some blue colouring of wood was present in one laminate of the
propeller boss, but there was no evidence to link this
discolouration with failure of the propeller. The specialist
reported that many of the wood failures, on the three wings of the
original manufacture, were brash rather than splintering,
indicating some embrittling associated with the age of the
wood.
Adhesive failure
The specialist also reported that some laminated members on the
three wings of the original manufacture and the propeller failed at
the glue line rather than in the wood. The failure was due to the
glue line being devoid of the adhering adhesive. The adhesive was
identified as Casein. The specialist reported that the observed
failure was typical of that of Casein that was exposed to attack by
micro-organisms and that the attack was evident all over the glued
area of the joint except on small areas close to the edges of the
glued components. No delamination was observed on the new wing
fitted in December 2001. This wing was manufactured using modern
synthetic resin adhesive.
Casein is a milk-based glue that was particularly popular around
the 1940s, when the three wings and the propeller were most likely
manufactured. Since it contains protein, it could be subject to
attack by micro-organisms and weakened if the moisture content of
the wood and the adhesive is allowed to increase above a certain
level. The Casein glued joints, however, do not degrade
instantaneously when wet. The amount of degradation is proportional
to the time the joint is allowed to remain moist.
The specialist advised that the attacks by micro-organisms
occurs when the wood and adhesive moisture content is approximately
18% or greater. A moisture content of 18% could be achieved if the
wood was exposed to a relative humidity of 85% or greater. While
such humidity is experienced in tropical Australia, the average
moisture content of the wood and the adhesive is not likely to
reach this level due to lower values during various times of the
day. The evidence, however, indicated that the glued joints were
probably subjected to a number of periods when the moisture content
was high, allowing micro-organisms to attack the adhesive. Such
periods were further evidenced by the presence of corrosion around
the nails, bolts and screws securing the joints.
However, the specialist concluded that there is little or no
evidence indicating that any single glue failure may have resulted
in a catastrophic failure of a major structural element of the
aircraft or the propeller.
Propeller
The numbers stamped on the propeller boss were consistent with
the propeller having been manufactured in the 1940s. The wooden
propeller consisted of a number of laminates. After the propeller
was manufactured, the centre part of each blade was wrapped with a
layer of `Irish linen'. The entire propeller was then coated with a
relatively thick coat of dark coloured cellulose-based paint.