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.
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.
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.
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.
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.
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.
As a result of this occurrence, the Australian Transport Safety Bureau issues the following safety recommendation:
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.
|Date:||16 February 2002||Investigation status:||Completed|
|Time:||1530 hours ESuT|
|Location:||2 km SW Williamtown, Aero.|
|State:||New South Wales||Occurrence type:||Loss of control|
|Release date:||27 February 2003||Occurrence class:||Operational|
|Report status:||Final||Occurrence category:||Accident|
|Highest injury level:||Fatal|
|Aircraft manufacturer||de Havilland Aircraft Pty Ltd|
|Type of operation||Private|
|Damage to aircraft||Destroyed|
|Departure point||Williamtown, NSW|
|Departure time||1426 hours ESuT|
|Role||Class of licence||Hours on type||Hours total|