Aviation safety investigations & reports

de Havilland Canada DH-82A, VH-TMK

Investigation number:
199800648
Status: Completed
Investigation completed

Summary

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.

Analysis

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.

Safety Action

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.

General details
Date: 28 February 1998   Investigation status: Completed  
Time: 1735 hours WST    
Location   (show map): Wellard    
State: Western Australia   Occurrence type: In-flight break-up  
Release date: 14 July 1999   Occurrence category: Accident  
Report status: Final   Highest injury level: Fatal  

Aircraft details

Aircraft details
Aircraft manufacturer de Havilland Aircraft Pty Ltd  
Aircraft model DH-82  
Aircraft registration VH-TMK  
Serial number T250  
Type of operation Private  
Damage to aircraft Destroyed  
Departure point Jandakot, WA  
Departure time 1715 hours WST  
Destination Jandakot, WA  
Crew details
Role Class of licence Hours on type Hours total
Pilot-in-Command Private 166.0 456
 
Injuries
  Crew Passenger Ground Total
Fatal: 1 1 0 2
Total: 1 1 0 2
Last update 13 May 2014