Cause
The cause of the accident was a fatigue failure in the right wing initiated by a crack in a span-wise stringer close to the wing root. The crack had been detected running through a slosh hole (an aperture in the wall of the stringer that allows fuel to flow from one side of the stringer to the other) and seemingly repaired earlier, but the repair was eventually to prove ineffective.
The Mallard was designed in the 1940s with a so-called 'wet wing' where the fuel tanks, instead of being separate items within the wing, are constructed from sealed-off portions of the wing structure itself. This eliminates the additional weight of the tanks and also allows more fuel to be contained within a given wing-size. The drawback of this form of construction is that all the joints around the tank seams have to be sealed, so as to make a fuel-tight tank. In addition, as the wing flexes to some extent during flight, the movement has a tendency over extended periods of time to open seams leading to fuel leaks. Grumman, the manufacturer, had issued warnings as early as 1963 about fuel leaks from the Mallard's wing being indicative of possible structural problems, however for unknown reasons the airline did not consider this particularly relevant to its own aircraft.
The accident aircraft had, over a period of several years, developed chronic fuel leakage problems from both wings that had been remedied by the operator by repeatedly applying a sealant to the inside seams of the fuel tanks. Unfortunately, in applying the sealant inside the right wing tank the operator had inadvertently applied the material over a damaged lower stringer - an important, load-bearing structural member of the wing - in such a way as to conceal a fatigue crack, which had earlier been discovered during maintenance and an attempt to repair made by grinding the crack out. In applying the sealant the operator was required to access the inside of the fuel tank using small removable inspection hatches in the top surface of the wing, and of necessity, this resulted in poor visibility and awkward conditions for working inside the tank. Unfortunately the sealant concealed the earlier repair on the stringer and made subsequent checking for further damage to this component, i.e., any crack growth, impossible.
The first outward sign of possible significant problems with the accident aircraft was when a chord-wise (i.e., running from the front of the wing towards the rear) crack was noticed in the skin of the lower surface of the right wing at the root, which, after several failed attempts at stopping the crack by drilling stop-holes, was then repeatedly repaired by affixing doublers - metal patches intended to take over the load from the damaged part, a normal repair procedure for minor skin damage - riveted over the cracked skin, however the crack continued to grow, requiring longer and longer doublers to be fitted. Although the skin crack was slowly getting longer it was not thought to be anything other than a skin problem, which could be dealt with in the normal way, i.e., affixing a doubler.
Like most aircraft of the period the Mallard was built using an aluminium stressed skin construction, in which, unlike earlier fabric covered aircraft where the fabric covering is merely for aerodynamic purposes and contributes little to the overall strength of the airframe, the metal skin itself carries part of the flight loads, i.e., is 'stressed'. Many such aircraft develop minor skin cracks over time and provided appropriate action is taken to repair any cracking then the safety of the aircraft need not be compromised.
Meanwhile the unseen cracked stringer allowed the right wing to flex more during flight which increased the bending forces at the root such that the visible skin crack slowly increased in length with each subsequent flight.
Eventually on the accident flight, the crack in the underside of the wing grew in length until the wing was so weakened it was unable to support the flight loads, and the wing separated. This caused the fuel contained within the wing to be released which then ignited, resulting in the fire that was seen by witnesses.
On examination of the wreckage investigators discovered that in addition to the external ones, internal doublers had also been affixed to the root area of both wings, although maintenance records for these repairs were not available. They also concluded that the cracked stringer that initiated the wing loss had probably failed completely some considerable time prior to the accident, leading to a substantial weakening of the wing structure.
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