Rosenker said. The safety board also said the Federal Aviation Administration (FAA) failed to detect and correct the airline's maintenance shortfalls. Regulations exempt older seaplanes from rigorous structural oversight. Chalk's had no comment on the safety board's findings. The FAA said it had no indication that the airline's maintenance program was in question. "The regulations are crystal clear that the carrier has primary responsibility for the airworthiness of (its) fleet and that includes making appropriate structural repairs," the agency said in a statement. Probable cause The NTSB determined that the probable 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 must be sealed in order to ensure the tank is fuel-tight. In addition, the normal flexing of the wing in flight has a tendency to open seams over extended periods of time, 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. The accident aircraft had, over a period of several months, suffered repeated fuel leaks from both wings that had been remedied by the operator by repeatedly applying a sealant to the inside seams of the fuel tanks. In applying the sealant inside the right wing tank the operator 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. The crack was discovered during previous maintenance, and an attempt was made to repair the crack by grinding it out. In applying the sealant, the operator was required to access the inside of the fuel