Complex sentence: 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.
Split into simpler sentences: 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.