Chinbu has hit the nail on the head. The most cost and time effective delivery of retardant to either a fire front or a containment line is the single engine air tanker (SEAT). They have constantly proven themselves in the face of the C214's, DC10, Convair's, etc.
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These aircraft are the present and future of Australian aerial firefighting, in conjunction with their rotary winged friends. The trials of the DC10, etc, have been expensive and unsuccessful political stunts.