In the mid-60s the local service airlines were well into their graduation from DC-3 centric fleets to those based predominantly around Martins and Convairliners and in fact most were beginning to look at jets as well. Piedmont had retired its last Dc-3 in 1963 and had been operating 8 Fairchild F-27s since 1958. It had added a sizeable number of Martin 404s since 1961, but was in the market for fleet renewal. It was interested in jets but also needed new prop-liners and found them in an unusual location.
"It did what it was designed to do - carry out heavy loads from mountainous airports with short runways and in all kinds of weather."
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2013. A few questions about the YS-11. Airliners.net