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Old Twin Otters May Never Die

New STC extends fuselage fatigue life

Twin Otter

December 1, 2011 – The de Havilland Twin Otters were certified with structural life fatigue limits that must have seemed to be longer than anyone could ever need. But nobody at de Havilland could have guessed how successful and irreplaceable the rugged high-wing twin turboprop would be and many of the airplanes are reaching life limits on the wing, and now the fuselage.

The wing box, the primary structural element of the wing, was originally certified for a maximum of 25,000 flight hours or 50,000 flight cycles. Many Twin Otters logged that much time and the RWMI division of Ikhana, a California-based aircraft services company, developed a life extension modification for the wing box in the 1990s. Under the RWMI STC the wing box life can become either 35,000 hours and 70,000 cycles, or a more extensive “re-life” modification can take the wing box out to as much as 49,000 hours and 98,000 cycles.

With the wing life extended, Twin Otter operators began to bump up against the fuselage fatigue life and a new RWMI mod can extend that to 66,000 hours and 132,000 cycles.

The life extension modifications are not cheap, but without them, the Twin Otter is nothing but scrap when it reaches certified fatigue life limits. Owners can opt to extend the life of the airframe, or complete extensive modifications up to and including a change to modern engines and systems, plus new interior and furnishings. The complete redo could cost several million bucks, but a new Twin Otter is around $6 million.

Viking Air is building new Twin Otters from scratch, complete with updated engines and systems, and there seems to be a solid market for the airplanes. Now RWMI, which is authorized by Viking/de Havilland, can keep the old utility twins flying longer. Will there always be a Twin Otter in the air somewhere? It sure looks like it.

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