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Air Moorea Flight 1121 crashed because its up-elevator cable broke, allowing the elevator to blow to its faired position and thus be unavailable to counteract the pitch-down moment caused by the flaps being retracted;1 the DHC-6 elevator control system has two cables (up-elevator and down-elevator, distinguished by what part of the aft-quadrant bellcrank they attach to), and, if one breaks, the tension in both is released and the elevator floats. No redundancy is provided in the form of a second or third set of elevator cables, unlike how most airliners have double- or even triple-redundant flight controls. Why does the DHC-6 have only a single set of elevator control cables, rather than two or three?

1: No matter that retracting one’s flaps should not do that.

Vikki
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The Twin Otter isn't certified in a category that requires duplicate control runs with split surfaces. It's more or less a single engine Otter with two PT-6s and a redesigned tail. Those requirements apply to heavier aircraft than the DHC-6.

John K
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  • If it were a one-person general-aviation aircraft, I'd buy that explanation, but this is an airliner we're talking about here (albeit a small one). – Vikki Mar 06 '19 at 03:44
  • The Beech 1900 is the Twin Otter's closest competitor in the commuter airline business, and it also has a single cable circuit. You don't see separate left and right elevator control systems with pitch disconnects until you are in the Transport Category airplanes of the Dash 8/ATR size. – John K Mar 06 '19 at 04:40
  • @Sean doesn't matter. The requirements are based on aircraft size, weight, etc. and not so much the purpose it's used for. – jwenting Mar 13 '19 at 04:50