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The Soloy Dual Pac apparently allows two engines to rotate one propeller -- here's a picture of it on an Otter:

Is this recognised as a centreline thrust twin engine aircraft, a "standard" twin engine aircraft or just an aircraft with a single engine for FAA certification? What about for pilot licensing?

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    $\begingroup$ I don't know the answer (and I'm not familiar with the Soloy Dual Pac), but isn't this situation similar to a twin-engine single-rotor helicopter, where two engines both drive the same rotor? Those are definitely considered multi-engine. (And the PT6 cores used in the Soloy Dual Pac are also commonly used in helicopters as the PT6B and PT6C variants.) $\endgroup$
    – TypeIA
    Apr 9, 2014 at 13:29
  • $\begingroup$ @TypeIA: I'd say it's even more similar to the Allison T40 - two power sections, two propellers (contra-rotating), but a single common gearbox, which was enough to have it considered a single engine. $\endgroup$
    – Vikki
    Mar 13, 2019 at 2:58

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According to the certificate is a

Twin Power Section Turboprop

and, later, note 7: (emphasis mine)

This engine is certificated as a unit comprising two separate power sections with the capability of single engine operation with either power section alone in multi-engine airplanes. The unit is also approved as a single engine with either or both engines operating continuously.

No explicit remark is given about pilot licensing, but given the note reported, I would say that a licence for a single engine is sufficient.

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    $\begingroup$ To me the bolded section sounds like it's main purpose is to imply single-engine rating is sufficient to operate it. $\endgroup$
    – Jan Hudec
    Apr 10, 2014 at 7:24
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    $\begingroup$ @JanHudec I thought I wrote exactly that, haven't I? $\endgroup$
    – Federico
    Apr 10, 2014 at 7:30
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Two engines with the crankshaft welded, would be classified as a single engine. But when both crankshafts are connected with "one way cluck", a pure bureaucrat would call it multi-engines. A logical person would ask, how different would be to fly an aircraft with the crankshaft welded as a single piece or interconnected with one way clutches?

Both would fly in the same way. We expect that an intelligent person would classify it as a single engine as far as only one control is used for both engines. A helicopter is different because it uses more than one control.

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    $\begingroup$ More than one control for what? $\endgroup$
    – Koyovis
    Jun 3, 2017 at 1:36

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