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I understand that a Eurocopter EC135 can fly on only 1 engine.

In this instance, is the dead engine power turbine still coupled to the rotor? Does it freewheel? Is there a clutch to disengage it? If not, does the power turbine draw air through the dead engine?

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    $\begingroup$ Welcome! Related free wheeling unit. This unit disconnects automatically the engine from the shaft. Twin-Pac is a system included in PT6T dual turbine (PT6 are not used on EC135). Using free turbines may also help (to be confirmed). $\endgroup$ – mins Jul 7 '16 at 1:47
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    $\begingroup$ Some use a combining transmission where each engine goes into a Sprag Clutch which each feeds an output shaft connected to a gear(box) driving the rotor. When an engine fails, the sprag clutch freewheels allowing the other side to spin faster than the failed side. This is also pretty much the same in single engine helicopters which is why the rotor doesn't stop spinning when an engine fails. $\endgroup$ – Ron Beyer Jul 7 '16 at 3:57
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    $\begingroup$ There's a very simplified diagram in this accident report, see page 11 of the full report. (Doesn't show the clutches though.) $\endgroup$ – Andy Jul 7 '16 at 8:39
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Multiple-engine configurations use multiple freewheeling units that couple the rotor with the engine and allow the first to revolve faster than the latter but not the other way around. If just one of the engines fails, the freewheeling unit for that particular engine will effectively disengage the engine from the rotor (as the rotor is now revolving faster than the failing engine), allowing the other engine(s) to drive it.

How a freewheeling unit functions on a mechanical level is described on page 6 of this document.

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