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I read from most of the article and books that counter-rotating propellers multi engine airplanes do not have critical engine, but it just happened to my friend who did his Commercial Multi Engine Rating check-ride that he answered there is no critical engine on counter-rotating propellers and the examiners failed him. And one of the instructor said that he should answered both engines are equally critical. is there any terms that stated that counter-rotating propellers has both engione critical?

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    $\begingroup$ It's just a matter of semantics. "There is no critical engine" simply means that neither engine is more, or less, critical than the other. Which is the same thing as saying "both engines are equally critical." This is a talking point to clarify during the exam, not failure criteria. I would file a complaint about this if it was the only reason for failing... $\endgroup$
    – Michael Hall
    Dec 6, 2019 at 19:15

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The term "critical engine" can actually refer to two very different factors:

  1. P Factor
  2. Engine driven accessories, depending on the airplane and how it's configured.

A twin with counter-rotating propellers doesn't have a critical engine from a controlability perspective, but very often there are important engine driven accessories, like hydraulic pumps, vacuum pumps, generators, etc. that are only installed on one engine, and if you lose that engine, well it's more of a problem than the other one.

So you may lose the left engine and it has no effect on control or performance compared to the right, but that engine has the only hydraulic pump, so now you have to do the engine-failure-on-a-missed-approach song and dance, all while hand pumping the gear up. Gets pretty busy. In such a situation I'd consider the left engine more critical than the right one even though it's not a stick-and-rudder problem.

I think that's what the examiner was looking for.

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    $\begingroup$ Excellent points, I didn't consider that in my comment above. Perhaps there is more to the story on this particular exam failure... Still, you would think the examiner would have explained this to the examinee as the specific reason for failing. $\endgroup$
    – Michael Hall
    Dec 6, 2019 at 20:45
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    $\begingroup$ Yes agreed thanks. I think part of the problem is students are learning on newer twins with dual everything and are unaware that there are older twins with single accessories that create that situation. My fav is the B-24, which only had one hydraulic pump, on engine #3. Shutdown that, and you lose your gear hydraulics and your brakes. They had a procedure where the waist gunners would hook their parachute harnesses to the waist gun pintle mounts or some other convenient structure and deploy them on landing to help slow down. $\endgroup$
    – John K
    Dec 6, 2019 at 20:52
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As an addition to John K's answer - There are twins that have propellers on both sides rotating 'outwards', thus both are critical aerodynamically.

Of course another possible answer is the critical engine is the one that that is still running after one of them fails.

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