Light general aviation piston twins have full-feathering props. It's normal to perform feathering checks by pulling the prop level all the way down to feather, briefly, and observing the changes. First, why is the feather check performed and what is the pilot looking for during this check? Also, how far should the RPMs be allowed to drop? Could allowing a large RPM drop cause mechanical damage?

I realize that the specific allowances may vary between Lycoming and Continental engines and between models, but an average number would be helpful for the sake of the question.

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    $\begingroup$ In my mind the prop check has a lot more to do with being a constant speed prop than being a feathering prop. The check is performed to check control and governor operation and (especially in frigid temps) to cycle warm oil into prop. Most modern designs don't need much of the later. $\endgroup$ – J Walters Feb 13 '16 at 19:34
  • $\begingroup$ On singles you don't pull it back to feather so you don't have the possibility of getting that "big RPM drop." But I totally get what you're saying. $\endgroup$ – ryan1618 Feb 13 '16 at 23:22
  • $\begingroup$ My Aztec AFM says do not exceed a 500 RPM max drop for the 1500 RPM feather check and 300 RPM max drop for the 2200 RPM governor check (4.17). In either case you are checking the same control system; the speed of the check determines what part of the control range you are checking. $\endgroup$ – J Walters Feb 13 '16 at 23:31
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    $\begingroup$ Nobody seems to have mentioned that you're also checking to make sure you don't get a windshield full of oil when you pull the RPMs back in flight. $\endgroup$ – Porcupine911 Feb 18 '16 at 23:59
  • $\begingroup$ @Porcupine911 That's a great point! But if you fly a Cessna 337, like me, you won't get a windshield full of oil when your rear prop governor takes a crap. ;) $\endgroup$ – ryan1618 Feb 19 '16 at 2:02

The check is performed to make sure the prop will actually feather when pulled past the feather gates. You'll be in a world of hurt if you are not able to feather a prop if the engine quits.

A couple hundeed RPM drop is acceptable (generally). So called "deep cycling" props does nothing for you. I suppose that there are some goofy mechanisms on oddball planes that require a deep cycle but the vast majority only require you to observe a small drop.

Remember, you are checking that the governor can control the prop, not how far the prop can travel.

Edit 1: I should add that you should just do whatever the POH/AFM tells you to do.

Edit 2: a pilot is typically looking for an oil pressure change, manifold pressure rise, and an RPM drop with each prop cycle.

  • $\begingroup$ While I voted for this answer, I'd like to add one small correction. I don't know that you'd actually be "in a world of hurt". Your single-engine performance will be significantly degraded, but, even with feathering props, you should still be familiar with single-engine operation of the aircraft when that prop does not feather. The Piper Aztecs I flew forever ago were notorious for not fully feathering. -- EDIT: Oops. Just realized this was an old answer. $\endgroup$ – Shawn Jan 2 '18 at 15:57
  • $\begingroup$ Fair point! A lot of twins will still fly, albeit with a higher Vmc and degraded climb performance. Change "Aztec" to "Apache" though... $\endgroup$ – acpilot Jan 3 '18 at 16:45
  • $\begingroup$ That's sad that Piper put bigger engines on the aircraft but still couldn't figure out how to make them feather properly. Other than the Cub, I was never fond of any of the Pipers I flew. Senecas weren't bad, but I absolutely hated flying the Azwreck. :-/ $\endgroup$ – Shawn Jan 3 '18 at 17:30

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