The PC6 is a popular skydiver carrier due to its ability to use steep nose down beta descent to get back to the ground quickly, even beating free falling skydivers.

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FAA AC25.1155x (I only saw the draft) proposes that a "means to prevent intentional or inadvertent selected of reverse thrust or propeller pitch below the flight regime" be required, which cannot be overridden. The intention is to prevent several accidents resulting from said deployment. The lockout means must not degrade landing performance, so the intent is not to eliminate beta entirely.

Does this mean that no new aircraft can be certified that has a descent mode from altitude similar to the PC6?

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    $\begingroup$ Are you sure it's Part 25? The one I found is Part 23 and the .1155 is the old section. $\endgroup$ – ymb1 May 11 '18 at 14:36
  • $\begingroup$ What I saw was Part 25. It was old as well. $\endgroup$ – Pilothead May 11 '18 at 15:36

I wouldn't call it a "descent mode from altitude". It's considered to be a dangerous and foolish practice to operate an engine in ground beta (generally coming back into DISCING - blades flat) while in flight to get steep descent rates. If the prop doesn't want to come out of ground beta, you are dead meat.

When the Twin Otter was in production, it was an open secret that demo pilots were using DISCING to get crazy steep approaches to impress customers. After a couple exciting events, flight ops finally put an end to it.

Some airplanes already have ground beta lockout systems. Is this a proposal to mandate a system for the PC6 specifically?

  • $\begingroup$ What I read was not PC6 specific. $\endgroup$ – Pilothead May 11 '18 at 23:49
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    $\begingroup$ If the prop doesn't want to come out of ground beta, you stop the fuel flow and glide to a landing. A better precaution against pilot incompetence is better training, not more system complexity. $\endgroup$ – Peter Kämpf May 27 '18 at 14:38
  • $\begingroup$ If you shut the engine down and the prop moves to feather on its own, the normal action, that's fine. If a prop gets stuck at or near DISCING and you shut it down, and the prop stays put, you will still be coming down like a crowbar with a flat pitch prop windmilling and the result will be unpleasant. That said, I agree that it's an airmanship issue (as in, a practice that is really stupid, airmanship wise) and not something that requires Beta lockouts on small turboprops. $\endgroup$ – John K May 28 '18 at 15:17

I found this FAA Working Group paper on it


(28 page paper, poorly scanned PDF, I am not copying any sections here.)

It looks to me under the Recommendation section that are proposing putting the plane into beta just be made a seperate distinct action such that the crew cannot accidentally go into beta. They must go to Flight Idle, then beta could be allowed.

I am not seeing any follow up either, so I don't know if the AC was published.

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    $\begingroup$ The document is from 1999, I don't think it's the new proposal. Also tip: the pdf is already OCR'ed, which makes copy and paste quite easy. $\endgroup$ – ymb1 May 11 '18 at 14:04
  • $\begingroup$ Most aircraft have a mechanical detent that provides an idle stop, and has to be disengaged, like a trigger or a gate. The Twin Otter has a gate at flight idle, basically a step in the track, and you rotate the hand grips to bypass it to come back into ground beta. I can't imagine an airplane with at least that kind of action required. $\endgroup$ – John K May 11 '18 at 16:06
  • $\begingroup$ @JohnK Can the gate be bypassed in the air or just on the ground? $\endgroup$ – Pilothead May 25 '18 at 17:25
  • $\begingroup$ In the air as well unless the aircraft has a Ground Beta lockout system which some larger complex turboprops have. Most I would say don't have it. A Twin Otter certainly doesn't and you can go into full reverse in the air if the mood suits you. $\endgroup$ – John K May 25 '18 at 23:34

I just came across this. AOPA is a reliable source but it is not a reference to a regulation.

Beta mode is only available for ground operations. Many single-engine turboprops have low propeller ground clearances, so it is vital to minimize beta thrust in contaminated areas to avoid engine and prop damage from dirt and debris. A few creative pilots have tried using beta thrust in flight to increase descent rates; however, some of those who have tried that trick wound up at the bottom of a smoking hole. The use of reverse thrust in flight is strictly prohibited in virtually every type of aircraft. That’s why most turboprop propeller controls have in-flight reverse-thrust lockout systems. Unless it’s approved for your aircraft, don’t even think about it.

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    $\begingroup$ Oh, come on. You can still stop the engine and glide to a safe landing. No smoking hole required in case of beta malfunction. $\endgroup$ – Peter Kämpf May 27 '18 at 14:37
  • $\begingroup$ @PeterKämpf I agree with that. After reading the quote a couple more times I still don't see that it's possible to get in flight beta approved for new aircraft, only that some aircraft have been approved. $\endgroup$ – Pilothead May 27 '18 at 20:59
  • $\begingroup$ Splitting hairs practically speaking, but there IS a flight Beta mode on turbo props now. The transfer of blade angle control from the prop governor to the power lever, and rpm from the prop governor to a fuel controller based rpm governor, which is what Beta mode is (similar to a turbine helicopter really), happens just ahead of IDLE. So when you are at or just above IDLE you are in "flight beta". $\endgroup$ – John K May 28 '18 at 15:26

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