Some aircraft have propellers capable of reversing pitch, which is also known as beta angle. The example I saw was an Aviat Husky.

(Youtube demonstration)

I want to know if there are any safety features that prevent this from activating while airborne. For example, what kind of controls activate it? How long does it take to actually reverse the blades, and how much physical effort does the pilot have to exert? Would it be impossible to force the reverse when at high cruising speed?

If it is a mechanically powered reverser, then is it possible for it to be inadvertently triggered?

And if it is activated, can it quickly be set back to original blade angle?

Note, I am interested in any propeller-driven aircraft, not just commercial ones, but smaller ones as well. Piston prop, turboprop, GA, anything.


Beta props are pretty rare on anything smaller than Twin Otter or King Air or Porter or Turbo Beaver. Can't speak for MT's system, but most beta mode systems use mechanisms to switch control of propeller blade angle from the propeller rpm governor to the throttle. Beta mode is the mode of throttle or power lever control of blade angle, with engine torque controlled by a governor system of some kind to regulate rpm.

Beta mode is very similar to turbine helicopter collective control, where a lever, the collective, directly changes rotor pitch and a governor system regulates rotor rpm by adjusting engine torque.

Beta mode is normally entered just above flight idle (flight beta) once the propeller speed governor is no longer regulating blade angle (blades at fine pitch stops).

There is normally some kind of detent or gate that requires a deliberate movement or unlatching to move below flight idle (the Twin Otter requires you to twist the throttle grips - on others there are finger triggers).

Also, some turboprops have beta lockout systems that prevent power lever movement below flight idle until you're on the ground. But a lot of airplanes don't and can be put into ground beta in flight if the flight idle gate or latch is deliberately operated by the pilot.

During the Twin Otter program there was some controversy at DeHavilland Canada because demo pilots were using ground beta (coming into DISC - blades flat, like a disc) to make crazy steep descents (I've also heard of Porter pilots doing it). A very dangerous thing to do, because if the prop doesn't want to come out of beta mode when you move the lever(s) back up, you are dead meat.

  • $\begingroup$ I have heard of C130 pilots (the ones who were flying the landings onto the Forrestal in particular, but perhaps this is more general) using beta/reverse basically in the flare to achieve very short landing distances, which makes me wonder how the Herc handles all this... $\endgroup$ – UnrecognizedFallingObject Jul 24 at 4:40
  • $\begingroup$ So once it's engaged, then at full throttle you're at full reverse pitch, and at half throttle you're at half reverse pitch? $\endgroup$ – DrZ214 Jul 24 at 5:59
  • $\begingroup$ More recently, the Let L410 has been having issues with beta mode engaging uncommanded, like in this accident: avherald.com/h?article=4b111511&opt=0 $\endgroup$ – AEhere Jul 24 at 9:34
  • $\begingroup$ @DrZ214 yes but the range is in the other direction from normal. At flight idle, the blades will be a the now pitch limit for constant speed operation. As you move the power lever AFT from flight idle, the blades get finer and finer in direct relation to the power lever movement. At DISC, the blades are flat. Farther back moves them into reverse pitch. You move the power lever/throttle and the blades move. Meanwhile, rpm is now controlled by a speed governor that regulates engine power to the propeller to keep a predetermined rpm schedule as the blades move into reverse. $\endgroup$ – John K Jul 24 at 15:07

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