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With flight control technology available today like fly-by-wire, digitally augmented command inputs etc., I wonder whether any technical solution exists (or could possibly exist) to detect and counter the occurrence and dangers of pilot-induced oscillations (PIO).

Consider for example a low visibility final approach scenario:

  1. Is it possible to detect an early onset of PIO and either augment pilot input to prevent further amplification of oscillations, or, less intrusive, give some (e.g. aural) warning to the pilot?

  2. Also, a fully developed PIO situation might be detected and a warning triggered to increase awareness or prompt the pilots towards an eventual go around, similar to windshear warnings.

If feasible, do such technical mechanisms exist, or would they even be considered helpful by pilots at all? I imagine a critical part to solve would be not to override an actually intended control input by the pilot (that is within the usual safe envelope).

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    $\begingroup$ I would think that Airbus flight laws make PIO rather unlikely (the aircraft auto-trims to fly straight when you release the stick and the rather low sensitivity leads to rather atypical piloting technique of quick nudges with releasing the stick in between). But that's just one specific kind of FBW and I have no idea what the others do. $\endgroup$
    – Jan Hudec
    Commented Jan 30, 2017 at 17:26
  • $\begingroup$ Actually the auto-trim is likely to make it more PIO prone if not designed properly. Often integrators are used to provide auto-trimming and this will introduce a lag in the system which will result in a system more susceptible to PIO. Careful design with a feed forward path is required to make it PIO resistant, the Gripen PIO accident was a problem with the early design and was easily corrected. $\endgroup$
    – Adrian
    Commented Jan 31, 2017 at 13:11

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PIOs occur because the pilot senses a rapid movement and feels the urge to counter it. Key is the frequency of a few Hertz, so the pilot's reaction kicks in with a phase shift such that it promotes the movement, and low damping such that the oscillation amplitude grows over time. With a working FCS this kind of rapid movement should not happen.

However, once the FCS is not tuned to the situation, it can actually aggravate the situation and promote PIOs. The best known case is probably the 1992 crash landing of Tom Morgenfeld with the YF-22 prototype.

Here the gains in the FCS were set too high with afterburner on and gear up at slow speed.

Another case happened in the early phases of flight testing the V-22 Osprey. Here two of the three gyros in the control system were incorrectly wired and the pilot made a wrong control input.

So it is less the ability of the FCS to detect PIOs than its improved damping which never lets a rapid oscillation happen in the first place.

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  • $\begingroup$ There have been some cases of PIO on Airbus A321 aircraft in certain conditions which then led to an update of flight control laws: tsb.gc.ca/eng/rapports-reports/aviation/2002/a02o0406/… $\endgroup$
    – Cpt Reynolds
    Commented Jan 31, 2017 at 18:34
  • $\begingroup$ @CptReynolds: As I read it, in both cases icing changed the aerodynamics. With a clean aircraft this would not had happened. $\endgroup$
    – Peter Kämpf
    Commented Feb 1, 2017 at 10:23
  • $\begingroup$ Sorry but I consider this to be quite a misleading description on the root cause of PIOs. PIOs occur due to a lag in the feedback system so that the aircraft is not responding as the pilot expects it to. Rapid movement is not a problem per se, there are very agile combat aircraft that are PIO resistant. Also you can get low frequency PIO problems, e.g. a phugoid PIO, the space shuttle early design had a PIO problem on approach which wasn't rapid movement. Finally the YF-22 was a rate limit PIO and not the FCS gains. When it went into rate limit it introduced a lag leading to the PIO. $\endgroup$
    – Adrian
    Commented Feb 2, 2017 at 17:04
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    $\begingroup$ @Adrian: This was not meant to be a definition of what a PIO is. Did you read and comprehend the question? It seems the asker is keenly aware of what a PIO is and wonders about the role of the FCS in it. $\endgroup$
    – Peter Kämpf
    Commented Feb 2, 2017 at 22:20
  • $\begingroup$ I did read the question. However my comment was in reply to your response not to the original question. If it was not meant to be a definition of what a PIO is then why start with "PIOs occur because"? With it becoming the accepted answer I think a little more rigour in the answer is required and in my opinion some of the description related to PIOs is misleading. $\endgroup$
    – Adrian
    Commented Feb 3, 2017 at 9:24
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Maybe, but it would require giving a special feedback to the pilot.

PIO occurs when the pilot input is lagging or leading the tendency the pilot is trying to correct, so the pilot ends up reinforcing it, instead of dampening the oscillation.

To break this loop there would need to be some kind of instrument or display that enabled the pilot to visualize the lag and also indicate how much control movement is necessary to correct it. Such a system would not be trivial to design.

One point of fly by wire systems is to make PIO unlikely because the pilot does not have to guess how much input will generate a certain result. The pilot just says, I want to fly level and the plane takes care of the rest. In other words the pilot does not need to guess how much to move the stick to level the plane. Thus, a fly by wire system should theoretically make what you are proposing unnecessary in the first place.

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    $\begingroup$ "To break this loop there would need to be some kind of instrument or display..." Or a pilot that realizes their mistake and stops making that mistake. A PIO only happens because the pilot "chases the ball", once you realize you are in that situation all you have to do is hold steady or go around. $\endgroup$
    – Ron Beyer
    Commented Jan 30, 2017 at 19:59
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    $\begingroup$ Fly by wire systems don't make PIO unlikely by pure virtue of being FBW. If the FBW emulates behaviour of mechanical controls like the Boeing one, PIO is just as likely. The flight laws is what might prevent PIO. $\endgroup$
    – Jan Hudec
    Commented Jan 31, 2017 at 18:52

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