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The FAA Pilot's Handbook of Aeronautical Knowledge says on page 5-20:

Because it is more desirable for the aircraft to have “spiral instability” than Dutch roll tendencies, most aircraft are designed with that characteristic.

What are the grounds for that?

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2 Answers 2

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Because spiral instability is easier to address by the pilot than Dutch Roll tendencies.

Spiral instability is instability about the longitudinal axis. For example, spiral instability means that if the right wing tip moves down, it continues to move down rolling the plane to the right. Its simple to detect and address with left aileron.

Additional information on spiral instability available here: http://www.faatest.com/books/FLT/Chapter17/SpiralInstability.htm

Dutch Roll causes cross-control-type movements across two axes, the vertical and longitudinal axis, such that when the airplane rolls right, it yaws to the left, then it swings back the other way and rolls left, while yawing to the right. This movement is difficult for the pilot to counteract and the inputs from the pilot can actually amplify these movements.

Additional information on Dutch Roll tendancies available here: http://www.faatest.com/books/FLT/Chapter17/FreeDirectionalOscillations.htm

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  • $\begingroup$ Thank you for your answer. Are there any recommended recovery procedures to break Dutch Roll, like "power idle, full opposite ruder, briskly yoke forward" for a spin recovery? $\endgroup$
    – lemonincider
    Jun 29, 2017 at 18:52
  • $\begingroup$ @lemonincider I've never flown a plane with Dutch Roll tendencies. I usually fly smaller airplanes. I believe that it is more of a problem for higher performance jets with sweptback wings. I believe reducing speed is how you address it. Pilot inputs often makes Dutch Roll worse. boldmethod.com/learn-to-fly/aerodynamics/dutch-roll $\endgroup$
    – Devil07
    Jun 29, 2017 at 18:57
  • $\begingroup$ Thank you very much for your informative replies and the link! $\endgroup$
    – lemonincider
    Jun 29, 2017 at 19:04
  • $\begingroup$ "Spiral instability is instability about the longitudinal axis" ... is it? The spiral mode is a lateral-directional eigenmode, decoupled from the longitudinal modes (short and long period). As you correctly state it causes increasing roll, so I find the longitudinal part misleading at best. $\endgroup$
    – AEhere supports Monica
    Aug 1, 2019 at 8:43
  • $\begingroup$ @AEhere, I think he is referring to motion in longitudinal AXIS, rather than longitudinal modes, in that case it’s roll. And that’s true it’s a lateral-directional mode of motion. $\endgroup$
    – Kolom
    Oct 17, 2019 at 9:57
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@Devil07 concentrated the answer on complexity of the movement (1D vs. 2D).

I'll talk about the response times, which will be different between isntabilities. Typically, the response time of the spiral mode will be longer than that of the dutch roll.

This makes it easier to control spiral for the pilot, which has more time to:

  • Identify the situation
  • Prepare the appropriate command mentally
  • Synchronize its commands to the phase of the instability
  • Apply the right amount of control force

In the case of faster instabilities, the pilot could have a hard time synchronizing its input to the right phase, and may in if failing to do so actually degrade the situation by adding pilot-induced perturbation.

Finally even when the airplane stays controlable and recovers, faster instabilities are more uncomfortable, as our incomfort "ceiling" seem to decrease with the square of the frequency of the oscillations. Since the spiral doesn't even always produce oscillation, the problem of comfort is never even brought up.

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    $\begingroup$ Well, spiral instability does not have any phase as it is simply diverging, not oscillating. So there is no need to synchronize the commands. $\endgroup$
    – Jan Hudec
    Oct 16, 2019 at 5:25

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