I recently received this question in my inbox from a website subscriber;
My instructor keeps mentioning pilot induced oscillations during my tailwheel training debriefs, and he does so without really explaining it clearly. Could you explain to me what pilot induced oscillations are and why they happen in the simplest terms possible?”
Simplest terms possible? Certainly!
Let’s take a step back to stability, when your instructor (hopefully) covered stability with you. He/she should have mentioned two types of stability; static (the initial reaction of an aircraft once it has been displaced from its respective axis) and dynamic (stability around a particular axis for a duration of time.)
Rarely do instructors mention stability over each type of axis to their students, but low and behold, the axis in which the aircraft pitches around, which is called the lateral axis, is quite the focus of aeronautical engineers. Since these engineers are focused on control feel and safety, they engineer our aircraft to usually have the least amount of control resistance and oscillatory nature as possible. However, each airplane is unique, and sometimes designers want some sort of pitch instability to reach some other goal, such as making an aircraft more maneuverable.
PIO is the result of the pilot attempting to correct oscillations, but in doing so their reaction time is shorter than the period of the oscillations (from their lowest point of the oscillation to their highest point.) The result is that the oscillations become even more unstable, possibly leading to a fatal situation if not handled properly.
(Damped Vs. Undamped Stability from the Pilot's Handbook of Aeronautical Knowledge.)
Well how do you handle it? Most aircraft designers suggest to stop fighting the oscillations and let the aircraft design do it’s thing. Sometimes this means either releasing the flight controls, or holding them in a neutral position until the oscillations dampen. However, sometimes extremely unstable aircraft PIO's can become even worse, and pilot intervention is absolutely necessary for the safest outcome.
Check out this video of an F-8 Crusader pilot encountering PIO during a test flight.
Note how in the video the oscillations start near the landing flare, increase their intensity as the pilot attempts to correct for them, and then decrease their intensity as the pilot seizes the corrections.
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