Recently I theorized that AF447 suffered a roll PIO condition that completely overload PF's ability to control other axes of the aircraft.
While looking for a definition of PIO to compare against the BEA data, I came across a relatively recent paper showing how thinking is moving on the subject of PIO. You can read the paper
here. Aircraft and Rotorcraft Pilot Couplings – Tools and Techniques for Alleviation and Detection (ARISTOTEL)
Among the key points in the paper are:
An aircraft- or rotorcraft-pilot coupling (A/RPC) is an unintentional (inadvertent) sustained or uncontrollable vehicle oscillations characterized by a mismatch between the pilot’s mental model of the vehicle dynamics and the actual vehicle dynamics. The result is that the pilot's control input is out-of-phase with the response of the vehicle, possibly causing a diverging motion.
and
It can also be said that in normal situation, the pilot drives the vehicle, whereas during an A/RPC event the situation is reversed. In that case, the pilot is driven by the vehicle due to this mental mismatch and actively tries to control it
Based on the many comments from the industry, [ref. 6] suggested the following ten features (definitions) characterizing virtually every APC documented in the open literature:
1.
PIO is a sustained or uncontrollable unintentional oscillation resulting from the efforts of the pilot to control the aircraft. This is the MIL-STD-1797A definition, with the word “unintentional” added.
2.
PIO occurs when a response state of the airplane is approximately 180 degrees out of phase with the pilot. It could be any response state of the airplane, the most common for fixed wing aircraft are pitch attitude, roll attitude, and load factor [ref. 6]
3.
PIO is an event that results from faulty aircraft design, extension of the airplane’s operational usage into an area for which it was not intended, or following a failure, and is not the fault of the pilot.
4.
PIO is commonly found to be related to deficiencies in basic flying qualities characteristics, though it should be treated independently from flying qualities. Most PIOs outside of the research world are related to rate limiting of a control effector or software element upstream of a control effector, but rate limiting can be both the cause of PIO and the result of it.
5.
PIO may be either constant-amplitude, convergent, or divergent with time.
6.
PIO may be any number of cycles of oscillation; there is no minimum number to declare it a PIO.
7.
PIO may occur at very low frequencies – near the phugoid mode in pitch – up to frequencies of around 3 Hz (“roll ratchet”). The most common frequency is in the range for pilot closed-loop control, typically 1/6 Hz to slightly above 1 Hz (1 rad/sec to 8 rad/sec), but frequency alone does not determine whether an oscillation is a PIO.
8.
High-frequency, small-amplitude oscillations in pitch (sometimes referred to as “pitch bobble”), and in roll (“roll ratchet”), may be considered a “mild” form of PIO, and may not even be judged as PIO in all cases. If the amplitudes of the oscillations become intrusive on the piloting task, they are PIOs.
9.
PIO that interferes with, but does not prevent, performance of a primary mission task is a “moderate” PIO; if a Cooper-Harper Handling Qualities Rating 7 is obtained, it is usually in the range of 4-6 (Level 2 by handling qualities specifications). In general, “moderate” PIO is associated with peak-to-peak angular rates of less than ±10 degrees/sec and control forces less than ±5 lb.8 “Moderate” PIO requires corrective action for normal operation of the airplane, but if it occurs in developmental testing the flight test program can continue.
10.
PIO that prevents performance of the task, or that requires the pilot to abandon the task in an attempt to stop the oscillation, is a “severe” PIO; if a Cooper-Harper Handling Qualities Rating is obtained, it is usually 7 or worse (Level 3 or unflyable by handling qualities specifications). Peak-to-peak angular rates are usually greater than ±10 degrees/sec, and control forces greater than ±10 lb, though rate limiting can attenuate the former and result in large increases in the latter.8 “Severe” PIO requires immediate changes to the airplane, and if it occurs in developmental testing the flight test program should be postponed or redirected until the corrections are made.