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NASA, Contractor Report - Unified Theory for Aircraft Handling Qualities and Adverse-Pilot Coupling

An adverse aircraft—pilot coupling (APC) or pilot-induced os—
cillation (PIO) can be defined as an unwanted, inadvertent, and
atypical closed-loop coupling between a pilot and the response vari-
ables of an aircraft.1 APC or PIO problems are not new phenomena;
indeed, they have been around since the Wright Brothers and have
been referred to as the senior handling qualities problem.2 McRuer2
gives a concise historical perspective of the PIO problem, including
a review and discussion of germane pilot behavior patterns.

Because of a strong correlation between APC/PIO susceptibility
and modern, full—authority control systems employing fly-by—wire
(FBW) technology, interest in studying the APC/PIO phenomenon
has been increasing. For example, NATO’s AGARD convened a
special workshop on P10,3 and NASA has sponsored a National
Research Council committee to study the problem of APC/PIO.4
US. Air Force interest in the APC/PIO problem has led to the pub-
lication of four reports under the general rubric of a Unified PIO
Study.5‘8

Despite the amount of research that has been directed toward so-
lution of the APCI’PIO problem, there appears to be little consensus
about the phenomenon itself in terms of the pilot behavior that initi-
ates and sustains the APC/PIO. There is general agreement that the
contributing factors are 1) a demanding flight task, 2) a vehicle with
unsatisfactory dynamics, and 3) a triggering event.2 The main thrust
of the research to be described is to suggest one possible human-
centered theory concerning the APC/PIO phenomenon. The theory
to be proposed will attempt to unify the topics of vehicle handling
qualities and APC/PIO, which have been somewhat disparate in the
past. Finally, although pertinent to the handling qualities of high—
performance aircraft, the phenomenon of roll ratchet9 will not be
discussed here.

Figure 1 shows what will be referred to here as the revised struc-
tural model of the human pilot. The model has its genesis in a
previously described structural model10 and in a later modifica—
tion of that model.11 As shown in Fig. 1, the model is describ-
ing compensatory pilot behavior, i.e., behavior involving closed-
loop tracking in which the visual input is system error. The ele-
ments within the dashed box represent the dynamics of the human
pilot.