The NTSB report continues:
The airplane was equipped with a Bendix/King 150 Series Automatic Flight Control System (AFCS), which was approved for
use in Piper PA-32R-301 model airplanes by the FAA on November 1, 1982. The AFCS provided two-axis control for pitch
and roll. It also had an electric pitch trim system, which provided autotrim during autopilot operation and manual electric trim
for the pilot during manual operation.
The AFCS installed on the accident airplane had an altitude hold mode that, when selected, allowed the airplane to maintain the altitude that it had when the altitude hold was selected. The AFCS did not have the option of allowing the pilot to preselect an altitude so that the autopilot could fly to and maintain the preselected altitude as it climbed or descended from another altitude. The AFCS had a vertical trim rocker switch installed so that the pilot could change the airplane's pitch up or down without disconnecting the autopilot. The rocker switch allowed the pilot to make small corrections in the selected altitude while in the altitude hold mode or allowed the pitch attitude to be adjusted at a rate of about 0.9 degree per second when not in altitude hold mode.
The AFCS incorporated a flight director, which had to be activated before
the autopilot would engage. Once activated, the
flight director could provide commands to the flight command indicator to maintain wings level and the pitch attitude. To satisfy the command, the pilot could manually fly the airplane by referencing the guidance received in the flight command indicator, or the pilot could engage the autopilot and let it satisfy the commands by maneuvering the aircraft in a similar manner via the autopilot servos.
According to an FAA Instrument Flying Handbook, Advisory Circular 61-27C
(AC) (Section II, "Instrument Flying: Coping
with Illusions in Flight"), one purpose for instrument training and maintaining instrument proficiency is to prevent a pilot from
being misled by several types of hazardous illusions that are peculiar to flight. The AC states that an illusion or false impression occurs when information provided by sensory organs is misinterpreted or inadequate and that many illusions in flight could be created by complex motions and certain visual scenes encountered under adverse weather conditions and at night. It also states that some illusions may lead to spatial disorientation or the inability to determine accurately the attitude or motion of the aircraft in relation to the earth's surface. The AC also states that spatial disorientation as a result of continued VFR flight into adverse weather conditions is regularly near the top of the cause/factor list in annual statistics on fatal aircraft accidents.
The AC further states that the most hazardous illusions that lead to
spatial disorientation are created by information received
from motion sensing systems, which are located in each inner ear. The AC also states that the sensory organs in these systems
detect angular acceleration in the pitch, yaw, and roll axes, and a sensory organ detects gravity and linear acceleration and that,
in flight, the motion sensing system may be stimulated by motion of the aircraft alone or in combination with head and body
movement. The AC lists some of the major illusions leading to spatial disorientation as follows:
"The leans - A banked attitude, to the left for example, may be entered
too slowly to set in motion the fluid in the 'roll'
semicircular tubes. An abrupt correction of this attitude can now set the fluid in motion and so create the illusion of a banked
attitude to the right. The disoriented pilot may make the error of rolling the aircraft back into the original left-banked attitude or, if level flight is maintained, will feel compelled to lean to the left until this illusion subsides.
Coriolis illusion - An abrupt head movement made during a prolonged
constant-rate turn may set the fluid in more than one
semicircular tube in motion, creating the strong illusion of turning or accelerating, in an entirely different axis. The disoriented
pilot may maneuver the aircraft into a dangerous attitude in an attempt to correct this illusory movement....
Graveyard spiral - In a prolonged coordinated, constant-rate turn, the
fluid in the semicircular tubes in the axis of the turn will
cease its movement...An observed loss altitude in the aircraft instruments and the absence of any sensation of turning may
create the illusion of being in a descent with the wings level. The disoriented pilot may pull back on the controls, tightening the spiral and increasing the loss of altitude....
Inversion illusion - An abrupt change from climb to straight-and-level
flight can excessively stimulate the sensory organs for
gravity and linear acceleration, creating the illusion of tumbling backwards. The disoriented pilot may push the aircraft abruptly into a nose-low attitude, possibly intensifying this illusion.
Elevator illusion - An abrupt upward vertical acceleration, as can occur
in a helicopter or an updraft, can shift vision
downwards (visual scene moves upwards) through excessive stimulation of the sensory organs for gravity and linear
acceleration, creating the illusion of being in a climb. The disoriented pilot may push the aircraft into a nose low attitude. An
abrupt downward vertical acceleration, usually in a downdraft, has the opposite effect, with the disoriented pilot pulling the
aircraft into a nose-up attitude....
Autokinesis - In the dark, a stationary light will appear to move about
when stared at for many seconds. The disoriented pilot
could lose control of the aircraft in attempting to align it with the false movements of this light."
The AC also states that these undesirable sensations cannot be completely prevented but that they can be ignored or sufficiently suppressed by pilots' developing an "absolute" reliance upon what the flight instruments are reporting about the attitude of their aircraft. The AC further states that practice and experience in instrument flying are necessary to aid pilots in discounting or overcoming false sensations.