In the parlance of flight instructors and flight researchers,
the term "graveyard spiral" refers to any high-speed
spiral dive, as distinguished from a "graveyard spin"
in which the airplane is actually stalled at a low airspeed. A
pilot can enter a graveyard spiral in many different ways, including
the scenario described by Gillingham and Wolfe (Spatial Orientation
in Flight, USAFSAM TR-85-31, Dec 1986), but most often the entry
is subtle rather than violent. The violence typically follows
a gentle, unnoticed entry into a banked attitude below the pilot's
threshold for angular acceleration, which calls for a brief account
of how the inner ear works.
The sensory nerves in the semicircular canals of the inner ear
are the receptors that provide inputs to our "sense of balance"
that, among other services, keeps us from falling in the shower
when we get soap in our eyes. Because these sensors respond only
to linear and angular accelerations, they tell the brain nothing
dependable about rates or positions, such as roll rates and bank
angles. Furthermore, these sensors have relatively high thresholds,
below which they do not sense accelerations. As a consequence
a pilot can gradually enter a bank without feeling any change
in aircraft attitude, and this is the most common beginning of
a graveyard spiral.
When the pilot notices the banked attitude on the artificial horizon
dicator, several bad things can happen:
If the pilot makes the correct aileron input to roll out of the
bank and the resulting roll acceleration is above the pilot's
threshold, the wings may be leveled, but the pilot will now "feel"
that the airplane is banked in the opposite direction. This experience
is known as having "the leans" because of the compelling
urge to lean in the direction of the original banked attitude
even though the wings-level attitude is maintained. If the illusion
is overpowering, the pilot will often roll back into the original
turn to restore a feeling of wings-level and become confused,
then totally disoriented.
If the pilot makes the incorrect aileron input, as occasionally
happens, the bank angle will start to increase, with the artificial
horizon bar rolling farther away from wings-level with a consequent
lowering of the nose of the airplane and a loss of altitude. The
pilot may be able to catch this mistake immediately and make the
proper responses to recover, but not always. The pilot may also
try to stop the rotation of the horizon bar by a hard-over rotation
of the control yoke in the opposite direction and pull back on
the yoke to stop the loss of altitude. This tightens the turn
and steepens the dive.
At this point the pilot has a fully developed horizon control
reversal and will hold full aileron and full rudder in the direction
of turn all the way to impact.
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Cockpit view of the last moment before impact after breaking out of IMC. By this time, it may be too late to exit the spiral. |
See an animation of the
three displays
Return to "Kennedy's
Design-Induced Spiral"
Return to "More on Horizon
Control Reversal"
Return to Capt. Kukar's B-747
Crash in AeroNews 1/96
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