In aviation, a graveyard spiral is a dangerous spiral dive entered into accidentally by a pilot who is not trained or not proficient in instrument flight when flying in instrument meteorological conditions (IMC). Other names for this phenomenon include "suicide spiral", "deadly spiral", "death spiral" and "vicious spiral".
Graveyard spirals are most common in nighttime or poor weather conditions where no horizon exists to provide visual correction for misleading inner-ear cues. Graveyard spirals are the result of several sensory illusions in aviation which may occur in actual or simulated IMC, when the pilot experiences spatial disorientation and loses awareness of the aircraft's attitude. In other words, the pilot loses the ability to judge the orientation of his aircraft due to the brain’s misperception of spatial cues.
The graveyard spiral consists of both physiological and physical components. Mechanical failure is often a result but generally not a causal factor, as it is the pilot’s sense of equilibrium which leads to the spiral dive. Flying by “the seat of the pants,” and failing to recognize and/or respond to instrument readings is the most common source of controlled flight into terrain where a plane controlled by a pilot impacts ground.
Physics of the Graveyard Spiral
The impression given by the senses in that situation would be level flight, with a descent indicated on the altimeter and vertical speed indicator. This usually leads to the pilot "pulling up" or attempting to climb by pulling back on the control yoke. In a banking turn, however, the plane is at an angle and will be describing a large circle in the sky. Pulling back on the control yoke has the effect of tightening that circle and causing the plane to lose altitude at an increasing rate. An increasing component of the lift being generated by the wings is directed sideways by the bank angle. At that point the aircraft is describing a descending circle or spiral. In the ever-tightening, descending spiral the aircraft eventually exits the base of the clouds and/or impacts the ground.
To avoid this situation, a student pilot or a pilot under instruction uses a view limiting device to learn instrument flying proficiency under the supervision of a flight instructor. Instrument-rated pilots also use view-limiting devices supervised by an instructor or a safety pilot to practice instrument flight and maintain instrument flying proficiency.
Vestibular Aspects of the Graveyard Spiral
The vestibular system, located in the inner ear, contains two distinct structures: the semicircular canals, which detect changes in angular acceleration, and the otolith organs (the utricule and the saccule), which detect changes in linear acceleration and gravity. Both the semicircular canals and the otolith organs provide information to the brain regarding the body’s position and movement. A connection between the vestibular system and the eyes helps to maintain balance and keep the eyes focused on an object while the head is moving or while the body is rotating. 
The semicircular canals are three half-circular, interconnected tubes located inside each ear, each of which corresponds to a different plane of motion: roll, pitch, or yaw. Each canal is filled with a fluid called endolymph and contains a motion sensor with little hairs whose ends are embedded in a gelatinous structure called the cupula. The cupula and the hairs move as the fluid moves inside the canal in response to an angular acceleration. In flight, when the head is still and the airplane is straight and level, the fluid in the canals does not move and the hairs stand straight up, indicating to the brain that there is no rotational acceleration. 
The Graveyard Spiral is associated with a return to level flight following an intentional or unintentional prolonged bank turn. For example, a pilot who enters a banking turn to the left will initially have a sensation of a turn in the same direction. If the left turn continues (~20 seconds or more), the pilot will experience the sensation that the airplane is no longer turning to the left. At this point, if the pilot attempts to level the wings this action will produce a sensation that the airplane is turning and banking in the opposite direction (to the right). If the pilot believes the illusion of a right turn (which can be very compelling), he/she will re-enter the original left turn in an attempt to counteract the sensation of a right turn. If the pilot fails to recognize the illusion and does not level the wings, the airplane will continue turning left and losing altitude until it impacts the ground. 
- Federal Aviation Administration. 2008. Aeromedical Factors. Pilot's Handbook of Aeronautical Knowledge. Oklahoma City, OK: FAA Flight Standards Service.
- Soderlind, Paul A. The Deadly Spiral. AVWeb.com, May 11,2000. http://www.avweb.com/news/airman/184306-1.html retrieved March 24, 2013.
- Naval Air Training Command. 2002. Joint Aerospace Physiology Student Guide: Spatial Disorientation. Corpus Christi, TX: AETC/BUMED Joint Specialized Undergraduate Pilot Training.
- Saladin, Kenneth S. Anatomy & Physiology: The Unity of Form and Function. 2012.
- Pilot Handbook. FAA. http://www.faa.gov/library/manuals/aviation/pilot_handbook/media/PHAK%20-%20Chapter%2016.pdf