Graveyard spiral

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 an airplane controlled by a pilot hits the ground.

The pilot mistakenly believes they are flying with the wings level, but 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 airplane 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 airplane to lose altitude at an increasing rate, like water swirling in a drain or funnel. An increasing component of the lift being generated by the wings is directed sideways by the bank angle, not only pushing the airplane "upward" into the turn, but reducing the amount of lift which is holding the airplane up. At that point the aircraft is describing a descending circle or spiral, with a flight path that again resembles being in a funnel. In the ever-tightening, descending spiral the aircraft eventually exits the base of the clouds and/or hits the ground.

...
Wikipedia