In aviation, a graveyard spiral, or death 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.
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 when the pilot is in IMC – Instrument meteorological conditions. That means flying in bad weather, when one can’t see the ground, or even horizon, and thus one needs to fly solely by using instruments.
In such conditions, it is possible to experience spatial disorientation and loses awareness of the aircraft’s attitude. In other words, the pilot loses the ability to judge the orientation of their aircraft due to the brain’s misperception of spatial cues.
The graveyard spiral consists of both physiological and physical components.
What is supposed to happen:
We think of our ear as an organ for hearing, but that’s only one small part of what it does. Your inner ear has a series of fluid filled tubes which sense orientation, acceleration, and up from down. It lets you tell whether you are standing up, or upside down, even if your eyes are closed.
Notice the three sets of fluid-filled tubes. They are like the motion detectors in a Wii controller
Notice the three sets of fluid-filled tubes (semicircular canals)
They are like the motion detectors in a Wii controller. Since they are all perpendicular to each other, they tell your brain about motion in the X, Y or Z direction.
Here you see what happens when you tilt your head down:
How does a pilot get disoriented, and tricked into performing a graveyard spiral?
These three sets of tubes are the equivalent of gyroscopes located in the X, Y and Z plane.
Each corresponds to the rolling, pitching, or yawing motions of an aircraft.
Ideally, as your airplane and body moves, your inner ear sends correct signals to the brain, which then correctly interprets them. Thus you should feel whether you are right side up, or upside down; whether you are banking right, or are flying level.
But this system evolved in our ancestors, for primates who lived on the ground or spent some time in trees; the vast majority of their motion was during day, or during night when the moon was out (which offers plenty of light.) Most motion of our ancestors was done with sight, not blind. But in this case we are dealing with pilots flying in IMC – Instrument meteorological conditions, and evolution didn’t prepare our species for this kind of motion.
So when flying blind, our inner ear & brain don’t work perfectly. They can get tricked. People can end up feeling like they are level, when they are really turning, or even feel right-side-up when they are upside-down! You can read more details here.
There is a solution. A pilot must consciously override our instinct to judge our orientation based on what we feel, and instead rely on the visual cues of horizon, and of the instruments in the airplane, until the brain once again adjusts.
Perception vs reality