In good visibility your eyes are the main guide, and they usually win over false sensations from your other senses. Take away the horizon and those false sensations can disorient you quickly.
During flight in visual meteorological conditions (VMC), the eyes are the major orientation source and usually prevail over false sensations from other sensory systems. When these visual cues are removed, as they are in instrument meteorological conditions (IMC), false sensations can cause a pilot to quickly become disoriented.
Pilots can rapidly become spatially disoriented when they cannot see the horizon. The brain receives conflicting or ambiguous information from the sensory systems, resulting in a state of confusion that can rapidly lead to incorrect control inputs and resultant loss of aircraft control.
Inside each canal is a section of fine hairs. When the hairs bend, the brain reads a movement.
Each canal is filled with fluid and has a section full of fine hairs. Acceleration of the inner ear in any direction causes the tiny hairs to deflect, which in turn stimulates nerve impulses, sending messages to the brain.
The canal wall turns with your head, and the fluid inside it can move differently from the wall. When it does, it bends the hairs. So the hairs sense the difference between the wall and the fluid, not the turn itself.
PREDICT FIRST
In cloud, you roll briskly into a steady right turn and hold it for a minute. What does your inner ear tell you during that minute?
PREDICT FIRST
After that minute you roll the wings level, briskly. What do you feel?
TRY IT
Inside the inner ear
Pick a turn and how briskly you roll into it, then play it or step through. The top panels compare what the aircraft does with what you feel. Below them, two canals: the grey marks turn with the wall and your head, the blue dots are the fluid, and the dark flap is the section of hairs.
The aircraft
Banked 25° right. Turning right, 5.0° per second.
What you feel
Banked 11° right. Turning right.
Each panel shows the aircraft from behind, and from above with an arrow for its turn. The small figure is you, seen from behind.
Yaw canal, seen from above
Hairs bent left. You feel a turn to the right.
Roll canal, seen from behind
No roll felt.
Grey marks turn with the canal wall and your head. Blue dots are the fluid. The dark flap is the section of hairs.
Turn rate, degrees per second
The aircraft's turn
What the yaw canal signals
Not felt: 2° per second or less
Bank, degrees
The aircraft's bank
The bank you feel
Turn
Roll into the turn
Jump to
Bank
25° R
Bank felt
11° R
Turn
5.0°/s R
Turn felt
2.9°/s R
Holding the turn. The aircraft: banked 25° right, turning right, 5.0° per second. In the yaw canal the hairs are bent left. You feel a turn to the right, and feel banked 11° right.
PHAK gives its threshold as a rotational acceleration of 2 degrees per second. Applying that figure to the rate each canal signals, in roll and in yaw, is the simulation's choice. The fluid's lag, the bank, the turn rate, the roll rates and how fast a felt bank fades are illustrative. Play runs at three times real speed.
A roll the other way
The roll canal works the same way. Roll in briskly, then drag the time to 8 or 9 seconds, just after the roll stops: for a moment the roll canal reports a roll the other way. PHAK describes a demonstration flight that gives a strong sense of rotation in the opposite direction. PHAK does not say what causes it, so the simulation's after-effect is one way to picture it, not PHAK's explanation.
While straight and level, with the pilot’s eyes closed, the instructor pilot smoothly and positively rolls the aircraft to approximately 45° bank attitude while maintaining heading and pitch attitude. This creates the illusion of a strong sense of rotation in the opposite direction.
Now roll into the turn gently. PHAK gives its threshold as a rotational acceleration of 2 degrees per second. Applying that figure to the rate each canal signals is the simulation's choice. Rolling in at 1° per second, neither signal reaches it, so in the simulation you never feel the roll or the turn.
A condition called the leans, is the most common illusion during flight and is caused by a sudden return to level flight following a gradual and prolonged turn that went unnoticed by the pilot. The reason a pilot can be unaware of such a gradual turn is that human exposure to a rotational acceleration of 2 degrees per second or lower is below the detection threshold of the semicircular canals.
Looking down at a map, you let the aircraft roll slowly into a left turn without noticing. You look up and roll the wings level abruptly. Which way do you feel banked, and which way may you lean?
CASA describes how it often starts, and why a quick correction sets it off.
The leans often happen when a pilot looks down at a map, radio or instruments, and the aircraft goes into a gentle, banked turn that is too slow for the vestibular system to detect. When the pilot looks up and corrects the bank, and when this correction is fast enough to be felt by the inner ear, visual and vestibular information get out of sync and the leans occur.
The false bank fades in time, but until it does it pulls at you to roll the wings back into the turn.
A disoriented pilot will feel the need to roll the aircraft to the left again to ‘level’ the wings. This may result in a further or steeper turn to the left, or if level flight is maintained, the pilot will feel compelled to lean in the perceived vertical plane until this illusion subsides.
A turn you no longer feel can also tighten. If the aircraft loses height in the turn, the pilot may feel only a descent and pull back.
Because an aircraft tends to lose altitude in turns unless the pilot compensates for the loss in lift, the pilot may notice a loss of altitude. The absence of any sensation of turning creates the illusion of being in a level descent. The pilot may pull back on the controls in an attempt to climb or stop the descent. This action tightens the spiral and increases the loss of altitude; this illusion is referred to as a “graveyard spiral.”
Once the fluid moves with the canal in a long turn, moving your head in a different plane, such as looking at something elsewhere in the cockpit, can set it moving again. PHAK calls the false sense of turning on another axis that follows the coriolis illusion.
The “coriolis illusion” occurs when a pilot has been in a turn long enough for the fluid in the ear canal to move at the same speed as the canal. A movement of the head in a different plane, such as looking at something in a different part of the flight deck, may set the fluid moving, creating the illusion of turning or accelerating on an entirely different axis.
These sensations are normal, and any pilot can be caught by them. The defence is the flight instruments, and the training to trust them over your body.
The sensations that lead to illusions during instrument flight conditions are normal perceptions experienced by pilots. These undesirable sensations cannot be completely prevented, but through training and awareness, pilots can ignore or suppress them by developing absolute reliance on the flight instruments.
8. Most importantly, become proficient in the use of flight instruments and rely upon them. Trust the instruments and disregard your sensory perceptions.
Whenever the natural horizon is not clear enough to control the aircraft by visual reference, such as often occurs in marginal VMC, all pilots, no matter what their flight experience level, are potential victims of spatial disorientation.