17.2 Spatial Disorientation, Illusions, and Vision

ACS PA.I.H.K1d, K1e, and K1k are spatial disorientation, motion sickness, and optical illusions. PHAK Chapter 17 and AIM 8-1-5 / 8-1-6 are the matching chapters. The knowledge-test item is almost always “which illusion is this, and what do you believe?” The correct belief is the attitude indicator, not the inner ear.

Three systems, one airplane

The body builds a picture of “which way is up” from three streams:

• Visual — the eyes. In VMC with a real horizon this stream usually wins, which is why you can ignore a lot of inner-ear noise on a clear day.
• Vestibular — the inner ear. Three semicircular canals (roughly roll, pitch, and yaw) sense angular acceleration. The otolith organs sense linear acceleration and gravity. They do not sense a steady-state turn or a constant speed. A rotational acceleration slower than about 2° per second can stay under the canal threshold, so a slow roll into a bank is invisible to the ear.
• Proprioceptive / somatosensory — nerves in skin, muscles, and joints. This is “seat of the pants.” The body cannot tell a G from gravity. A 1.1 G pull in a coordinated turn feels like sitting a little heavier, not like “we are banked 25 degrees.”

Spatial disorientation is losing the plot about the airplane’s attitude, position, or motion. It is not the same as being lost on a sectional. It is thinking the wings are level when they are not. Once outside vision goes away — night over water, a haze layer, a climb into a cloud — the vestibular and proprioceptive streams will invent a story. Unless you have instrument training and use it, do not fly there.

AIM 8-1-5’s prevention sentence is the whole skill: prevent spatial disorientation only by visual reference to reliable, fixed points on the ground or to flight instruments.

Vestibular illusions — learn the names

False horizon and autokinesis are listed with the vestibular set in AIM 8-1-5 because they produce the same end state — a dangerous attitude — but they start in the eyes. A sloping cloud deck, city lights mixed with stars, or a shoreline can become a horizon that is not the horizon. Autokinesis: stare at a single static light in the dark and, after many seconds, it appears to wander; the pilot chases it with the airplane.

Coriolis is why instrument instructors bark about not making sudden, extreme head movements in a prolonged turn, especially in IMC. Cross-checking the fuel selector by craning under the panel in a standard-rate turn is how you buy that illusion.

Optical / landing illusions — PA.I.H.K1k

These do not spin the semicircular canals. They lie about height and distance so you fly the wrong glidepath. AIM 8-1-5 and PHAK use the same list.

• Runway width. A narrower-than-usual runway looks as if you are higher than you are, so the unrecognized illusion produces a low approach — objects on short final, or a landing short. A wide runway does the opposite: you flare high and land hard or long.
• Runway or terrain slope. Upslope runway or terrain looks high, so you go low. Downslope looks low, so you stay high.
• Featureless terrain / black-hole approach. Water, snow, or a dark area with no texture makes you feel high; you again fly low. A night arrival over unlit terrain to a bright runway is the classic black-hole setup.
• Haze. Traffic and the runway look farther away than they are; the usual error is a low, dragged-in approach. Haze also feeds empty-field myopia (below).
• Rain on the windscreen can make you feel high (refraction). Fog can create a sudden pitch-up illusion; the pilot steepens the approach, often abruptly.
• Ground lighting. A straight road or a train can look like a runway. Bright approach lights against dark terrain can make the runway look closer, so you fly high; a dark hole around the field makes you fly low.

Prevention is boring and correct: anticipate the illusion at an unfamiliar field, fly a visual inspection when you can, use VASI / PAPI or an electronic glide slope, and believe the altimeter and airspeed, not the picture you wanted.

Night vision — rods, cones, and a 30-minute clock

The retina has cones (center / fovea: color and sharp daylight detail) and rods (periphery: motion and dim light, no color). Daylight is photopic (cones). Dawn, dusk, and moonlight are mesopic (both). Deep night is scotopic: cones quit, detail collapses, color dies, and a night blind spot opens in the exact center of vision because that is where the cones live and the rods do not.

So the night scan is not a stare. PHAK: to see an object at night, look 5° to 10° off center and keep moving; an off-center image that you freeze for more than about two or three seconds fades. Overlap each scan stop. Do not drill a hole in the windshield at the spot you “know” the traffic is.

Dark adaptation is the rods resetting. AIM 8-1-6: complete adaptation needs exposure to darkness for at least 30 minutes. You can get a usable amount in about 20 minutes under dim red cockpit lighting. White light ruins it in a few seconds. Red light preserves rod sensitivity somewhat, but it wrecks chart colors and makes focusing inside the cockpit harder, so you still need a dim white light for maps and then protect the other eye if you must look at something bright. Closing one eye while you glance at a landing light or a phone preserves some adaptation in that eye.

Dark adaptation is impaired by cabin altitudes above 5,000 feet, by carbon monoxide (smoking or exhaust), by Vitamin A deficiency, and by a day spent in bright sun without sunglasses. AIM wants sunglasses that absorb at least 85 percent of visible light with neutral color. Night vision is an early hypoxia victim; AIM encourages supplemental oxygen above 5,000 feet at night even though 91.211’s hard numbers are higher (those numbers live in the part 91 chapter).

Empty-field myopia is what happens when haze or a cloud top gives the eyes nothing at infinity to grab. They relax to a comfortable 10 to 30 feet and you look without seeing. Focus on a distant object or a wingtip-to-horizon scan on purpose.

Day collision-avoidance scanning (AIM 8-1-6) is a different technique you still need: short stops, each eye movement not more than about 10 degrees, at least a second per sector, and no more than about ¼ to ⅓ of the time inside on the panel.

Motion sickness — PA.I.H.K1e

Motion sickness is the brain receiving conflicting messages — usually the inner ear reporting motion that the eyes have not confirmed, or the reverse. Nausea, sweating, pallor, and vomiting follow. PHAK’s in-airplane kit is not a pill: open the vents, put the eyes on a fixed outside reference (the horizon), and stop unnecessary head movement. Have the sick passenger fly a little if they are a student and it is safe; giving the hands something true to do often helps. Do not self-medicate with over-the-counter motion-sickness drugs. Those antihistamines are the exact 91.17(a)(3) “drug that affects the faculties contrary to safety” problem in 17.3.

Scenario: Luis, a black-hole night, and the leans

Luis is inbound at night to a rural strip. The town is behind him. Ahead is dark terrain and a narrow, well-lit runway. He feels high, so he eases the nose down. The VASI — when he finally looks at it — is all red. At the same time a slow, unnoticed left bank has developed. When he snaps the wings level, his body insists he is in a right bank. He starts to put the left wing back down.

Two lies at once. The narrow, bright runway in a black hole made him feel high, so he flew low (featureless-terrain / runway-width / ground-lighting stack). The leans then invited him to re-enter the bank. The fix is not “which feeling is louder.” Level the wings with the attitude indicator, get back on the VASI / glideslope and altimeter, and stop staring at the threshold lights. If he has no real horizon and is not on instruments, the private-pilot answer was not to be in that soup of dark and lean in the first place.