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Visual Scanning and Collision Avoidance

An airplane on a collision course with you does not move across the windshield. It sits still and grows — which is precisely what the human eye is worst at noticing.

Technical Subject Areas — FI.II.B

The traffic that hits you is the traffic that isn't moving

The useful form of that fact is a test you can run in the air. Once you have a target, pick a fixed reference on the airframe — a rivet line, the edge of a windshield post — and watch whether the other airplane drifts against it. Drift means the two flight paths diverge and you will pass. No drift over several seconds means the geometry is closing on a single point in space, and the only question left is which of you changes something.

Growth is the only warning a target like that gives, and growth is nearly nothing for a long time and then everything at once. That is backwards from what the eye catches. Motion pulls your attention; the target that matters has none. Nothing about seeing traffic is automatic, which is why scanning is taught as a procedure rather than left to instinct.

What the eye can and cannot do

Sharp, colored vision comes from cones packed into the fovea at the center of the retina, and they need decent light to work. Cone density falls away steeply outside that center and rods dominate the periphery, so what you get out there is motion and dim light rather than color and detail. The piece of sky you are seeing clearly at any instant is small — far smaller than it feels, because your brain fills in the rest.

You also have a true blind spot where the optic nerve leaves each retina. Prove it on the ground. Put two marks on a page about three inches apart, cover your left eye, hold the page at arm's length, and stare at the left mark with your right eye. Keep staring at the left mark and bring the page slowly toward you: somewhere around a foot the right-hand mark disappears completely, then comes back as you keep going. Swap eyes and the marks swap roles — with the left eye you fixate the right mark and lose the left one. The side matters, because the blind spot sits toward your temple in each eye. Nothing warns you when the mark goes. Your brain does not report the gap, it paints over it.

The last trap has no symptoms. Given an empty, featureless sky, your eyes relax to a short focal distance instead of holding infinity — empty-field myopia. You are staring at the traffic and not focused on it. Give your eyes something real every so often: a wingtip, a ridgeline, a cloud edge, then back out.

Everything that degrades the picture

Start with yourself. The better condition you are in, the better you see. Vision is among the first things hypoxia takes, and fatigue, poor sleep, alcohol, tobacco, medication and general health all degrade it long before you would call yourself unfit to fly.

Then the air: haze, smoke, mist, dust and cloud all cut contrast and let distant traffic disappear into the background, and sunrise and sunset put glare exactly where you need to look. Then the airplane — a windshield full of bugs, dirt and fingerprints is the cheapest fixable hazard in aviation. Leave the polarized sunglasses on the ground. FAA guidance advises against polarized lenses in the cockpit: they can cancel the glint of sun off another airplane that often gives it away, they produce blotchy patterns through laminated and plexiglass windshields, and they wash out LCD instrument and tablet displays.

When your senses lie

Visual illusions mostly bite on approach. A narrow runway or upsloping terrain feels high, so you fly low; a wide runway or downsloping terrain feels low, so you fly high. A black-hole approach over featureless terrain, an approach over water, and haze all push you low, while fog feels like a pitch-up that pilots steepen into. A false horizon — a sloping cloud deck, a line of ground lights — tempts you to level the wings on the wrong reference. The defenses are the visual glideslope indicators, the altimeter, and knowing the airport.

The inner ear tells two different kinds of lie, from two different organs. The fluid-filled semicircular canals sense angular acceleration — the onset of a rotation — by deflecting fine hairs, and they are honest about that onset and useless about rotation that is sustained. The leans, the most common, follows a slow unnoticed bank: roll level and it feels like a bank the other way. The graveyard spiral shares that root — the sensation of a long steady turn fades, so a pilot who rolls out, feels a turn the other way and rolls back in then sees altitude falling and pulls, tightening the spiral. Coriolis follows a head movement in a prolonged turn and feels like rotation about a different axis.

The otolith organs sit alongside the canals and sense linear acceleration and the pull of gravity, and they cannot tell the two apart. That is where the remaining two come from. Somatogravic gives nose-up on acceleration and nose-down on deceleration — the classic being a night takeoff where the acceleration feels like a steep climb and invites a push. The elevator illusion turns the upward acceleration of an updraft into a false climb, and invites the same push.

The fix for all of them is short: stay out of cloud unless you are trained, current and equipped, and when your body disagrees with the instruments, believe the instruments.

See and avoid, and whose job it is

Under 14 CFR 91.113(b), when weather conditions permit, vigilance must be maintained by each person operating an aircraft so as to see and avoid other aircraft — whether the flight is IFR or VFR. 14 CFR 91.3 puts final responsibility for the operation of the aircraft on the pilot in command. Regulations change and the CFR is the authority, so read the current text of the section rather than anyone's summary of it, this one included.

Flight following, a clearance, an approach controller and an ADS-B display do not move that job to anyone else. Use every one of them, but as additions to the scan rather than substitutes: plenty of VFR traffic is talking to nobody and transmitting no position at all. Position reports, yours and theirs, build a map of who is where, at what altitude, going which way, so your eyes know where to look first.

How to scan

Effective scanning is a series of short, regularly spaced eye movements that bring successive pieces of sky into central vision. Each movement should be no more than about 10 degrees, and each sector held for at least one second — that is what the eye needs to detect something. Most pilots prefer a horizontal back-and-forth pattern, but which pattern matters less than holding to one. Sweeping your eyes smoothly across the windshield feels thorough and finds nothing.

Night changes the technique, because the center of your vision is the part that fails in low light. The way to see a dim object after dark is to look a little to one side of it — roughly 5 to 10 degrees off — so the light falls on rods rather than on the fovea, and to keep moving your eyes in small steps, because a faint image held steady in one place fades out on you within seconds. Keep interior lighting low to protect dark adaptation, and turn the exterior lights on so somebody else's scan can find you.

Either way, attention is divided with the greater share outside, and that starts before engine start: an organized flight deck, charts and checklists off the windshield, means less time head-down.

Blind spots and clearing procedures

Every airplane has places you cannot see, and the answer is to move the airplane so that you can. A high wing hides the inside of a turn and everything above it; a low wing hides below and behind. In both, a large volume behind and beneath you is never visible from the seat, and clearing turns do not remove that void — they relocate it long enough to look.

A second set of eyes is always worth having, and simulated instrument flight requires one. 14 CFR 91.109(c) is the section: it requires a safety pilot who holds at least a private pilot certificate with category and class ratings appropriate to the aircraft, an aircraft with fully functioning dual controls, and adequate vision forward and to each side of the aircraft, with a limited exception for approved throwover controls and a provision for a competent observer. Read the section itself before you fly with a hood on, and read the current text — the CFR is the authority and it changes.

Speed, and how little time you have

The faster the closure, the less time between the first speck on the windshield and impact. Think of a bicycle and a fast motorcycle on the same road when a dog steps out: same road, different outcome. Now let the dog be another airplane coming the other way.

Seeing is not reacting, either. The AIM breaks that sequence down — noticing the object, recognizing it as an aircraft, realizing it is on a collision course, deciding, moving the controls, the airplane responding — at roughly 12.5 seconds end to end. That budget is why a target growing without moving gets an immediate change of altitude or heading rather than another look.

Where the risk concentrates

Midairs are not random. They happen where airplanes are funneled together: traffic patterns, especially arriving at a busy field with no picture of who is already there; over VORs and along popular Victor airways; at the common departure and arrival times. Add the clear, calm weekend morning, when every airplane on the field is flying.

Where you meet it next

An examiner can ask about this as a knowledge task, but the real evaluation runs the whole ride: clearing turns before every maneuver, a visible scan on every leg of the pattern, and heads-up discipline while you are being asked to do something else. Failing to clear the area is disqualifying however well the maneuver itself was flown.

The lessons next door pick up the pieces this one only touched. Aeromedical factors go further into hypoxia, fatigue and the rest of what takes your eyesight before it takes anything else. Night operations covers dark adaptation and off-center viewing as flying technique rather than as physiology. Airport traffic patterns and radio procedure are where most of your collision exposure actually lives.

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