LESSONS · HUMAN FACTORS · 15 MINUTES · AUSTRALIA AND NZ
See and avoid.
An aircraft heading for the same point as you can be one of the hardest to see. Watch why from above and from your seat, then learn how a scan and a radio call help.
In modern aviation, see-and-avoid is the last line of defence, but usually not the only mechanism for avoiding a collision or an Airprox event.
You see the most detail in a small area at the centre of your vision.
There is a small pit called the fovea where almost all the light sensing cells are cones. This is the area where most “looking” occurs (the center of the visual field where detail, color sensitivity, and resolution are highest).
Outside that centre, your vision is good at noticing movement, and movement is usually the first sign of a threat.
Peripheral vision can be most useful in spotting collision threats from other aircraft. Each time a scan is stopped and the eyes are refocused, the peripheral vision takes on more importance because it is through this element that movement is detected. Apparent movement is usually the first perception of a collision threat and probably the most important because it is the discovery of a threat that triggers the events leading to proper evasive action.
You fly straight and level. Another aircraft, at your height, will reach the same point as you at the same moment. Both of you hold your speed and track. Over the next minute, what does it do on your windscreen?
PREDICT FIRST
On that course, the other aircraft is 2 NM away. When it is 1 NM away, how wide does it look compared with before?
TRY IT
Converging traffic, from your seat and from above
Fly the minute before you would meet. Watch the other aircraft on your windscreen, from above and on the width chart. Then give it a different speed so it is no longer on a collision course, or change the angle until it hides behind a window post.
From your seat
Drawn to scale: 1° is the same width anywhere across the view, and the other aircraft is drawn as wide as it looks. Anything under 2 pixels is drawn 2 pixels wide.
From above, north up
Navy is you, red is the other aircraft. The thin lines show where you saw it every 10 seconds. On the collision course they stay parallel.
How wide it looks
Its width on your windscreen against time to the meeting point. The dashed line marks 12.5 seconds to go: the time AC 91-14 cites for a pilot to avoid a collision after seeing the other aircraft.
Jump to
The other aircraft's speed. Yours is 100 kt.
Distance
2.36 NM
Bearing
45° R
Moved in 10 s
0.0°
Width
0.13°
It sits 45° right of the nose and has moved less than half a degree in the last 10 seconds. It is 0.13° wide.
On this course both aircraft reach the meeting point at the same moment. Same height, straight and steady. The speeds, the 10 m wingspan and the window posts are illustrative.
Why it is hard to see
On a collision course the other aircraft gives your eyes little to work with. It does not move, and for most of the approach it is tiny. Of size and contrast, AC 91-14 says size matters more.
Size and contrast are the two primary factors that determine the likelihood of detecting other aircraft, size being the more important factor.
An aircraft on a steady course and speed can be harder to pick out than one that keeps changing.
Gliders and aircraft conducting aerobatics can sometimes be observed more easily because of their constantly changing profile direction, attitude and altitude. These aircraft generally do not remain on a fixed course for a long time, which would provide the human eye the opportunity to more easily obtain a fix than is the case where speed and direction remain constant.
A window post can hide it too. In the simulation, set the angle between your tracks to 70°: on its steady bearing the other aircraft sits behind the window post for the whole minute. PHAK adds that a post can hide a target from one eye and leave it in the other eye's blind spot. AC 91-14 says to move your head to see beyond window posts.
On the other hand, where the field of vision of one eye is obstructed by an object (windshield divider or another aircraft), a visual target could fall in the blind spot of the other eye and remain undetected.
Workload mismanagement can lead to excessive ‘head in the cockpit’ with less time spent looking outside the aircraft during busy periods. Pilots need to move their head to see beyond window posts and any other obstructions, such as pilots or passengers in the adjacent seat(s).
AC 91-14 describes a scan of 10° sectors, looking at each for one second. How long does it take to scan 180° across and 30° up and down?
The FAA's flying handbook describes the same scan: short, regular eye movements that bring each part of the sky into your central vision.
Effective scanning is accomplished with a series of short, regularly-spaced eye movements that bring successive areas of the sky into the central visual field. Each movement should not exceed 10 degrees, and each area should be observed for at least 1 second to enable detection.
An empty sky gives your eyes nothing to focus on, so they relax and focus short. Refocus at intervals.
The human eyes tend to focus somewhere, even in a featureless sky. In order to be most effective, the pilot should shift glances and refocus at intervals.
Empty-field myopia is a condition that usually occurs when flying above the clouds or in a haze layer that provides nothing specific to focus on outside the aircraft. This causes the eyes to relax and seek a comfortable focal distance that may range from 10 to 30 feet. For the pilot, this means looking without seeing, which is dangerous.
Alerted or not, seeing has limits. In the ATSB's investigation of a midair collision at Caboolture, Queensland, in July 2023, trees blocked the two pilots' view of each other, and one pilot appeared to see the other aircraft late.
A stand of trees between the runways prevented the Pawnee and Jabiru pilots from being able to see one another’s aircraft once the Jabiru had taxied onto the runway heading.
While the Jabiru pilot appeared to see the Pawnee late in the sequence and attempted to evade it, the 2 aircraft collided near the runway intersection at approximately 130 ft above ground level.