The Big Sky Theory

• At the front of the eye is the cornea, a thick, transparent tissue that forms the outer coat of the eyeball and covers the iris, the colored part of the eye. The pupil, the circular opening in the center of the iris, allows light to enter the eye. The iris and pupil rest against the front of the lens, which is held in place by thousands of elastic fibers. These fibers, and the muscles to which they are attached, enable the lens, by changing shape, to focus on objects at varying distances.
• The retina, the inner layer of the back of the eye, contains more than 125 million light-sensitive receptor cells that receive information about an object being viewed.
• Rods and cones are the two main types of light-sensitive cells found in the retina. Rods, which are approximately 20 times more numerous than cones, respond to darkness, faint light, shape and movement. Thus rods, with their light-sensitive pigment (rhodospin), are responsible for adaptation to darkness (night vision) and perception of shades of gray.
• Cones, on the other hand, are stimulated by bright light and are responsible for our ability to perceive colors. Cones are concentrated in the highly sensitive central section of the retina, the fovea. Light entering the eye is focused directly on the fovea, making it the site of greatest visual acuity (sharpness of vision) and providing the ability to distinguish fine details.
• Visual acuity depends not only on the proper focusing of the image on the retina, but also on the ability of the retina to distinguish between objects that are extremely close together.
• In this area of maximum resolving power, there are approximately 170,000 receptor cells per square millimeter (10.6 million receptor cells per square inch). This vast number of receptors makes it possible to discern tightly spaced, minute objects as separate visual targets.
• The optic nerve, which consists of some one million nerve fibers, connects each eye to the brain and supplies blood to the retina. The retina transforms the information about the patterns of light and dark received by the rods and cones into electrical impulses that travel through the optic nerve to the brain, where they are interpreted as an image.

Source: Flight Safety Digest

The optic nerve is joined to the eye in the retina at a point called the optic disk. Because the optic nerve contains no light-sensitive receptor cells, it is considered “blind” and renders the optic disk blind, as well — creating the area commonly referred to as the “blind spot.” Normally, the blind spot is between five degrees and 10 degrees wide. The small size of the blind spot may make it sound insignificant, but it is enough to allow an aircraft to disappear from view, often before the eyes have detected it.

Source: Flight Safety Digest

The eye has an inbuilt blind-spot at the point where the optic nerve exits the eyeball. Under normal conditions of binocular vision the blind spot is not a problem as the area of the visual field falling on the blind spot of one eye will still be visible to the other eye. However, if the view from one eye is obstructed (for example by a window post), then objects in the blind spot of the remaining eye will be invisible. Bearing in mind that an aircraft on a collision course appears stationary in the visual field, the blind spot could potentially mask a conflicting aircraft.

Source: Limitations of the See-and-Avoid Principle, ¶2.5.1

The average person has a field of vision of around 190 degrees, although field of vision varies from person to person and is generally greater for females than males (Leibowitz 1973). The field of vision begins to contract after about age 35.

Source: Limitations of the See-and-Avoid Principle, ¶2.2

Although our eyes accept light rays from an arc of nearly 200°, they are limited to a relatively narrow area (approximately 10 – 15°) in which they can actually focus on and classify an object. Anything perceived on the periphery must be brought into that narrow field to be identified.

Source: Collision Avoidance, pg. 7

Visual acuity is greatest for objects that are directly in front of the eye. But the fovea is a mere two degrees wide, which results in a very narrow high-acuity detection area and leaves as much as 178 degrees of the detection area in the realm of peripheral vision. This is one reason that we often tend to spot traffic or obstacles out of the “corner” of our eye.

Source: Flight Safety Digest

• The quality of vision varies across the visual field, largely in accord with the distribution on the retina of the two types of light sensitive cells, rods and cones. Cones provide sharp vision and colour perception in daylight illumination and are concentrated at the fovea, the central part of the retina on which an object appears if it is looked at directly. Rods are situated on the remainder of the retina surrounding the fovea on an area known as the peripheral retina. Although rods provide a black and white image of the visual field, they continue to operate at low light levels when the cones have ceased to function.
• Vision can be considered to consist of two distinct systems, peripheral and foveal vision. Some important differences between the two systems are that colour perception and the detection of slow movement are best at the fovea, while detection of rapid movement is best in the periphery. In daylight, acuity (sharpness of vision) is greatest at the fovea, but with low light levels such as twilight, acuity is fairly equal across the whole retina. At night, acuity is greatest in the peripheral retina.
• As figure 5 shows, acuity in daylight is dramatically reduced away from the direct line of sight, therefore a pilot must look at or near a target to have a good chance of detecting it.

Source: Limitations of the See-and-Avoid Principle, ¶2.2

Motion or contrast is needed to attract the eyes’ attention, and the field of vision limitation can be compounded by the fact that at a distance an aircraft on a steady collision course will appear to be motionless. The aircraft will remain in a seemingly stationary position, without appearing to move or to grow in size, for a relatively long time, and then suddenly bloom into a huge mass, almost filling up one of the windows.

Source: Collision Avoidance, pg. 8

When the eye has nothing to specifically focus on, which happens at very high altitudes, but also at lower levels on vague, colourless days above a haze or cloud layer with no distinct horizon, people experience something known as ‘empty-field myopia’, and opposing traffic entering the visual field is just not seen.

Source: Collision Avoidance, pg. 7

In the absence of a visual stimulus (for example, empty airspace), the muscles in the eye relax, preventing the lens from focusing. This creates a problem for a pilot who is attempting to scan for traffic in a clear, featureless sky. Because the eye cannot properly focus on empty space, it remains in a state of unfocused, or blurred, vision. This phenomenon, known as “empty-field myopia,” hinders effective search and detection.

Source: Flight Safety Digest

• In the absence of visual cues, the eye will focus at a relatively short distance. In the dark the eye focuses at around 50 cm. In an empty field such as blue sky, the eye will focus at around 56 cm (Roscoe and Hull 1982). This effect is known as empty field myopia and can reduce the chance of identifying a distant object.
• Because the natural focus point (or dark focus) is around half a metre away, it requires an effort to focus at greater distances, particularly in the absence of visual cues. However, the ability to accommodate to greater distances can be improved by training (Roscoe and Couchman 1987).

Source: Limitations of the See-and-Avoid Principle, ¶2.5.4

The human eyes tend to focus somewhere, even in a featureless sky. If there is nothing specific on which to focus, your eyes revert to a relaxed intermediate focal distance (10 to 30 feet). This means that you are looking without actually seeing anything, which is dangerous. In order to be most effective, the pilot should shift glances and refocus at intervals. Most pilots do this in the process of scanning the instrument panel, but it is also important to focus outside to set up the visual system for effective target acquisition.

Source: AC 90-48D, ¶4.2.2.

One inherent problem with the eye is the time required for ‘accommodation’ or refocusing. Our eyes automatically accommodate for near and far objects, but the change from something up close, like a dark instrument panel, to a bright landmark or aircraft several miles away, takes one to two seconds. That can be a long time when you consider that you need 10 seconds to avoid a mid-air collision.

Source: Collision Avoidance, pg. 7

Pilots should also realize that their eyes may require several seconds to refocus when switching views between items in the cockpit and distant objects.

Source: AC 90-48D, ¶4.2.3.

• Another aspect of eye functioning that is relevant to visual searching is saccadic eye movement. When they are not tracking a moving target, the eyes do not shift smoothly; they shift in a series of jerky movements or “jumps” called saccades. As a result of saccadic eye movements, it is not possible to make voluntary, smooth eye movements while scanning featureless space.
• A study conducted at the U.S. Naval Aerospace Medical Research Laboratory (NAMRL) showed that when the eyes are in saccadic movement, visual acuity decreases sharply, leaving large gaps in the distant field of vision.

Source: Flight Safety Digest

Glancing out and moving your eyes around without stopping to focus on anything is practically useless; so is staring out into one spot for long periods of time.

Source: Collision Avoidance, pg. 11

If you are scanning for an aircraft you know is below, at, or above your altitude, it would be helpful to know where that is in relation to your perception of level in your cockpit. Most cockpit windows tend to bend light downward, so at your level actually looks to be below you. You can get a sense of this in your hangar by picking an object on the wall you know is the same height as your eyes while seated, and then looking at the same point from the cockpit. The next time ATC calls out traffic from a distance, see how its perspective changes as it comes nearer.

Source: Collision Avoidance, pg. 8

Research has shown that the average person has a reaction time of 12.5 seconds. This means that a small or high-speed object could pose a serious threat if some other means of detection other than see and avoid were not utilized, as it would take too long to react to avoid a collision.

Source: AC 90-48D, ¶4.2.1

Visual search at night depends almost entirely on peripheral vision. This is due in part to the night blind spot that involves an area between 5 and 10 degrees wide in the center of the visual field. By looking approximately 10 degrees below, above, or to either side of an object, “off center” viewing can compensate for this night blind spot.

Source: AC 90-48D, ¶4.2.6.

The night blind spot occurs when the fovea becomes inactive under low-level light conditions. The night blind spot involves an area from 5 to 10 degrees wide in the center of the visual field. If an object is viewed directly at night, it may not be seen because of the night blind spot; if the object is detected, it will fade away when stared at for longer than two seconds. The size of the night blind spot increases as the distance between the eyes and the object increases.

Source: Aeromedical Training for Flight Personnel, ¶ 8-26

For improved safety and to aid in collision avoidance, the following safety equipment is recommended:

1. High-intensity anticollision white strobe lights visible from all directions.
2. Pulse light (collision avoidance) systems for the aircraft landing lights.

Source: AC 90-48D, ¶4.5.1

You must ensure the operation of your transponders, even when you are outside radar coverage, in order to enable Traffic Alert and Collision Avoidance System (TCAS)-equipped aircraft to identify conflicting traffic.

Source: AC 91-70B, ¶E.5.4

From January 2009 through December 2013, a total of 42 midair collisions occurred in the United States. During this same time period, there were 461 reported NMACs. Statistics indicate that the majority of these midair collisions and NMACs occurred in good weather and during daylight hours.

Source: AC 90-48D, ¶3.1

When weather conditions permit, regardless of whether an operation is conducted under instrument flight rules or visual flight rules, vigilance shall be maintained by each person operating an aircraft so as to see and avoid other aircraft.

Source: 14 CFR 91, ¶91.113(b)

A study of over two hundred reports of mid-air collisions in the US and Canada showed that they can occur in all phases of flight and at all altitudes. However, nearly all mid-air collisions occur in daylight and in excellent visual meteorological conditions, mostly at lower altitudes where most VFR flying is carried out. Because of the concentration of aircraft close to aerodromes, most collisions occurred near aerodromes when one or both aircraft were descending or climbing, and often within the circuit pattern. Although some aircraft were operating as Instrument Flight Rules (IFR) flights, most were VFR.

Source: U.K. Safety Leaflet, ¶1.c.

Prior to taxiing onto a runway or landing area for takeoff, scan the approach areas for possible landing traffic by maneuvering the aircraft to provide a clear view of such areas. It is important that this be accomplished even though a taxi or takeoff clearance has been received.

Source: AC 90-48D, ¶4.3.1.

During climbs and descents in flight conditions which permit visual detection of other traffic, execute gentle banks left and right at a frequency which permits continuous visual scanning of the airspace about them.

Source: AC 90-48D, ¶4.3.1.

In normal flight, most collision threats will come from an area within 60° left and right of your flight path. However, do not forget the rest of the sky around you. You should also scan at least 10° above and below the projected flight path of your aircraft, because collision threats may be climbing from below or descending from above.

Source: Collision Avoidance, pg. 11

Concentrate your search on the areas most critical to you at any given time. In the circuit especially, always look out before you turn and make sure your path is clear. Look out for traffic making an improper entry into the circuit.

Source: Collision Avoidance, pg. 11

During that very critical final approach stage, do not fix your eyes on the point of touchdown, but scan all around. Another pilot may be aiming for the same point!

Source: Collision Avoidance, pg. 11

Execute appropriate clearing procedures before all turns, abnormal maneuvers, or acrobatics.

Source: AC 90-48D, ¶4.3.1.

• The ability of pilots to sight other airplanes was evaluated during two test programs conducted by the Lincoln Laboratories of the Massachusetts Institute of Technology (MIT). These tests were part of a general research project and were not conducted as a result of this accident. In addition to counting the number of times that these pilots either acquired or failed to acquire an intruder airplane visually, the tests determined the distance at which the targets were acquired.


Probability of seeing the other aircraft as a function of time, (NTSB Aircraft Accident Report, Figure 3)

• One test evaluated pilot performance during unalerted search. The tests were conducted during a series of triangular round robin flights from Hanscom Field, Massachusetts, using two VORTACS near, but not inside, the Boston TCA as waypoints. The subject pilots were not alerted that there would be intruder aircraft or that scanning behavior was the focus of the study. Each leg was flown at a different altitude and the pilot was required to perform his own navigation and answer various questions asked by a the evaluator during the flight. The planned angles of the intercepts were head-on, 90, and 135’, and the intercepts were predominantly from the left (the pilot’s side of the airplane). Data were obtained for 64 unalerted encounters. Visual acquisition was achieved in 36 encounters (56 percent of the total), and the median acquisition range for these 36 encounters was .99 nmi. The greatest range of visual acquisition was 2.9 nmi.


The effect of TCAS-type alert, (NTSB Aircraft Accident Report, Figure 4)

• The other test program evaluated the performance of pilots who had been alerted to the presence of an intruder airplane. Data for 66 encounters were collected during the testing of the TCAS II. The subject pilots were aware that intercepts would be conducted and they received traffic advisories on a TCAS II cathode ray tube (CRT) display. The subject pilots acquired the intruder visually in 57 of the 66 encounters (86 percent of the total). In five of the nine failures, the failure was partially due to the pilot’s response to a TCAS resolution advisory. The median range of the visual acquisitions was 1.4 nmi.
• The performance of the pilots was used to provide data for a mathematical model of visual acquisition. This model is based on the experimental observation that the probability of visual acquisition in any instant of time is proportional to the product of the angular size of the visual target and its contrast with its background. The cumulative probability of visual acquisition is obtained by integrating the probabilities for each instant as the target approaches.
• The data cited herein were developed by a project leader on the Air Traffic Control Division, Lincoln Laboratories, MIT, who had conducted research on human visual performance and flight testing of collision avoidance systems. At the Safety Board’s request, the project leader constructed Probability of Visual Acquisition Graphs based on the extrapolation of pertinent data contained in the facts and circumstances of the collision between the Piper PA-28 and flight 498 with the data described above. (See figures 3 and 4.) The graphs are based on the closure rate between flight 498 and the Piper and on the results achieved by pilots having an unobstructed view of the intruder. The graphs do not account for such limiting factors as cockpit structure and the visibility that the airplanes might be positioned so that they can be seen with only one eye e However, the information in this report is of significance in that it provides a baseline for further evaluation.

Source: NTSB Aircraft Accident Report, PB87-910409, pgs. 14 - 15

Cockpit workload and other factors reduce the time that pilots spend in traffic scans. However, even when pilots are looking out, there is no guarantee that other aircraft will be sighted. Most cockpit windscreen configurations severely limit the view available to the pilot. The available view is frequently interrupted by obstructions such as window-posts which totally obscure some parts of the view and make other areas visible to only one eye.

Source: Limitations of the See-and-Avoid Principle, pg. vii

Visual scanning involves moving the eyes in order to bring successive areas of the visual field onto the small area of sharp vision in the centre of the eye. The process is frequently unsystematic and may leave large areas of the field of view unsearched.

Source: Limitations of the See-and-Avoid Principle, pg. vii

The physical limitations of the human eye are such that even the most careful search does not guarantee that traffic will be sighted. A significant proportion of the view may be masked by the blind spot in the eye, the eyes may focus at an inappropriate distance due to the effect of obstructions as outlined above or due to empty field myopia/ in which, in the absence of visual cues/ the eyes focus at a resting distance of around half a metre. An object which is smaller than the eye’s acuity threshold is unlikely to be detected and even less likely to be identified as an approaching aircraft.

Source: Limitations of the See-and-Avoid Principle, pg. vii

The human visual system is better at detecting moving targets than stationary targets, yet in most cases, an aircraft on a collision course appears as a stationary target in the pilot’s visual field. The contrast between an aircraft and its background can be significantly reduced by atmospheric effects, even in conditions of good visibility.

Source: Limitations of the See-and-Avoid Principle, pg. vii

An approaching aircraft, in many cases, presents a very small visual angle until a short time before impact. In addition, complex backgrounds such as ground features or clouds hamper the identification of aircraft via a visual effect known as ‘contour interaction’. This occurs when background contours interact with the form of the aircraft, producing a less distinct image. Even when an approaching aircraft has been sighted, there is no guarantee that evasive action will be successful.

Source: Limitations of the See-and-Avoid Principle, pg. vii

A traffic search in the absence of traffic information is less likely to be successful than a search where traffic information has been provided because knowing where to look greatly increases the chance of sighting the traffic (Edwards and Harris 1972). Field trials conducted by John Andrews found that in the absence of a traffic alert, the probability of a pilot sighting a threat aircraft is generally low until a short time before impact. Traffic alerts were found to increase search effectiveness by a factor of eight. A traffic alert from ATS or from a radio listening watch is likely to be similarly effective (Andrews 1977, Andrews 1984, Andrews 1987).

Source: Limitations of the See-and-Avoid Principle, ¶2.6.1

Following the AIM, chapter 4, Air Traffic Control, section 3, execute pattern entries and departures for the runway in use appropriate to the airport configuration and information depicted.

Source: AC 90-48D, ¶4.3.1.

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.

Source: AC 90-48D, ¶4.2.4.

• Anticipate the target in the location and ranges you are searching;
• Locate a sizable, distant object (e.g., a cloud formation, mountain peak, prominent landmark, building or pier) that is within the range of the anticipated target, and focus your eyes on it as you begin each scan pattern;
• Refocus frequently on a distant point as you begin each new scan;
• Allow three to five seconds for your saccadic eye movement to suppress before shifting your search to the block of airspace around the object; and,
• Vary distances to ensure a thorough scan and to reduce visual fatigue.

Source: Collision Avoidance, pg. 6

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 almost always 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. It is essential to remember, however, that if another aircraft appears to have no relative motion, it is likely to be on a collision course with you. If the other aircraft shows no lateral or vertical motion, but is increasing in size, take immediate evasive action.

Source: AC 90-48D, ¶4.2.5.

Most importantly, the eye is vulnerable to the vagaries of the mind. We can ‘see’ and identify only what the mind permits us to see.

Source: Collision Avoidance, pg. 7

To accept what we see, we need to receive cues from both eyes (binocular vision). If an object is visible to only one eye, but hidden from the other by a windshield post or other obstruction, the total image is blurred and not always acceptable to the mind. Therefore, it is essential that pilots move their heads when scanning around obstructions.

Source: Collision Avoidance, pg. 7

Glare occurs when unwanted light enters the eye. Glare can come directly from the light source or can take the form of veiling glare, reflected from crazing or dirt on the windscreen. Direct glare is a particular problem when it occurs close to the target object such as when an aircraft appears near the sun. It has been claimed that glare which is half as intense as the general illumination can produce a 42 per cent reduction in visual effectiveness when it is 40 degrees from the line of sight. When the glare source is 5 degrees from the line of sight, visual effectiveness is reduced by 84 per cent (Hawkins 1987). In general, older pilots will be more sensitive to glare.

Source: Limitations of the See-and-Avoid Principle, ¶2.3.3.

The eye, and consequently vision, is vulnerable to many things including dust, fatigue, emotion, germs, fallen eyelashes, age, optical illusions, and the effect certain medications. In flight, vision is influenced by atmospheric conditions, glare, lighting, windshield deterioration and distortion, aircraft design, cabin temperature, oxygen supply (particularly at night), acceleration forces and so forth.

Source: Collision Avoidance, pg. 7