Visual Scanning and Collision Avoidance

Visual scanning is the disciplined way a pilot moves the eyes to search the sky for other aircraft and to read the instruments. It is the working half of see and avoid, the oldest collision-avoidance rule. A note in ICAO Annex 2, repeated in the guidance material to the EU's Standardised European Rules of the Air (SERA), stresses that vigilance for potential collisions must be kept on board every aircraft, whatever the type of flight or class of airspace, and 14 CFR 91.113(b) requires every pilot, under IFR or VFR, to keep watch so as to see and avoid other aircraft whenever the weather permits.

The difficulty is that the human eye is a poor collision detector. It sees sharply over a tiny area, sees almost nothing while it moves, and is drawn to movement, which is exactly what a converging aircraft does not show. A good scan is therefore a technique to be learned, not a natural talent, and it is examined in the Human Performance papers from PPL to ATPL and in the FAA knowledge tests.

Limits of see-and-avoid

The FAA's Aeronautical Information Manual (AIM) notes that the eyes take in an arc of about 200° of the horizon at a glance, but only the fovea, a very small area at the centre of the retina, sends sharp images to the brain. It gives an example: an aircraft that could be recognised at 7 miles in foveal vision would have to be within about 0.7 of a mile to be recognised outside it. EASA training texts put the same point in terms of acuity: 20/20 (6/6) vision exists only at the fovea, falls to 20/40 at 5° from it and to about 20/200 at 25°. Reading an eye chart therefore says little about spotting traffic.

Geometry makes it worse. EASA texts give the case of two aircraft closing head-on at 800 kt: the other aircraft subtends only about 0.5° three seconds before impact and 1° at one and a half seconds, and becomes large only in the last 1.4 seconds. In a US experiment quoted in the same texts, pilots who had been warned of converging traffic in good visibility detected it in time to avoid a collision in only about half of the encounters.

Windscreen pillars, wings and door frames hide parts of the sky, and dirt, insects, haze, smoke, rain and flying towards the sun all reduce detection. The AIM observes that most collisions happen in good weather and that traffic clusters near VORs and in the busy airspace around aerodromes. Other defences therefore supplement the eye: exterior lights, which increase conspicuity by day and night, traffic information from ATC, and traffic displays and ACAS. FAA guidance is clear that traffic displays support the visual scan and do not replace it.

Saccades and fixations

Unless it is following a moving object, the eye does not sweep smoothly. It moves in a series of rapid jumps called saccades, each followed by a brief rest, or fixation, during which it samples the new field. EASA texts give about a third of a second for a saccade and its rest. Little useful information is gathered during the jump itself; the visual cortex joins the fixations into what feels like continuous vision, which is why the jumps are not noticed.

The operational consequence is simple. A continuous sweep across the windscreen is a string of blurred saccades and detects very little. Detection happens during the fixations, so a scan must be built from deliberate stops.

Lookout scanning technique

The recommended visual scanning technique, often called a block scan, divides the sky into sectors and brings each into central vision in turn:

• Move the eyes in steps of no more than about 10°, so that successive areas overlap.
• Hold each position long enough to detect a target. EASA texts recommend about 2 seconds per sector; the FAA's AIM asks for at least 1 second per area.
• Cover above and below the horizon as well as ahead and to the sides.
• Move the head, and if necessary the body, to see around pillars and door posts, and when needed manoeuvre the aircraft, for example by lifting a wing, to see into the area it hides.

The AIM suggests starting over the left shoulder and working across the windscreen to the right, because the brain is used to taking in information from left to right. It also recommends looking out along the left wing and past the wingtip into the first sector, then returning inside along the right wing, which helps the eyes refocus between the panel and the distance. Whatever pattern is chosen, it should be used consistently.

For judging a sighted aircraft, the AIM gives a simple rule: an aircraft seen above the horizon is probably at a higher level, and one below the horizon probably lower.

At night the rods, which give low-light and movement detection, lie away from the fovea, so a dim light stared at directly can disappear. The night scan uses short movements and viewing slightly to one side of the area of interest; a single light stared at for many seconds may also appear to wander (autokinesis, see visual illusions and the eye and vision).

Each eye also has a blind spot, where the optic nerve leaves the retina and there are no light-sensitive cells. The other eye normally covers it, but if a windscreen pillar blocks that eye's view, an aircraft can sit unseen in the gap. EASA texts point out that traffic on a constant bearing can remain there until impact, which is one more reason to move the head.

Instrument scan patterns

The same principles govern the instrument scan or cross-check. In the radial scan, also called the hub-and-spoke scan, the attitude indicator is the hub: it is the only instrument that shows pitch and bank directly and without lag, so each glance at a performance instrument (airspeed, altimeter, heading, vertical speed) starts from it and returns to it. The FAA's Instrument Flying Handbook calls it the selected radial cross-check and also describes inverted-V and rectangular cross-checks as alternative patterns.

Three scanning errors are classic: fixation, staring at one instrument, typically the localiser or glide path needle near minima, while others drift; omission, leaving an instrument out of the scan; and emphasis, relying too heavily on one instrument. With practice the scan becomes largely automatic, skill-based behaviour that uses little attention, which is why a beginner becomes overloaded where an experienced pilot has capacity to spare. In visual flight the instrument scan is interleaved with the lookout, not substituted for it.

Empty-field and instrument myopia

The relaxed eye does not focus at infinity. With nothing to focus on, such as a cloudless sky at altitude, a haze layer, uniform overcast or darkness, it drifts to a resting focus a short way ahead. EASA texts put this at just under 1 m to about 1.5 m; the FAA's AIM gives a comfortable focal distance of 10 to 30 ft. Either way distant objects fall out of focus and the pilot looks without seeing. This is empty-field myopia.

Instrument myopia is the closely related effect after a long spell head-down: the eyes stay focused near and need several seconds to refocus on distant objects, and they tire quickly from repeated changes of focus. The remedy for both is to focus deliberately on a real, distant object, such as a wingtip, a cloud edge or a ground feature, before starting each outside scan.

View full schematic

Managing head-down time

The more time spent looking out, the better the chance of seeing a threat, but the instruments, charts and avionics also need attention. The AIM cites studies showing that time on tasks inside the cockpit should be no more than a quarter to a third of the time spent outside, which it expresses as no more than 4 to 5 seconds on the panel for every 16 seconds of outside scan.

Head-down time is mostly created before the flight. Studying charts, noting frequencies, organising the cockpit and programming the navigator and flight management system on the ground all reduce the time needed in the air. The FAA's 5P single-pilot risk review (plan, plane, pilot, passengers, programming) treats programming the avionics as a risk area of its own, because the head-down time it takes is itself a hazard. In flight, a task that captures attention, such as reading a chart when uncertain of position or troubleshooting an indication, can leave the aircraft effectively unwatched for minutes; the defence is to fly the aircraft first and deal with the problem in short glances (see attention, vigilance and distraction). In a crew, head-down tasks are shared so that one pilot keeps the lookout while the other works inside.

Constant bearing and collision risk

Two aircraft flying straight at constant speeds towards the same point meet there only if the bearing of each from the other stays the same while the range decreases, a situation that seafarers call constant bearing, decreasing range. Seen from the cockpit of an aircraft holding its heading, the other aircraft keeps a constant relative bearing: it stays on the same spot of the windscreen. EASA texts describe the two aircraft as converging on a line of constant bearing (LCB).

This is the most dangerous geometry for the eye. The conflicting aircraft produces no sideways or vertical movement, the cue that peripheral vision detects best, and its image grows very slowly until the final seconds, when it suddenly "blossoms". Traffic that moves across the windscreen, horizontally or vertically, will pass clear as long as both aircraft hold their tracks and speeds.

The AIM's rules follow from this:

• Any aircraft that shows no relative motion and stays in one scan sector is likely to be on a collision course. If it grows larger without lateral or vertical movement, take evasive action.
• On an obvious collision course, take immediate evasive action, where possible in accordance with the right-of-way rules: aircraft approaching head-on both turn right, and of two aircraft converging at about the same level the one that has the other on its right gives way (see rules of the air).
• Expect the other pilot to manoeuvre too, watch the other aircraft during the manoeuvre, and then resume the scan, because there may be more traffic.

A small change of heading or altitude also creates relative motion and can reveal traffic that has been hiding in plain sight. An ACAS resolution advisory, where fitted, is followed even against an ATC instruction.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.