Scan Aviation

[Rosette Allaband]

Despitethe value of electronic means of conflict detection (e.g. ACAS and STCA) visual lookout remains an important defence against loss of separation for all classes of aircraft. This is particularly true for pilots of light aircraft, many of which are single-pilot operated, are not equipped with ACAS or transponders and frequently operate VFR outside ATC radar cover and at low altitude. It is essential that these pilots develop sound visual scanning techniques.

This article is composed of extracts from the UK CAA Safety Sense Leaflet No 13 - 'Collision Avoidance' (see further reading). Although this article is directed towards pilots of light aircraft it is equally valid for all pilots, including those of large commercial aircraft.

To avoid collisions you must scan effectively from the moment the aircraft moves until it comes to a stop at the end of the flight. Collision threats are present on the ground, at low altitudes in the vicinity of aerodromes, and at cruising levels.

Before take-off, check the runway visually to ensure that there are no aircraft or other objects in the take-off area. Check the approach and circuit to be sure of the position of other aircraft. Assess the traffic situation from radio reports. After take-off, continue to scan to ensure that there will be no obstacles to your safe departure.

Especially during climb or descent, listen to radio exchanges between air traffic and other aircraft and form a mental image of the traffic situation and positions of aircraft on opposing and intersecting headings, anticipating further developments. Scan with particular care in the area of airway (route) intersections and when near a radio beacon or VRP. You should remain constantly alert to all traffic within your normal field of vision, as well as periodically scanning the entire visual field outside the aircraft to ensure detection of conflicting traffic. Remember that the performance capabilities of many aircraft, in both speed and rates of climb/descent, result in high closure rates, limiting the time available for detection, decision, and evasive action.

The best way to develop effective scanning is by eliminating bad habits. Naturally, not looking out at all is the poorest scan technique! Glancing out at intervals of five minutes or so is also poor when considering that it takes only seconds for a disaster to happen. Check the next time the aircraft is climbing out or making an approach to see how long you spend without looking outside.

Learn how to scan properly by knowing where and how to concentrate your search on the areas most critical to you at any given time. In the circuit especially, always look out before you turn to make sure your path is clear and that nothing is approaching from an area that will be hidden as you turn. Look out for traffic making an improper entry into the circuit.

During that very critical final approach stage, do not forget to scan all around to avoid tunnel vision. Pilots often fix their eyes on the point of touchdown. You may never arrive at the runway if another pilot is also aiming for the same runway threshold at that time!

In normal flight, you can generally avoid most of the risk of a mid-air collision by scanning an area at least 60 left and right of your intended flight path. Be aware that constant angle collisions often occur when the other aircraft initially appears motionless at about your 10 o'clock or 2 o'clock positions. This does not mean you should forget the rest of the area you can see. You should also scan at least 10 above and below the projected flight path of your aircraft. This will allow you to spot any aircraft that is at an altitude that might prove hazardous to you, whether it is level with you, climbing from below or descending from above.

The more you look outside, the less the risk of a collision. Certain techniques may be used to increase the effectiveness of the scan. To be most effective, the gaze should be shifted and refocused at regular intervals. Most pilots do this in the process of scanning the instrument panel but it is also important to focus outside the cockpit or flight deck to set up the visual system for effective target acquisition. Looking well ahead for weather and pre-planned navigation features can help. Proper scanning requires the constant sharing of attention with other piloting tasks, thus it is easily degraded by such conditions as distraction, fatigue, boredom, illness, anxiety or preoccupation.

Effective scanning is accomplished by 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, and each area should be observed for at least one second to enable detection. Although horizontal back-and-forth eye movements seem preferred by most pilots, each pilot should develop the scanning pattern that is most comfortable and then keep to it. Peripheral vision can be useful in spotting collision risks. 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 horizontal or vertical motion on the windshield, but is increasing in size, take immediate evasive action.

Two scanning patterns described below have proved to be very effective for pilots and involve the 'block' system of scanning. This system is based on the premise that traffic detection can be made only through a series of eye fixations at different points in space. In application, the viewing area (windshield) is divided into segments, and the pilot methodically scans for traffic in each block in sequential order.

Start at the far left of your visual area and make a methodical sweep to the right, pausing very briefly in each block of the viewing area to focus your eyes. At the end of the scan, return to and scan the instrument panel and then repeat the external scan.

Start in the centre block of your visual field (centre of front windshield); move to the left, focusing very briefly in each block, then swing quickly back to the centre block after reaching the last block on the left and repeat the action to the right. Then, after scanning the instrument panel, repeat the external scan.

There are other methods of scanning, of course, some of which may be as effective as the two described above. However, unless some series of fixations is made, there is little likelihood that you will be able to detect all targets in your scan area. When the head is in motion, vision is blurred and the mind will not register potential targets.

A VFR pilot shouldn't really be "scanning" the instruments in the sense that an IFR pilot does; he should be scanning the sky for traffic (see my first answer) while using his peripheral vision to maintain attitude. Yes, he should periodically (and briefly) check the instruments to make sure nothing is amiss, but then go back to scanning for traffic. I wasn't taught any particular "scan" for that, just to look at whichever instruments seem most useful for what I'm doing at the moment.

If you're having trouble holding proper altitude and airspeed as a VFR student pilot (from a comment to other answer), you may be spending too much time looking at the instruments rather than not enough. That may seem counter-intuitive, but the key to both is attitude. Get the right attitude, using the artificial horizon or other instruments at first if necessary, then look outside at the real horizon (in particular, the relation of your nose to it) and then just maintain that picture while you do your traffic scan.

The only time a VFR pilot is supposed to be "scanning" the instruments is during hood training. For that, I was taught the "T" scan: assuming you have a six-pack in the standard arrangement, you focus on the AI (top center) and then briefly look left at airspeed, then back, then down at heading, then back, then right to altitude, then back. Repeat until the hood comes off.

The method I was taught is to hold your eyes steady on each clock position for 1-3 seconds. This takes advantage of how your peripheral vision is more sensitive to movement. If your eyes are constantly moving, everything you see is moving, so your brain filters it all out.

Note that objects coming directly toward you don't move, which is why you need to scan with your detailed center vision to detect size changes. That also works better with the clock scan rather than continuous scan, but I'm not as sure why.

A proper scan optimizes our vision for collision avoidance. However, the term may be a misnomer; scan implies a sweep of the eyes, while the correct scan for conflicting traffic is actually a sequence of intense, fixated observations. The eyes need one to two seconds to adjust before they can focus; a continuous sweep blurs the vision.

MACs are frequently the result of one aircraft overtaking another, so check for overtaking aircraft after every few scans, especially during approach and landing when midair collisions are most likely to occur.

Haze, flight over open water, or an obscured horizon can make it difficult to see distant objects, impairing the ability to refocus. The same phenomenon can occur when flying over a haze or cloud layer with a high overcast layer above. Focus on the farthest point visible, even the wing tip, to overcome the problem. In poor visibility, repeat refocusing every minute or so.

In addition to atmospheric conditions and optical illusions, irritants in the air, fatigue, age, residual alcohol in the bloodstream, and lower oxygen levels can all impact the ability of your eyes to perform at the optimum level.

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