Protective Measures against Illusions

Aviation in Cognition 10

•That head movements, fatigue, night and conditions of reduced visibility are all factors that can promote visual illusions.

•That manufacturers and certifying authorities also have a role in providing protection.

Collision and the Retinal Image

If a closing aircraft remains on the same spot on the windscreen then it is maintaining a Line of Constant Bearing (LCB), and a collision risk exists. If it appears to move across the windscreen, vertically or horizontally, then no collision risk exists while both aircraft maintain their current tracks. The blind spot is of special significance when flying visually in VMC as it is possible that conflicting traffic will remain permanently on this area and undetected.

High speed traffic on a converging track from ahead may produce a very small angular picture on the retina until it is very close. Typically with two aircraft flying at 800 knots closing speed, approaching head on, each aircraft will only subtend a retinal angle of 0.5° with 3 seconds to impact and only 1° at 1.5 seconds to collision. Because of the aircraft’s small retinal size, it is quite possible that the pilot will not notice it at all if there is no relative movement across the windscreen. In the last 1.4 seconds the image of the converging aircraft will grow very large indeed, but by this time it will be far too late to take avoiding action.

At 3 seconds, ½°

At 1.5 seconds, 1°

At 0.75 seconds, 2°

At 0.38 seconds, 4°

Figure 10.11 The retinal size of an approaching aircraft before impact

Figure 10.11 shows the rate of increase for an aircraft approaching at a closing speed of 800 knots. It can be seen that the image of the aircraft remains very small until just before impact.

At any stage of flight there is a risk of a mid air collision. With the sophistication of modern radar and air traffic control systems the risk is greatly reduced when operating in controlled airspace, but it can never be completely eliminated. It is essential that all pilots maintain a good scan of the world outside as well as of the cockpit instruments.

However conscientiously the pilot maintains a lookout it is probably true that detection of all possible collision risks cannot be guaranteed with purely visual scanning. In an experiment in the USA with aircraft flying in good VMC conditions, pilots were warned of aircraft on collision headings from ahead. Even with almost perfect visibility, and with prewarning, only 50% of the pilots were able to detect the approaching aircraft in sufficient time that would have enabled them to take avoiding action.

Human Performance Cognition in Aviation

To summarize:

The two problems facing the pilot in recognizing the immediate danger are:

•Retinal Size

As the aircraft presenting the hazard approaches, its retinal size increases. The rate of such increase has been plotted in Figure 10.11 above for an aircraft with a closing speed of 800 kt.

It can be seen that the image remains very small until very shortly before impact.

•LCB

If the conflicting aircraft remains on a constant relative bearing danger is at a maximum because there will be no movement cue aiding detection. The situation is worsened if the other aircraft is positioned, from the pilot’s perspective of view, behind a windscreen support or in his/her blind spot.

Saccade

There are problems associated with the way that the eye moves and how it takes in information. Unless following a moving object the eye does not move smoothly but in a series of jerks. Each movement is known as a saccade and is followed by a rest period when the eye samples its new field. Smooth vision is achieved by the visual cortex of the brain so that the observer is never conscious of saccade.

The average saccade and rest period takes about one third of a second. As the area within which we have good visual acuity is small, it means that eye movements must be frequent and small, and the rest periods of short duration to ensure that the sky is covered.

ScanningTechnique

Recommendations for a successful scanning technique are:

•Each movement should be of 10° at the most and each area should be observed for at least 2 seconds to allow detection.

•Airspace above and below the aircraft must be covered.

•The sky should be covered in overlapping sectors of about 10°.

Cognition in Aviation 10

Aviation in Cognition 10

•Peripheral vision can be vital in spotting collision threats. Each time the scan is stopped and the eyes refocused, the peripheral vision takes on more importance, because it is through this that movement is best detected - this is particularly important by night. In fact, by night it may not be possible to identify an object by looking directly at it and it may only be spotted by looking slightly to one side of the object thus utilising the rods and peripheral vision for detection.

•It is best to move the body as well as the head to see around physical obstructions in the cockpit (doors and windscreen posts). Together these can cover a considerable amount of the sky and a small head or body movement may uncover an area that is concealing a threat.

•Particular care must been taken prior to take-off, landing, ascent or descent and a meticulous scan must be carried out even though clearance may have been received from ATC.

•If another aircraft shows no lateral or vertical movement, but is increasing in size, immediate evasion action must be taken.

Special Situations

Introduction

There are several special situations which are mainly caused by environmental factors which mislead our perception. These have resulted in a number of major accidents. Among these are:

Rain on the windscreen

•Refraction

Due to the refraction of the raindrops collecting on the cockpit windscreen the eye sees the runway lower than it is. This tends to make the pilot carry out a shallower than normal approach. This is true both by day and night.

•Rain on the windscreen

Rain on the windscreen at night can also make runway lights bloom and, as a result, the runway appears closer than it really is; subsequently the pilot has the impression that the aircraft’s closing speed is faster than it is in reality. The pilot may subsequently adjust the approach resulting in an adoption of a too shallow approach angle.

•Weather

A heavy rain storm between the aircraft and the airfield will make the field seem more distant.

Water and Height Judgment

Flying over a smooth water surface makes it extremely difficult to judge height due to the lack of visual cues. This has the effect of the aircraft flying too low - especially over smooth water.

There have been a number of instances of helicopters flying into the sea during night approaches onto offshore rigs. Multi-engine aircraft have hit the water with their propellers when attempting to fly at fifty feet over a calm sea or lake.