Cognition in Aviation 10
Vertical Separation
A common problem in flight is the evaluation of the relative altitude of approaching aircraft and the assessment of a potential collision risk. At a distance an aircraft may appear to be at a higher level but may eventually pass below the observer. Mountains or clouds at a distance tend to appear to be above the aircraft but often pass below.
Holding
In civilian transport flying the normal manoeuvres are unlikely to create significant vestibular illusions. However, prolonged turning as in a holding pattern can create an illusion if the head is moved while still turning (vertigo - sometimes referred to as the somatogyral illusion - or coriolis effect).
Approach and Landing
50% of all airline accidents occur on the approach and landing and, of all the phases of flight, this is the one most prone to human error (73%). In the final stages of a flight the pilot has to cope with the most critical visual tasks, and these may be divided into 3 stages:
•Initial judgement of glideslope.
•Maintenance of the glideslope.
•Ground proximity judgements.
Initial Judgement of Appropriate Glideslope
Visual Angle
The judgement of the glideslope can be made easier by the use of VASIs or PAPIs at the airfield, or by positioning the aircraft at a predetermined height above known ground features. However, the judgement must frequently be made without such aids. To judge the approach path, normally 3°, the pilot is attempting to establish an angle. This angle is the visual angle, (see Figure 10.5) and is measured at the pilot’s eye down from the horizon to the visual aiming point on the runway. This, of course, is equal to the approach angle.
HORIZON |
PILOT'S |
EYE |
VISUAL AIMING |
POINT |
Figure 10.5 The visual angle
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Sloping Ground
A sloping terrain approach or a sloping runway may produce an incorrect estimate of horizon location and an incorrect approach slope judgement may be made (see Figure 10.6). The length of a sloping runway may also be misjudged.
Upsloping Runways
If the runway slopes up, the pilot will be encouraged by the threshold visual cues to descend. This leads to an approach which will tend to be too low. With a lower than normal approach, if continued, the aircraft wheels will contact the runway at an increased distance behind the visual aiming point with the possibility of touching down in the undershoot area. A sloping up runway will appear to be shorter than its actual distance, making it appear closer and creating a mistaken belief of a need to descend.
Aviation in Cognition 10
Downsloping Runways
If the runway slopes down, the pilot will be encouraged by the threshold visual cues to climb. This leads to an approach which will tend to be too high. If the approach is too high then the aircraft may land further into the runway than planned giving a reduced runway landing and breaking distances available. A down sloping runway will appear to be longer than its actual distance, making it appear to look further away and creating a mistaken belief of not needing to descend yet.
Figure 10.6
Width of Runways
The width of the runway may also cause incorrect height judgements on the final approach. A pilot used to a standard width runway 150 ft (46 m) may, when approaching an unfamiliar airfield with a narrow runway, judge he is too high and therefore round out on too low an approach. At an airfield with a wider runway than the pilot is used to the tendency will be to roundout too high on an approach to match the visual scene to the pilot’s expectation.
This phenomenon will be particularly noticeable for students from Kidlington when landing for the first time at Filton. They will be accustomed to the narrow runway width at Kidlington 75 ft (23 m) and will tend to flare much too high on their first landing at Filton where the runway width is 300 ft (92 m).
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The Black Hole Effect (The Kraft Illusion)
When approaching an airfield at night over water, jungle or desert the only lights visible may be the distant runway or airfield lights, with a black hole intervening. This absence of visual cue leads to an illusion that the aircraft is too high and as a result the approach path may be flown at too shallow an angle - the aircraft touching down short of the runway. This phenomenon is also important to consider when designing simulator visual presentations to ensure that enough intermediate texture is available on the screen to allow correct height judgement.
Figure 10.7 The Kraft illusion
Maintenance of the Glideslope
Aiming Point and Aircraft Attitude Pitch Angle
Once the aircraft is established on the glidepath and is in the landing configuration, it is relatively easy to visually maintain the glidepath by keeping the aiming point at a fixed position on the windscreen.
Inadvertent Speed Loss
A situation can arise on the approach that with an inadvertent speed loss and a gradual loss of altitude, the runway could remain in the same position on the windshield, giving the impression of a safe, stabilized approach until touching down some distance before the threshold. In particular this should be remembered when visual flying at night around airports where the surrounding terrain is without lights.
Clearly this technique (using visual angle) will only work providing the speed and configuration are maintained. If the speed decreases, the nose-up attitude of the aircraft will increase and the selected spot on the windscreen will move towards the overshoot end of the runway. The danger is to descend to regain its position on the windscreen whilst maintaining the lower speed and visa versa.
Texture andTexture Flow
The approach is progressing normally as long as visual texture flows away from the aiming point and the visual angle between this point and the horizon remains constant. The texture change is particularly important when landing on a grass field .
Figure 10.8 Visual texture flow away from the aiming point
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Ground Proximity Judgements
Aviation in Cognition 10
In the final stage of the approach the pilot will be required to make an assessment of his height above the ground to initiate the flare or power reduction. The pilot will use a number of cues in this height assessment among which will be the:
•Apparent speed of objects on the ground will increase as the height reduces.
•Size of objects, such as runway lights etc. will increase with decreasing distance. (Both height and range of the aircraft).
•Apparent width of the runway will increase.
•Texture of the ground will change. At height, grass will appear only as a green surface, only at low level does it look like grass. There are recorded cases where pilots have mistaken the green jungle canopy for a flat green field and attempted to land with engine out.
Wheels andTouchdown Point
Because most large aircraft have their main undercarriage a long way below and behind the pilot’s eye level, it is self-evident that the wheels will not touch down at the visual aiming point but some way short. See Figure 10.9.
Figure 10.9 Wheels & touchdown point
The shallower the approach path then the greater the distance between the aiming point and the actual touchdown. If a pilot has been misled by sloping terrain or runway to fly a lower than normal approach, and accurate height cues are missing, there is a danger of the aircraft touching down before the prepared surface. This is one of the reasons for the recommended approach for large aircraft being with automatic ILS coupled to radio altimeter input of wheelheight above touchdown whenever possible.
Missed Approach - Somatogravic Illusion
A missed approach in poor visual conditions involves linear acceleration and this causes an illusion of pitch up or climb. This is due to the inputs from the otoliths and has been discussed in Chapter 4.
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