Cognition in Aviation — Page 193, Lesson 306

Let’s start with the vertical separation problem. A common difficulty in flight is judging whether another aircraft is above or below you, especially when it’s still far away. At a distance, an aircraft may appear to be at a higher level than you, but it can eventually pass below. The same illusion happens with mountains or clouds — they tend to appear above the aircraft when seen from a distance, but often pass below. So the visual cue of height at a distance is unreliable, and you have to be aware of that. Now, holding. In civilian transport flying, the normal manoeuvres you perform — turns, climbs, descents — are unlikely to create significant vestibular illusions. That means your inner ear balance system won’t usually be tricked by routine flying. However, there’s an important exception: prolonged turning, like what you do in a holding pattern. If you move your head while still turning, you can create an illusion. That illusion is called vertigo, sometimes referred to as the somatogyral illusion, or the Coriolis effect. So in a hold, keep your head still during the turn to avoid that false sensation of motion. Now we come to approach and landing — and this is a critical part of the syllabus. 50% of all airline accidents occur on the approach and landing. Of all the phases of flight, this is the one most prone to human error — 73% of approach-and-landing accidents involve human error. In the final stages of a flight, the pilot has to cope with the most critical visual tasks. Those tasks can be divided into three stages: 1. Initial judgement of glideslope. 2. Maintenance of the glideslope. 3. Ground proximity judgements. Let’s look at the first stage: initial judgement of the appropriate glideslope. The judgement of the glideslope can be made easier by using VASIs or PAPIs at the airfield — those are the visual approach slope indicator systems you see beside the runway. You can also position the aircraft at a predetermined height above known ground features. But frequently, you have to make that judgement without such aids. To judge the approach path — normally 3 degrees — the pilot is trying to establish an angle. That angle is called the visual angle. Let me define that precisely. The visual angle is measured at the pilot’s eye, down from the horizon to the visual aiming point on the runway. And that visual angle is equal to the approach angle. So if your approach angle is 3 degrees, the visual angle from your eye down to where you’re aiming on the runway is also 3 degrees. That’s the geometry you’re working with. That figure shows exactly what I just described — the horizon line, the pilot’s eye, the visual aiming point on the runway, and the visual angle between them. So when you’re on final approach without external aids, you’re essentially trying to hold that visual angle constant to maintain the correct glideslope.