Cognition in Aviation — Page 199, Lesson 315

I want to walk you through a critical part of collision avoidance that every pilot needs to understand — how your eyes and brain can actually work against you when you're looking for other traffic. This is about the retinal image and what happens when another aircraft is on a collision course with you. Let's start with the key concept: the Line of Constant Bearing, or LCB. If you see another aircraft through your windscreen and it stays fixed on the same spot — it doesn't move left, right, up, or down relative to the windscreen frame — then that aircraft is maintaining a Line of Constant Bearing relative to you. And here's the danger: if it's on an LCB, a collision risk exists. The two of you are on a converging path that will bring you together at the same point in space. Now, if that aircraft appears to move across your windscreen — either vertically or horizontally — then as long as both of you maintain your current tracks, no collision risk exists. The aircraft will pass ahead, behind, above, or below you. That relative movement is your visual cue that you're safe. But there's a special problem I need to highlight: the blind spot. Every human eye has a blind spot where the optic nerve leaves the retina — there are no light-sensitive cells there. When you're flying visually in VMC — Visual Meteorological Conditions — this blind spot is especially significant. It's possible that conflicting traffic will sit permanently on that blind spot and go completely undetected. You won't see it at all, even if you're looking in that general direction. Now let's talk about high-speed traffic on a converging track from ahead. This is where the numbers get really sobering. Imagine two aircraft approaching each other head-on at a combined closing speed of 800 knots. That's about 920 miles per hour. At that speed, the approaching aircraft produces a very, very small angular picture on your retina until it's frighteningly close. Let me walk you through the numbers from Figure 10.11. At 3 seconds before impact, the other aircraft subtends a retinal angle of only half a degree — that's about the size of the full moon, but remember, you're looking for a small aircraft shape, not a bright disc. At 1.5 seconds to collision, it's still only 1 degree. At 0.75 seconds, 2 degrees. At 0.38 seconds, 4 degrees. And at 0.1 seconds — well, the book says 'very big indeed.' But by that point, it's far too late to take any avoiding action. Because the aircraft's image on your retina stays so small for so long, it's quite possible that you simply won't notice it at all — especially if there's no relative movement across the windscreen. If it's on an LCB, it's not moving relative to the windscreen, and your brain has no motion cue to alert you. The image only grows dramatically in the last 1.4 seconds, but by then you have no time to react. So here's the bottom line for you as a pilot: at any stage of flight, there is a risk of a mid-air collision. Modern radar and air traffic control systems greatly reduce that risk when you're operating in controlled airspace, but they can never completely eliminate it. It is absolutely essential that you maintain a good scan of the world outside as well as of your cockpit instruments. Your eyes are your last line of defense, and understanding these limitations — the blind spot, the tiny retinal image at high closing speeds, and the deceptive stillness of an LCB target — is what keeps you alive.