As I stare with one eye covered at an image in an aviation text held at arm’s length, a remarkable thing happens.
I lean in closer, and the small airplane on the left side of the image remains visible, but a black X on the right side of the image vanishes, then reappears. Quite a convincing demonstration of the eye’s blind spot, as presented on pages 17-21 and 17-22 of the Pilot’s Handbook of Aeronautical Knowledge.
Nice, but a pilot is not going to fly along with an eye covered, so what’s the big deal?
Well, suppose the field of vision of one of your eyes is obstructed by something like a windscreen divider. Then, according to the handbook, “a visual target could fall in the blind spot of the other eye and remain undetected.”
Who knew see-and-avoid was so scientific? Clearly visual scanning has drawbacks and limitations important to understand. Pilots also learn that visual acuity declines with available light until low-light conditions call for using a different scanning technique—off-center vision—for detecting objects.
This technique—you look five to 10 degrees off center of the object in view—helps overcome a nighttime central blind spot that occurs because of the distribution of the eye’s rods and cones, which have different sensitivities to light. (Rods are far more light sensitive than cones at night.)
You can demonstrate this effect too, using a dim light in a darkened room. “When looking directly at the light, it dims or disappears altogether. When looking slightly off center, it becomes clearer and brighter,” the chapter explains.
If your flight plans for December include nailing down your required night flight, make a point of reviewing how to adapt your eyes to darkness, and how to protect them from sudden exposure to bright light once airborne.
For flying a night cross-country flight, it’s natural to build an extra safety margin into your altitude selection, but remember too that without supplemental oxygen, night vision “declines measurably at pressure altitudes above 4,000 feet” as available oxygen is reduced.