Vision

The human eye utilizes two different types of photoreceptor cells to process the light that the brain converts into images. Rods are the most sensitive of these cells and can be activated by a single photon. At low light levels, vision is derived solely from signals coming from the eye’s rods. Cone cells produce the highest visual acuity and resolution and are also responsible for color vision, but they require significantly brighter light (more photons) to become activated. Cone cells populate the center of the retina directly behind the eye’s lens in an area called the fovea. Across the rest of the retina, rods and cones are intermingled. For this reason, it can be difficult to spot an airport at night when looking directly at it. The periphery of the visual field (populated by rods) has the most sensitivity. In order to spot something at night the best technique is to scan an area for brief moments using the peripheral vision rather than staring straight ahead. Once detected, a target should be maintained in the field of view using the off center vision technique: glance 10 degrees above, below, or to the sides of the target for no more than three seconds.

The eye requires more oxygen by weight than any other part of the body and vision is diminished when flying at higher altitudes without supplemental oxygen. For this reason, the FAA recommends (but does not require) the use of oxygen at altitudes above 5000 feet MSL when flying at night.

Illusions

An illusion called autokinesis can be caused by staring at a single point of light against a dark background. Involuntary muscle twitches in the eye can make the light appear to move. The best way to prevent this illusion is to maintain a constant scan and focus on a variety of objects. False horizon is an illusion created when the real horizon is obscured or when ground lights and stars blend together. The effect is especially pronounced when flying toward a shoreline. The dark water can make shore lights appear to be stars and pilots may compensate by aligning the aircraft nose with the shore. To prevent this illusion, make pitch adjustments based on the attitude indicator. A number of landing illusions can also occur at night. A dark runway environment can lead to a lower-than-normal approach. Bright lights, steep terrain, or wide runways can lead to a higher-than-normal approach. Highway lights can also be mistaken for runway lights at night.

Fatigue

Missing one night of sleep has the same effect on vision, decision-making, and reflexes as a .08 blood alcohol level (the level at which an automobile driver would be charged with DUI). Night flying often constitutes the last legs of a long cross country flight that began much earlier in the day. Pilot performance during this very demanding stage of the journey can be considerably impaired. It is far better to stop for the night and continue the flight the next day after obtaining sufficient rest.