Dark Adaptation and the Bright Tablet on a Night Approach
A 52-year-old pilot is flying night VFR in a remote area with no moon and few ground lights. Thirty-five minutes after sunset he uses a tablet at full white brightness to check the destination chart and airport frequencies. He then looks outside for the unlit runway, which has only low-intensity edge lights and no approach lighting or glidepath indicator. He has difficulty judging the runway position and distance, sees the edge lights late, and finds himself high and fast. He also switches on a white cockpit dome light for several seconds to find a dropped pen. He has no significant eye disease and passed his last medical, but he reports increasing difficulty with night vision over the past two years. The aircraft is 4 nm from the runway, descending through 1,200 ft, and the surface wind is calm.
1. Explain how the tablet and dome light affected his night vision and why the effect is operationally significant.
Read the worked answer
Night vision depends mainly on rods, which contain rhodopsin. Rhodopsin is bleached by bright white light and regenerates slowly, so full dark adaptation may take 20 to 30 minutes or longer. A few seconds of white dome light or a bright tablet screen can destroy much of that adaptation even though the pilot feels only briefly blinded. Red light is preferred because it preserves rod function better, although very bright red light can also degrade adaptation. The operational significance is that the pilot loses the ability to detect low-contrast features: unlit terrain, runway edges, and dim lights. He may see the runway late, misjudge height and distance, and descend too low. The problem is not corrected by simply looking harder; the retina needs time to recover.
2. Describe the cockpit lighting and visual scanning technique he should use for the approach.
Read the worked answer
He should set cockpit lighting to the lowest level that still allows instrument reading, use red or dim white light, and avoid full-brightness screens. The tablet should be dimmed and, if possible, set to a red or night mode. He should not use the dome light in flight unless there is an emergency; if a light is essential, use a dim red torch and shield it. Outside, he should use off-centre viewing: looking slightly to one side of a dim object allows the more sensitive rods in the peripheral retina to detect it. He should scan in sectors rather than staring at one point, and allow several minutes for dark adaptation before the final approach. He should also use the instruments and a stabilised approach profile because visual cues are unreliable.
3. Given that he is high and fast at 4 nm, what decision should he make?
Read the worked answer
He should go around. Being high and fast at 4 nm with poor visual cues, low-intensity edge lights, and degraded dark adaptation is an unstable approach. Continuing would require a steep descent or excessive speed reduction close to the ground, increasing the risk of a hard landing, runway excursion, or controlled flight into terrain. A go-around allows him to reset, re-brief the approach, restore dark adaptation, and consider a different runway or a diversion. He should apply go-around power, establish a positive climb, and announce his intention. The decision should not be based on embarrassment or schedule pressure. If he still cannot see the runway environment adequately on the next approach, he should divert.