Ernest Hemingway said it a lot better: “The things of the night cannot be explained in the day, because they do not then exist.” Night flying is really IFR flying and we need to use all of our IFR skills to assure safety during nighttime ops.
In the last few articles we’ve been talking about our most important physical sense, our vision, and how visual illusions can lead us astray. In the dark of night, when we can’t see outside the cockpit, the risks of being tricked by our senses are much greater. We talked about daytime visual illusions that are caused when there is a disconnect between what our physical senses are telling us and reality. This risk gets worse at night since our visual acuity is markedly reduced and we have almost no color perception. With less visual acuity and no color distinctions, the ability to see contrast differences between an image and the background is totally degraded. That’s really important since the contrast between objects is a central element for visual acuity and anything that diminishes contrast makes it hard to distinguish objects that we need to orientate ourselves in space. When we lose outside visual clues there is also a higher risk of spatial disorientation (SD) than during daytime flight. This makes sense knowing that SD is caused by the same disconnect of our senses from reality. The tragedy of John F. Kennedy Jr.’s crash is a famous example. The NTSB report conclusions were that the crash was caused by the “pilot’s failure to maintain control of the airplane during a descent over water at night, which was a result of spatial disorientation. Factors in the accident were haze, and the dark night.” A military study of SD in fighter pilots confirmed a clear prevalence of night SD events over incidents in daylight hours. All of these factors are consistent with FAA statistics that show nighttime accidents are almost twice as common as daytime accidents.
We talked about how night vision depends mostly on rod cells and that these cells are located on the perimeter of the retina. That means in order to have the best possible night vision, you have to offset your scan slightly so light is focused on the edge of the retina. Ophthalmologists call this “eccentric fixation.” It’s essential to maximize visual acuity in low levels of light. You have to practice this to get it right and force your focus slightly above, below, or to either side of a night target. In addition, to see anything at night those rod cells have to go through a process called “dark adaption.” It’s a real chemical reaction that is needed to change the structure of photosensitive chemicals in the rods. Rods require 20 to 30 minutes, or sometimes even longer, in absolute darkness to attain maximum dark adaptation after exposure to bright light. This entire process can be reversed and night sight can be lost in a few seconds with exposure to light, so it’s really important to avoid bright lights when flying at night. Another way to lose dark adaptation is light flashing from wing strobes reflecting off of clouds or haze on a dark night. Strobe lights need to be turned off in the clouds at night since the pulsing light will not only be disorienting, it will also ruin your night vision. (This is also why LASER beams flashed at planes is so dangerous as pilots will their lose dark adaptation and with it their night vision.) Other things you can do to enhance night vision is to keep the instrument panel illumination at the lowest level consistent with safe operations, and if you have to light up a map or something on the flight deck use a red lens and keep it as dim as possible for the shortest possible time.
Dark adaptation is an independent process in each eye. Even though a bright light may shine in one eye the other will retain its dark adaptation if it is protected from the light and that means you can preserve some night vision and dark adaptation in one of your eyes by simply closing it.
Another important factor for dark adaptation is the effects of nighttime hypoxia, since low oxygen levels have a much greater detrimental effect on night vision than day visual acuity. Without breathing supplemental oxygen in an unpressurized airplane at night there is at least a 35% reduction in visual acuity even at lower altitudes. It’s necessary to start using supplemental oxygen at only a few thousand feet MSL to preserve your nighttime visual acuity. For similar reasons cigarette smoking degrades dark adaptation for night vision since the hypoxic effect of carbon monoxide is additive to hypoxia as altitude increases. That’s why the USAF actually prohibits smoking for three hours before a night mission.
We talked about approach and landing visual illusions last month, and the darkness at night accentuates these illusions and increases the risks. The simplest nighttime illusion is “ground light confusion,” which happens when a pilot confuses the lights on the ground with light up in the sky from the stars. It’s risky and this type of confusion has led to CFIT (controlled flight into terrain) accidents. It happens when pilots try to fly by visual references and apply VFR standards to night flight. The opposite illusion is the “black hole approach,” which can happen on final approach on a dark night over unlit terrain when visual cues are not available to orient yourself in space. When there are no external lights for a frame of reference the runway seems to float around in space on its own. This illusion leads to a tendency to lower the aircraft nose to look into the “black hole” that may end up with spatial disorientation or a crash landing short of the runway. The same solution applies to both illusions; fly like you’re in IMC, stay on the dials and load the approach and fly it right down to the runway. If you do become disoriented or feel unsure about your rate of descent, go around. Never try to force a potentially unsafe landing just to get on the ground sooner.
At night, sloping cloud formations and patterns of light of stars or ground lights on hilly terrain in an otherwise dark visual field can all play tricks on your eyes. This is especially dangerous when you see something that you perceive as the horizon and orient yourself accordingly, only to find out too late that what you saw was not the horizon at all. It’s known as the “false horizon” illusion, which can cause disorientation leading to an unperceived bank angle that can also end with a CFIT accident. Avoid falling victim to this illusion by continually cross-checking what you are seeing and what you think it means with your instruments, especially the altitude and attitude indicators. Again, trust your instruments and stay on the dials.
Disorientation and that spinning feeling of vertigo can also happen to you during night flight because of the way our eyes work. “Flicker vertigo” is a type of vertigo pilots can experience at night thanks to anti-collision lights, strobe lights, cockpit lights or other aircraft lights especially reflected off clouds or haze. It can also appear when you take off from a well-lit runway and make the transition into the dark night sky. I’ve experienced a mild sensation of flicker vertigo flying a night approach in my single engine Saratoga when looking at the runway lights through the spinning prop blade. It can really affect your senses as you slow down or speed up the prop adjusting for the descent and landing. Look slightly off to the side of the prop to get rid of this feeling.
Another nighttime illusion is called “autokinesis.” It’s an apparent wandering around in space of an object or a light when you see it against a visually unstructured background like the dark night sky. When it happens, a bright star may be seen as moving in a circle or moving linearly. Again, trying to control your airplane by a visual reference to a light that’s floating around in the sky can lead to a loss of control. This illusion occurs because motion perception is always relative to some fixed reference point, and in dark sky there are few if any reference points, so the position of the single point of light is undefined. This illusion can make traffic avoidance difficult at night and lead to trouble distinguishing relative movements of your own and other aircraft. To counteract this you should avoid staring at a single light for more than a few seconds and move your gaze around frequently. Be sure to cross-check traffic with your TCAS (traffic collision avoidance system) if you have it on your plane.
There’s way too much information on night vision and visual illusions that I can cover in this overview. As part of your preparation for night flight there are a number of good references linked here that you should review. The FAA review of nighttime flight safety is also a good overview and can be found at this link. I love flying at night; the air is usually smooth, the frequencies aren’t as busy, and the view is spectacular. Just remember, it’s not the same as flying during the day and it’s a skill that needs to be continually practiced. The way to avoid the added risks of nighttime flight is to stay on the instruments and stay current with your night hours and instrument proficiency training. Fly safe, but be extra careful at night because as old Ernie Hemingway said, “Everything’s different.”