Except for a relative few of us who live near the north or south poles, our perception is that on a daily basis, night falls predictably. For those living in the mid-latitudes of North America, that predictable rate falls somewhere between 700 and 900 miles per hour. While it sounds as if the process occurs at a staggering speed, from personal experience we know the dying light of day lingers for quite some time before total darkness overtakes us.
However, everything is not as it appears when we transition from the light of day into the gloom of night. Our measurable reality tells us that the slow progression of night we enjoy so much is actually occurring at a Mach number most pilots have never experienced. The darkness itself, well, that’s another story entirely.
It is in the best interest of every pilot to have a well-informed understanding of how our human systems adjust and adapt to the dark. While sport pilots are prohibited from flying at night, private pilots are required to have specific training in night operations. Those requirements include a minimum of three hours of night flight training, with at least one cross-country flight of more than 100 nautical miles total distance, and no fewer than 10 takeoffs followed by a circuit around the traffic pattern and 10 landings to a full stop.
The FAA defines night as, “the time between the end of evening civil twilight and the beginning of morning civil twilight, as published in the American Air Almanac, converted to local time.” While that definition is tremendously specific, it doesn’t exactly leap off the page and impart instant wisdom.
Evening civil twilight begins when the sun disappears over the horizon. It ends when the center of the sun falls to 6 degrees below the horizon. Morning civil twilight begins in the opposite fashion, when the center of the sun is 6 degrees below the horizon. When the sun reaches the horizon, morning civil twilight ends. In the official sense, it’s the time between the ending of one, and the beginning of the next, that we call night.
CFIs have historically encouraged students to begin logging their flight time as night time one hour after sunset, right through to one hour before sunrise. This isn’t a particularly scientific rule, but it works, and it’s easy to remember.
For certificated pilots, the FAA formalizes that idea in Federal Aviation Regulation 61.57(b), which requires the pilot in command—before carrying passengers during the period beginning one hour after sunset and ending one hour after sunrise—to have logged, within the preceding 90 days, at least three takeoffs and landings to a full stop during that period. (One other time period to remember is sunset to sunrise; that’s when your aircraft’s position lights must be illuminated.)
In the old days a student pilot could opt out of the night training requirement. That is not an option any longer. The only exception to the night training requirements these days comes in the form of a temporary limitation, and a short stay. Applicants who train and live in Alaska, where night is considerably less consistent on a daily basis than it is in the lower 48, can be issued a private pilot certificate with a limitation reading, “night flying prohibited.” But even with that limitation on their certificates, Alaskan private pilots are required to receive and log the appropriate night training within 12 months after the certificate is issued. If they fail to comply, their pilot certificate becomes invalid until they complete that training.
In addition to those basic night training requirements, there are two additional numbers that are important for pilots to remember when flying at night. The first number is, 10 seconds. The second is, 30 minutes.
It takes approximately 10 seconds for the average pilot’s eyes to adjust from the darkness to the light. That can be good and it can be bad, depending on the situation. If you’re walking out of a dark theatre into the street, it’s good. That ability to quickly adapt to light keeps you from walking into the street, where you might be run over by a car. On the other hand, if you were sitting in the cockpit all acclimated to the dark, and you were to inadvertently stare into the landing light of another aircraft taxiing nearby, your night vision could be affected considerably. You would have to begin the process of acclimating to the darkness all over again.
That’s where the 30 minutes comes into play. It can take up to 30 minutes for the eye to fully adapt to the low light levels available at night. That sluggish rate of adaptation suggests that pilots should think carefully and conduct themselves with caution when preparing to fly at night.
After sunset, it’s not a good idea to walk directly from a well-lit FBO out onto the ramp, climb into the airplane, and fire up the engine. While the instruments may indicate all systems are in the green, our own systems are not. Our eyes will still be adjusting to the darkness when we leave the ground—which will leave us unable to reliably distinguish between the lake we normally rely on as a landmark, and the mountain up ahead.
A half-hour is a long time to fly semi-blind through the night. Slow down, let your eyes adapt to the darkness, keep your cockpit lighting low, and avert your eyes from bright lights that might adversely affect the physical process of adapting to the darkness. Remember, while it takes our eyes as much as 30 minutes to adapt to the dark, looking directly into a bright light can reset our eyes back to daylight mode in a matter of seconds—forcing us to repeat the entire process.
In the cockpit we can prevent ourselves from unintentionally ruining our night vision by taking some basic precautions, and thinking about the environment we will be operating in before we ever leave the ground. A flashlight fitted with a red lens can go a long way toward preventing our eyes from being exposed to light that will adversely affect our night vision. However, that red lens will cause any red marks you’ve made on your sectional chart or note pad to disappear. Make sure your marks and notes are in black ink so they will show up clearly under the less intense, rose-colored lighting offered by your modified flashlight.
Another aspect of night vision that is not well understood by student pilots is the blind spot at the center of our field of vision. While we have a tendency to look directly at something that attracts our attention in good lighting, that practice can be problematic when flying at night. In fact, looking directly at an object at night can tend to make it disappear, or become less distinct. Even dim lights can tend to flicker or go out, when we look directly at them in the darkness.
This anomaly of optical clarity occurs because our eyes register light using different sensors in low light than the ones they use when operating in bright light. Those low-light sensors, called rods, work well. However, their physical arrangement within the eye leaves a blind spot that can be tremendously frustrating to compensate for if you aren’t aware of how the eye works at night.
The best way to see an object more clearly at night is to look near it, but not directly at it. Moving your gaze five to 10 degrees from the object makes it easier to see, track, and identify an object at night.
Darkness can lead to a wide variety of optical illusions that can cause us to make errors in the cockpit. Even something as seemingly simple as picking out the horizon can be a challenge at night. Dots of light on the ground can be difficult to differentiate from the dots of light given off by stars. This can result in a false horizon illusion that can cause a pilot to misalign the aircraft in flight, pitching or rolling into an unusual attitude based on the assumption that the aircraft is oriented correctly to a horizon that isn’t really the horizon at all.
Proper training can help us to overcome the limitations and illusions that come with night operations. Many have come before you, and they found a way to work well in the dark.
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