Night flight can be a whole lot of fun, especially on a clear moon-lit night, but night flight also presents additional hazards to the pilot over that of day-time flying.
These hazards mostly pertain to the reduced environmental cueing in darkness; cues that are heavily relied on by VFR pilots during daylight hours. This month I’ll go over night flying in terms of the pilot and the aircraft equipment.
Psychologically, night time flying doesn’t usually present new problems for the pilot to deal with. However, physiologically speaking, the pilot will experience a few changes that can significantly impact flight safety. Without a doubt, these changes all revolve around the eyes.
Night time flying presents new challenges for the eyes in several ways: 1) the eye is less adept (less visual acuity) at seeing objects at a distance, 2) the best area for viewing is no longer straight ahead (foveal view), 3) adapting to dark conditions takes time, and 4) there are several new illusions at night that can disorient the pilot. Awareness of these limitations is the first step to improving night time vision capability.
Vision is made possible by the light and colour receptors located on the retina at the back of the eye. Located more centrally near the optic nerve are the cones. The cones primary job is colour detection, but they also facilitate seeing distant objects and can focus to sharp detail.
The rods are located mainly in the periphery of the optic nerve, in a band around the cones and are most useful for locating peripheral movement. Fine detail and colour are not detected by the rods, so only shades of gray are seen. As a result of the characteristics of the rods and cones, a lack of normal light means that the rods are almost entirely responsible for visual identification of objects.
But this presents a problem since the center of viewing, right behind the pupil, does not support night time viewing, when the rods are doing most of the work.
Most people grow up being unaware that their eyes are not well designed to see straight ahead in dark or poorly lit environments. In fact, most people probably never figure out that night time vision can be measurably enhanced by using slightly off-centered viewing.
While an object can be best seen during the daytime by looking straight-ahead at it, at nighttime, it is best seen by moving the head slightly away from the object so that the image falls offcenter in the back of the eye (hence making better use of the rods).
Ignoring for a moment that everyone has a anatomical blind spot due to the location of the optical nerve precluding the existence of rods and cones at that location of the retina (rarely a problem due to there being two eyes), it is entirely possible to not see an object directly ahead if it is dark. A slight shift of the head while keeping the eyes focused to the original area of interest will then allow the object to be seen.
Clearly, off-centered viewing is an important practice to keep in mind for effective night time viewing.
Although the rods are much more capable of facilitating vision in dark conditions, they do not adjust to the lack of light as fast as the cones. The cones will adjust to dim lighting in five to 10 minutes and be about 100 times more sensitive to light than prior to experiencing dark condition. The rods on the other hand take up to 30 minutes to fully adapt to the dark, but the end result is sensitivity 100,000 times better than in lit conditions.
This process of adaptation can be experienced by walking into a dark room and sitting down; initially, very little if anything will be visible in the room around you. Within about 10 minutes a noticeable improvement in acuity will have occurred. Continue to sit for another 20 minutes and you’ll generally be able to see shapes and objects well enough to navigate around them.
Once the eyes adjust, the process can be quickly reversed by entering a well lit area. Reentering a dark environment will then cause the adaptation process to start all over again. The nature of this process, and its reversal, requires the pilot to be careful about exposure to bright lights once the dark adaptation has begun.
Temporary blindness is possible if exposed to a short burst of bright light (such as accidentally staring into a flashlight). While the eyes are recovering from a flash of light, visual illusions may occur, further exacerbating the recovery. Recognizing that illusions may occur is the only way to combat this potentially dangerous phenomenon.
If you think you might be exposed to a bright light during night flight, sometimes closing one eye will help. This way, you still have one eye adapted while the other is recovering.
There are several common night time illusions that pilots might experience. One such illusion is referred to as the autokinesis effect. If a pilot stares too long at a light (such as a lit object on the horizon) it will appear to move. The apparent movement might then distract the pilot and lead to spatial disorientation.
Likewise, even distant stationary lights can be confused with stars or other aircraft when seen on a clear night. Darker, lower visibility nights eliminate the horizon from view. As a result of these problems, reliance on the instruments for attitude orientation is often required.
One very dangerous illusion is referred to as the black-hole approach. This type of approach occurs when approaching a welllit airport from non-lighted terrain. With the runway as the primary source of visual cues, disorientation is common.
Typically, the pilot will perceive their glide path to be too high and adjust to a lower than safe glide path and impact terrain or land short of the runway. Clearly, electronic or visual approach slope indications need to be followed when approaching a runway under these conditions.
An additional problem, regardless of the terrain lighting on approach, is that bright airport or runway lighting will make the runway appear closer than it actually is. The obvious ramification of this is that the pilot might let down too early. Again, rely on the electronic or visual approach slope indicators.
In the absence of those devices, preplan your approach profile by noting 1) the airport touchdown zone elevation, 2) terrain clearance information, and 3) how far out a standard decent should be started to arrive at the runway safely. If possible, at controlled airports, ask the controller to turn down the runway lights to the lowest step that still allows you to adequately see the dimensions of the runway.
One last illusion is called the moth effect. This effect is exactly what it sounds like: flying towards light! Since runway edge lighting is the key visual cue for runway alignment, more attention is given to the lights than where the centerline should be (unless the centerline is lit as well).
Under these conditions the pilot will have the tendency to drift off centerline towards the lights. Extra care should be taken to assure that the airplane evenly bisects the edge lights on the runway (i. E. on centerline!), and the best way to accomplish this is by looking farther down field during landing and takeoff (You can try this in your car on a road with no traffic; try to stay in the middle of your lane by looking just over the hood at the roadside edge line. I’ll bet you have a hard time keeping the car centered! Now if you drive while looking farther ahead toward the center of your lane, you’ll be better able to keep the car centered).
Beyond the limitations of the human eye, the aircraft can present additional night time challenges if not properly equipped. Aircraft lighting is probably the most important equipment to consider.
If you’ve ever tried landing without a landing light at night, you know how tricky it can be. In airplanes that have both a taxi light and a landing light, I’ll try to use only the taxi light until I absolutely need the landing light for takeoff and landing. This preserves the landing light for when it is needed most. But in a pinch, most taxi lights can double for a landing light should it burn out unexpectedly.
Although the landing light may not be specifically required for certain flights (as is the case in the United States) it also provides the benefit of adding visibility of your aircraft to others who might need to see and avoid.
Likewise, operating position lights is a must when it’s dark. Since most position lights are difficult to see from the cockpit, a lights-on walk-around is necessary prior to night flight.
Finally, some sort of anti-collision light is also a must. Either strobes or a rotating beacon, depending on how your aircraft was originally certified, can suffice for anti-collision. My family’s aircraft had both for added safety!
In addition to aircraft lighting, consider the regulatory requirements that may exist for your aircraft and/or your type of mission. For instance, in some cases, spare fuses (for older aircraft) may be required. The pilot should also have two hand-held light sources available. A D-cell white light flashlight for the preflight inspection and a red light flashlight for cockpit use are recommended.
Be sure to always check battery condition prior to intended use! And remember that red markings on charts will not show up well under red lighting so you may want a small white or bluelight flashlight with an adjustable aperture so you can limit the amount of light and reduce the risk of destroying your night vision.
Finally, consider using supplemental oxygen (if equipped) for night flights above 5,000 MSL. Studies have shown that there is a significant reduction in visual capability above this altitude at night, and since the cones are already down for the count, you can’t really afford any additional vision losses.
And since the eyes are extremely sensitive to reductions in oxygen, consider that anything that reduces the availability or transportation of oxygen will have an equally deleterious effect on night vision.
Included in the list of ways to reduce oxygen to the eyes are smoking, drinking, and certain drug use. Pilots don’t drink and fly, so we can safely rule out that as a troublemaker (I hope!), but smoking can easily raise the physiological altitude of the pilot several thousand feet.
Drug use, prescription and non-prescription alike, can also limit or inhibit oxygen transportation to the eyes. A little research online will generally yield good information on side-effect and adverse reactions to common drugs, but please consult your aeromedical doctor prior to flying with any new drugs.
Avoiding the problems of night time illusions is mainly accomplished by being aware of their existence, and the limitations of sight in dark conditions. Having the right aircraft lighting equipment and personal lighting equipment is also a must.
Next month, in part 2, I’ll go over some night time recommendations for specific phases of flight.