Night flying hazards of spatial D

Combat Edge, Feb, 2005 by Brian Farrar

Night flying--or some it's the best time to be airborne (i.e., less air traffic, the concealing cover of darkness, inertial navigation system direct whenever you ask for it, and quiet radios). However, it's also the perfect time to experience spatial disorientation. Disorientation in flight is extremely hazardous because a pilot can make erroneous control inputs in response to his or her false perception and that can lead to a mishap. Even though aircrew members receive training on recognizing spatial disorientation during physiological refresher courses and routinely pre-brief the mere existence of such hazards when night flying, current statistics on nighttime aircraft accidents indicate we should review the specific nature of these pitfalls more frequently. The following visual illusions are prevalent occurrences during night flying. Let's look at each one by describing the illusion, explaining its effects, and reviewing methods to prevent or recover from visually-induced disorientation

Distance Illusions

Assuming that the dots in Figure 1 represent illumination from aircraft beacon lights, which aircraft is closer?

Actually, there isn't enough visual information available to answer the question. During the daytime, reflected light provides detail for the human brain to assess distance. However, during night flying, most of the light received by the human eye is transmitted directly from illumination sources as in the example above. If Aircraft A in the given example has low-illumination lighting and Aircraft B has very bright lighting, Aircraft B may actually be farther away.

This illusion is common when engaging or rejoining an aircraft without knowing its distance. Implementing training rules that forbid visual-only engagements and require a positive source of range information, such as radar, has drastically reduced the potential for midair collisions. Distance illusions can cause accidents during visual formation flying. If visual lookout is the primary means of maintaining formation position, the previously described lack of distance cues may cause horizontal distance to increase without notice. If the trailing pilot maintains a continuous line of sight to the lead aircraft and if that line of sight is relatively high, then vertical separation between aircraft will also increase as horizontal separation inadvertently grows. If the trailing pilot fails to monitor actual altitude, the potential exists for "impact with the ground" or another aircraft at a lower altitude. Stacking high on the lead aircraft, cross-checking actual altitude, and ensuring a stable distance between aircraft by using radar or air-to-air tactical air navigation prevents this illusion from taking over the formation.

[FIGURE 1 OMITTED]

Visual Autokinesis

Autokinesis, sometimes called autokinetic effect, is perceived movement exhibited by a static dim light when it is stared at in the dark. Air Force Research Laboratory scientists assess that after 6 to 12 seconds of visually fixating on a light, it appears to move up to 20 degrees per second in a particular direction or several directions in succession, and that the larger and brighter the object, the less the autokinetic effect. Autokinesis is most common in very dark conditions with only one or two lights present and is uncommon with three or more lights present.

The exact physiological cause of autokinesis remains unknown, but it is believed to be related to tiny fixation movements of the eye and the loss of the surrounding references (i.e., peripheral or ambient vision), which normally stabilize visual perception. The dark, empty environment conducive to autokinesis does not allow one's ambient vision the opportunity to establish spatial orientation while the brain attempts to resolve distance to the light source. To counter or minimize the effect, a pilot should shift his gaze frequently to avoid prolonged fixation on the light; view the source beside or in reference to a relatively stationary structure such as a canopy frame; make eye, head, and body movements to destroy the illusion; and monitor flight instruments to prevent or resolve any perceptual conflict.

False Horizon Illusions

To maintain horizontal and vertical orientation, the human brain sub-consciously uses the visual system to monitor the Earth's horizon or lines relative to the known horizon. These cues are compared to those encountered by the vestibular (inner ear) and somatosensory (seat-of-the-pants) systems in order to provide positional orientation. Because there is little reflective light at night to monitor the horizon, "any straight line will do." In the absence of any discernible horizon, starlight can look like ground lighting; starlight reflecting off of water can confuse the visual picture; and in northern regions, the aurora borealis causes similar disorientation.

The best defense against misinterpreting a perceived horizon cue is to depend upon the attitude indicator. A good calibration check during ground operations prior to takeoff and a continuous cross-check in flight to confirm correct operations will help you "trust your instruments" even when your eyes and brain tell you "up" is the other way.