NVG Operator Basics: A Study in Contrast
An important lesson I emphasize with aviators who are learning to optimize night vision goggle (NVG) operations—especially in a helicopter hoisting community that demands precise hovering—is sensitivity to contrast, or the lack of it. The concept of contrast, in simple terms, is the presence of differences in shading through the goggles, which allows a pilot to detect helicopter movement. The ability to evaluate—and when possible, deliberately create—contrast on scene immediately improves risk management and aircraft control. Over time, I have appreciated understanding the difference between hovering on NVGs in a high-contrast environment, where visual cues are abundant, and a low-contrast environment, where visual cues are limited or misleading.
Contrast is generally the result of four factors:
1. Surface Relief, Texture, and Shape – anything with variation in height, texture, or shape provides contrast.
High-contrast examples:
- Tree limbs and brush
- Whitecaps on water
- Buildings, cliffs, or rocky terrain
Low-contrast examples:
- Large expanses of calm water
- Open areas covered by heavy snow that smooths vegetation and eliminates edges
A flat, uniform surface removes critical hover cues and increases the probability of undetected drift.
2. Delineation Between Light and Dark – contrast is enhanced when there is a clear separation between light and dark areas.
High-contrast examples:
- Painted lines on pavement
- Dark puddles against a light surface
- Light sand interspersed with dark rocks
Low-contrast examples:
- A large, uniform sand beach
- A field of the same type of grass
Uniform coloration limits the pilot’s ability to perceive movement or depth.
3. Ambient Light Level – without sufficient light, it does not matter how much texture or relief exists you will not see it. Instead, the image degrades into scintillation.
I have often experienced this offshore under a marine layer with ceilings greater than 600’ and visibility over 3 miles. Over open water there is no cultural lighting, and when lower thick overcast blocks celestial light, there may be nothing visible through the goggles. In these conditions, usable visual cues may not appear until you descend low enough for aircraft lighting to reflect off the water—often while already in a hover.
The environment may feel no different than flying in fog. Failure to recognize this loss of visual cues and transition mentally and procedurally to an instrument scan has contributed to serious mishaps and numerous near-miss events in maritime helicopter rescue operations.
4. Angle of Light – the angle of illumination has a major effect on contrast.
A high, bright moon often produces little to no shadowing, significantly reducing contrast. For example, hovering over a field of similarly colored boulders under a high moon can be deceptively difficult due to the lack of shadows.
Conversely, a lower moon positioned behind the aircraft can cast strong shadows, dramatically increasing contrast and improving hover references. However, this benefit can quickly become a liability. If you have ever flown below a ridgeline enjoying good contrast with a low moon at your back, then turned 180 degrees, you have likely experienced the downside: a bright moon above the ridgeline washing out the goggles (de-gaining the NVGs) and turning the terrain you need to see through the goggles into a black void.
The first time I encountered this scenario unexpectedly, my planned 180-degree turn quickly became a climbing 360-degree turn to regain visual separation from terrain.
Creating and Managing Contrast
Depending on the environment, aircraft lighting can either create or destroy usable contrast. In some situations, contrast can also be deliberately added to the scene.
Anyone who has tried to hover over a uniform snowfield—such as a glacier—knows how valuable it can be to introduce reference points, even in daylight. In snowy, above-tree-line rescue environments, crews have strategically positioned ground party members within the pilot’s field of view. When that was not possible, pine boughs gathered at lower elevations have been placed to provide contrast.
Coast Guard crews have also used:
- Streamers from message blocks tossed onto snow
- Jettisoned trail lines with weight bags
- In extreme cases, they have deployed pump cans or life rafts
If you operate in an area with the potential for “field of snow” hoisting or landing environments, carrying simple items that can be safely thrown from the aircraft to create reference points is an easy and effective risk mitigation strategy.
When Too Much Contrast Becomes a Problem
There are also situations where excessive or poorly placed contrast is counterproductive.
Flying through snow or rain on NVGs with a searchlight on is a classic example. The light illuminates the precipitation so effectively that it obscures the horizon and masks obstacles. Similarly, during low-wind overwater hovers, rotor wash can create a “milk bowl” effect—swirling water vapor illuminated by the searchlight—which can be disorienting. In these situations, turning the searchlight off often restores a usable horizon.
Likewise, during whiteout or brownout operations on NVGs, turning off landing, hover, and search lights can reduce disorientation by preventing illumination of swirling obscurant and preserving external visual references.
Final Thought
NVG hovering can be managed effectively by understanding visual cues, deliberately manipulating contrast and acknowledging when visual cues are degrading. When contrast disappears, pilots must be ready to transition to an instrument scan—and when possible, take proactive steps to create usable lasting references before they are needed.
