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AIM

10/12/17

7−5−11

Potential Flight Hazards

7−5−13. Flying in Flat Light, Brown Out

Conditions, and White Out Conditions

a. Flat Light. Flat light is an optical illusion, also

known as “sector or partial white out.” It is not as

severe as “white out” but the condition causes pilots

to lose their depth−of−field and contrast in vision.

Flat light conditions are usually accompanied by

overcast skies inhibiting any visual clues. Such

conditions can occur anywhere in the world,

primarily in snow covered areas but can occur in dust,

sand, mud flats, or on glassy water. Flat light can

completely obscure features of the terrain, creating an

inability to distinguish distances and closure rates.

As a result of this reflected light, it can give pilots the

illusion that they are ascending or descending when

they may actually be flying level. However, with

good judgment and proper training and planning, it is

possible to safely operate an aircraft in flat light

conditions.

b. Brown Out. A brownout (or brown−out) is an

in−flight visibility restriction due to dust or sand in

the air. In a brownout, the pilot cannot see nearby

objects which provide the outside visual references

necessary to control the aircraft near the ground. This

can cause spatial disorientation and loss of situational

awareness leading to an accident.

1. The following factors will affect the

probability and severity of brownout: rotor disk

loading, rotor configuration, soil composition, wind,

approach speed, and approach angle.

2. The brownout phenomenon causes accidents

during helicopter landing and take−off operations in

dust, fine dirt, sand, or arid desert terrain. Intense,

blinding dust clouds stirred up by the helicopter rotor

downwash during near−ground flight causes signifi-

cant flight safety risks from aircraft and ground

obstacle collisions, and dynamic rollover due to

sloped and uneven terrain.

3. This is a dangerous phenomenon experienced

by many helicopters when making landing approach-

es in dusty environments, whereby sand or dust

particles become swept up in the rotor outwash and

obscure the pilot’s vision of the terrain. This is

particularly dangerous because the pilot needs those

visual cues from their surroundings in order to make

a safe landing.

4. Blowing sand and dust can cause an illusion

of a tilted horizon. A pilot not using the flight

instruments for reference may instinctively try to

level the aircraft with respect to the false horizon,

resulting in an accident. Helicopter rotor wash also

causes sand to blow around outside the cockpit

windows, possibly leading the pilot to experience an

illusion where the helicopter appears to be turning

when it is actually in a level hover. This can also cause

the pilot to make incorrect control inputs which can

quickly lead to disaster when hovering near the

ground. In night landings, aircraft lighting can

enhance the visual illusions by illuminating the

brownout cloud.

c. White Out. As defined in meteorological

terms, white out occurs when a person becomes

engulfed in a uniformly white glow. The glow is a

result of being surrounded by blowing snow, dust,

sand, mud or water. There are no shadows, no horizon

or clouds and all depth−of−field and orientation are

lost. A white out situation is severe in that there are

no visual references. Flying is not recommended in

any white out situation. Flat light conditions can lead

to a white out environment quite rapidly, and both

atmospheric conditions are insidious; they sneak up

on you as your visual references slowly begin to

disappear. White out has been the cause of several

aviation accidents.

d. Self Induced White Out. This effect typically

occurs when a helicopter takes off or lands on a

snow−covered area. The rotor down wash picks up

particles and re−circulates them through the rotor

down wash. The effect can vary in intensity

depending upon the amount of light on the surface.

This can happen on the sunniest, brightest day with

good contrast everywhere. However, when it

happens, there can be a complete loss of visual clues.

If the pilot has not prepared for this immediate loss of

visibility, the results can be disastrous. Good

planning does not prevent one from encountering flat

light or white out conditions.

e. Never take off in a white out situation.

1. Realize that in flat light conditions it may be

possible to depart but not to return to that site. During

takeoff, make sure you have a reference point. Do not

lose sight of it until you have a departure reference

point in view. Be prepared to return to the takeoff

reference if the departure reference does not come

into view.

2. Flat light is common to snow skiers. One way

to compensate for the lack of visual contrast and

3/29/18

AIM