AIM
8/15/19
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11
Potential Flight Hazards
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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