•FM 3-04.202 (1-202) ENVIRONMENTAL FLIGHT
•AERONAUTICAL INFORMATION MANUAL
• The mountain environment requires
special flying techniques due to:
– Its severe and rapidly changing weather
– Impacts on aircraft performance capabilities
– Acceleration of crew fatigue
• Winds associated with mountains can be broken
down into three main categories.
– Prevailing winds: upper-level winds flowing
predominately from west to east in the continental
– Local winds: also called valley winds, are created
by convection heating & cooling. They flow parallel to
larger valleys. During the day, these winds tend to
flow up valley; at night, they flow down valley.
– Surface wind: the layer of air which lies close to the
ground. It is less turbulent than prevailing & local
• Demarcation line: the point which
separates the up flow from the down flow
– It forms from the highest point of the mountain
& extends diagonally upward.
– The velocity of the wind and the steepness of
the uplift slope will determine the position of
the demarcation line.
– Generally, the higher the wind speed, &
steeper the terrain, the steeper the
PREVAILING WINDS TURBULENCE
• Light winds: 1-10 knots
– Accelerates slightly on the upslope, giving rise
to a gentle updraft.
– Follows the contour of the terrain feature over
– At some point past the crest, turns into a
• Moderate wind: 11 to 20 knots
– Will increase the strength of the up drafts and
downdrafts and create moderate turbulence.
– An updraft will be experienced on the lee (the
side sheltered from the wind) slope near the
crest of the mountain.
– The demarcation line forms closer to the hill
crest and is steeper
• Strong winds: above 20 knots
– The demarcation line will move forward to the
leading edge of the hill crest
– Becomes progressively steeper and the
severity of updrafts, downdrafts, and
turbulence will also increase.
– Under these conditions, the best landing spot
is close to the forward edge of the terrain
• A phenomenon that occurs when the
airflow over mountainous terrain meets
– Low-level layer of unstable air
– Stable layer of air above the lower levels
– Wind direction fairly constant with altitude
– Wind speed increasing w/ altitude
– Mountain lying perpendicular to the airflow
• The following conditions can exist in a
– Vertical currents of 2,000fpm are common,
– Moderate to severe turbulence
– Wind gusts up to 22 knots
– Altimeters errors up to 1,000 feet
– Icing can be expected
• When proper conditions exist, clouds will
form that provide visible indications of the
existence of a mountain wave. Three
types of clouds may form as a result of a
– Lenticular Clouds
– Rotor Clouds
– Cap Clouds
• Lenticular Clouds:
– lens shaped, high altitudes, 25,000-40,000’. Form in bands or as single
clouds, located above and slightly downwind from the ridge of the
mountain, turbulence may be encountered under the cloud
• Rotor Clouds:
– downwind from the ridge, several rows lying parallel to the ridge, bases
at or below ridge level, up/down drafts in excess of 5,000fpm. Short
duration & tend to disappear rapidly.
• Cap Clouds:
– formed primarily from vertical updrafts, up/down as they pass over the
mountain. Part of the cloud extends upwind, with finger-like extensions
running down the slope on downwind side of the ridge.
SLACK WINDS STABLE LAYER
WIND ACROSS A RIDGE
• Smooth air and updrafts will be experienced on the windward side of
the ridge and downdrafts on the lee side.
• The steeper the updraft slope & the higher the wind velocity, the
more severe the updrafts.
• As the air flows over the crest, a venturi effect is created. An area of
low pressure develops on the lee side of the mountain.
• Where the ridge line is irregular, a funneling of air through the gaps
will occur, causing a mixing of air on the lee side. This condition
tends to increase the turbulence.
• Wind striking the ridge at less than 90° produces fewer updrafts and
WIND ACROSS A RIDGE
WIND ACROSS A SNAKE RIDGE
• Down drafts and turbulent air may be
encountered on the windward slope of
• The severity will be determined by the distance
between the ridges, the depth of the valley, and
the angle the wind strikes the slope.
• The closer the ridges are together and the closer
the wind is to 90° to the slope, the updrafts and
turbulence will be more severe.
• Greater turbulence will be experienced on the
downdraft slope of succeeding ridges due to
turbulent air flowing over the ridge.
WIND ACROSS A CROWN
• Airflow in the vicinity of a crown is normally
lateral around its outer edges and over the
• Turbulence will develop on the lee side of
the hill, but will not extend too far out from
WIND ACROSS A CROWN
• The airflow around a shoulder is extremely
turbulent regardless of the wind direction.
• Extreme downdrafts may be experienced if
the shoulder is located on the lee side of
• Rotary turbulence may be experienced on
the uplift side of the shoulder.
WIND ACROSS A CANYON
• Usually the lower winds flow parallel to the
canyon floor. The degree of turbulence in
the low areas of a canyon depends on the
width & depth of the canyon and the wind
• In a narrow canyon, the most severe
turbulence is in the low area
• In a wide canyon, the low area may be
WIND ACROSS A CANYON
• During Mountain Flying the aviator’s senses are
• A natural tendency is to judge airspeed as too
slow and altitude too high.
• Difficulty may be experienced in maintaining the
proper flight altitude.
• Frequent reference should be made to the flight
• Update the PPC to compensate for gross weight
changes and center of gravity.
• Hover power check should be conducted.
• PPC: as a minimum: max torque available,
go/no-go torques, predicted hover torque
• When performing a mountain takeoff, apply
torque as necessary to gain forward airspeed
while maintaining sufficient altitude to clear any
obstacles until climb airspeed is reached.
• Where drop-offs are located along the takeoff
path, the aircraft may be maneuvered
downslope to gain airspeed.
AIRSPEED OVER ALTITUDE
FLIGHT ALONG A VALLEY
• Aircraft should be flown in the smoother upflowing air on
the lifting side of the valley.
• Requires less power and gives the aircraft a safe flight
• The velocity of the wind will determine how close you will
fly to the lifting side.
• In strong winds, it is advisable to avoid flying close to the
slope because of turbulence caused by irregular
projections may be encountered.
• In light winds, aircraft should be flown closer to the side
of the valley to allow for maximum horizontal clearance
for a 180° turn
FLIGHT ALONG A VALLEY
• Crossing at a 45° angle facilitates turning
away from the ridge should the helicopter
be carried below the crest by a downdraft.
180° TURN OR EARLY CLIMB
FACTORS IN THE
CONSIDERATION OF AN
• Wind direction and velocity
• Vertical air currents
• Escape routes
• Terrain contour & obstacles
• Position of the sun
• Approach paths and areas to be avoided
TYPE OF APPROACH
• There is no standard type of mountain approach.
• Light wind/when demarcation line is shallow:
– A relatively low angle of descent or flat approach should be
used- requires less power & control movement. If downdrafts are
encountered insufficient altitude may be available to continue the
• Stronger wind/steeper demarcation line:
– Steeper approach angle. Higher rate of descent & requires more
power to terminate the approach. Provides more terrain
clearance if downdrafts are encountered.
• Running landing:
– Used if insufficient torque to make a normal or shallow approach
and landing area is suitable. Effective translational lift is
maintained until contact with the ground.
FIGURE EIGHT CIRCULAR
• Avoid descents greater than 700fpm
• Normally, pattern altitude will not exceed 500
feet above the touchdown point.
• Mountain approach:
• When 50’ above the touchdown point begin
losing effective translational lift. Do not hover
OGE. Prior to reaching the near edge of the
landing area, the descent should be stopped &
forward airspeed reduced to a brisk walk.
AREA TO BE AVOIDED
SETTLING WITH POWER
•Vertical / Near Vertical Descent
at least 300fpm
•Low Forward Airspeed
•Using some of available Engine Power
HOVER IGE HOVER OGE
-Reduced rotor tip -Large blade-tip
-Reduced velocity of -High velocity of
induced airflow induced airflow
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