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AIM 2/19/04
7−5−4 Potential Flight Hazards
j. Mountain Wave. Many pilots go all their lives
without understanding what a mountain wave is.
Quite a few have lost their lives because of this lack
of understanding. One need not be a licensed
meteorologist to understand the mountain wave
phenomenon.
1. Mountain waves occur when air is being
blown over a mountain range or even the ridge of a
sharp bluff area. As the air hits the upwind side of the
range, it starts to climb, thus creating what is
generally a smooth updraft which turns into a
turbulent downdraft as the air passes the crest of the
ridge. From this point, for many miles downwind,
there will be a series of downdrafts and updrafts.
Satellite photos of the Rockies have shown mountain
waves extending as far as 700 miles downwind of the
range. Along the east coast area, such photos of the
Appalachian chain have picked up the mountain
wave phenomenon over a hundred miles eastward.
All it takes to form a mountain wave is wind blowing
across the range at 15 knots or better at an intersection
angle of not less than 30 degrees.
2. Pilots from flatland areas should understand
a few things about mountain waves in order to stay
out of trouble. When approaching a mountain range
from the upwind side (generally the west), there will
usually be a smooth updraft; therefore, it is not quite
as dangerous an area as the lee of the range. From the
leeward side, it is always a good idea to add an extra
thousand feet or so of altitude because downdrafts
can exceed the climb capability of the aircraft. Never
expect an updraft when approaching a mountain
chain from the leeward. Always be prepared to cope
with a downdraft and turbulence.
3. When approaching a mountain ridge from the
downwind side, it is recommended that the ridge be
approached at approximately a 45 degree angle to the
horizontal direction of the ridge. This permits a safer
retreat from the ridge with less stress on the aircraft
should severe turbulence and downdraft be experienced.
If severe turbulence is encountered, simultaneously
reduce power and adjust pitch until aircraft
approaches maneuvering speed, then adjust power
and trim to maintain maneuvering speed and fly away
from the turbulent area.
7−5−6. Use of Runway Half−way Signs at
Unimproved Airports
When installed, runway half−way signs provide the
pilot with a reference point to judge takeoff
acceleration trends. Assuming that the runway length
is appropriate for takeoff (considering runway
condition and slope, elevation, aircraft weight, wind,
and temperature), typical takeoff acceleration should
allow the airplane to reach 70 percent of lift−off
airspeed by the midpoint of the runway. The “rule of
thumb” is that should airplane acceleration not allow
the airspeed to reach this value by the midpoint, the
takeoff should be aborted, as it may not be possible to
liftoff in the remaining runway.
Several points are important when considering using
this “rule of thumb”:
a. Airspeed indicators in small airplanes are not
required to be evaluated at speeds below stalling, and
may not be usable at 70 percent of liftoff airspeed.
b. This “rule of thumb” is based on a uniform
surface condition. Puddles, soft spots, areas of tall
and/or wet grass, loose gravel, etc., may impede
acceleration or even cause deceleration. Even if the
airplane achieves 70 percent of liftoff airspeed by the
midpoint, the condition of the remainder of the runway
may not allow further acceleration. The entire length
of the runway should be inspected prior to takeoff to
ensure a usable surface.
c. This “rule of thumb” applies only to runway
required for actual liftoff. In the event that obstacles
affect the takeoff climb path, appropriate distance
must be available after liftoff to accelerate to best angle
of climb speed and to clear the obstacles. This will, in
effect, require the airplane to accelerate to a higher
speed by midpoint, particularly if the obstacles are
close to the end of the runway. In addition, this
technique does not take into account the effects of
upslope or tailwinds on takeoff performance. These
factors will also require greater acceleration than
normal and, under some circumstances, prevent
takeoff entirely.
d. Use of this “rule of thumb” does not alleviate the
pilot’s responsibility to comply with applicable
Federal Aviation Regulations, the limitations and
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