Aeronautical Information Manual (AIM), page 533

Index   532 -- Page 533 -- 534

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 experi-
enced. 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.

7-5-7. 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
performance data provided in the FAA approved
Airplane Flight Manual (AFM), or, in the absence of
an FAA approved AFM, other data provided by the
aircraft manufacturer.

In addition to their use during takeoff, runway
half-way signs offer the pilot increased awareness of
his or her position along the runway during landing

Potential Flight Hazards 7-5-5

Page 533 of the Aeronautical Information Manual (AIM.pdf)
AIM: Official Guide to Basic Flight Information and ATC Procedures

Index   532 -- Page 533 -- 534